BP503.T Pharmacology-II Unit 1 PDF

BP503.T. PHARMACOLOGY-II (Theory) UNIT-I 1. PHARMACOLOGY OF DRUGS ACTING ON CARDIO VASCULAR SYSTEM INTRODUCTION TO HEMODYNAMIC AND ELECTROPHYSIOLOGY OF HEART Hemodynamics is a...

BP503.T. PHARMACOLOGY-II (Theory) UNIT-I 1. PHARMACOLOGY OF DRUGS ACTING ON CARDIO VASCULAR SYSTEM INTRODUCTION TO HEMODYNAMIC AND ELECTROPHYSIOLOGY OF HEART Hemodynamics is a physical and physiological principle of blood flow (circulatory system) in the body. The main functions of blood are  transportation of blood gases, nutrients, wastes  homeostasis (regulation) of pH, body temperature, water content  Protection The forces involved with the movement of blood throughout the human circulatory system include:  Kinetic and potential energy provided by the cardiac pump  Gravity  Hydrostatic Pressure (weight of the liquid acting on a unit area at that depth plus any pressure acting on the surface of the liquid)  Pressure gradients or differences between two any points  Properties of blood itself that affect its flow: o Viscosity o Inertial mass (mass of an object measured by its resistance to acceleration) o Volume of blood to be moved  Factors that affect the motion of blood through the vascular channels include: o Size of the blood vessel o Condition of blood vessel o Smoothness of lumen o Elasticity of muscular layer (tunica media) o Destination of blood (vascular bed)  Pressure: Force per unit area (Unit- Newtons/m2, Pascal(Pa), mmHg)  Flow rate: Amount of fluid passing a given point over a given period of time  Viscosity: The internal friction between adjacent layers of fluid. Blood is 1.5 times as viscous as water and its viscosity.   Kinetic Energy: Active energy, the energy of motion as forward movement of blood. It is transformed in to potential energy when it produces a lateral pressure or stretching of vessel walls during systole.  Potential energy: Stored energy, it is converted back in to kinetic energy when the arterial walls rebound during diastole. Cardiac Output: Cardiac output is defined as the amount of blood flowing from the heart (from the left ventricle in to aorta) over a given period of time (or in one heart beat). Cardiac Output = Stroke volume x Heart rate = 70 ml x 72/min = 5040 ml/min = About 5 litre /min Where Stroke volume = Volume of blood pumped by heart/heart beat Heart rate = Ventricular systole/min CARDIAC ELECTROPHYSIOLOGY The properties which are especially important for understanding drug action on heart are: A. Impulse generation: Electrophysiologically, two types of myocardial fibres can be distinguished (Fig. 1.1). Fig. 1.1. Transmembrane potential of automatic (red) and nonautomatic (purple) myocardial fibres recorded through intracellular electrodes I. Nonautomatic fibres: These are the ordinary working myocardial fibres; cannot generate an impulse of their own. During diastole, the resting membrane potential remains stable (approximately 90 mv negative inside). When stimulated, they depolarize very rapidly (fast 0 phase) with considerable overshoot (+30 mv) rapid return to near isoelectric level (phase-1) maintenance of membrane potential at this level for a considerable period (phase-2, plateau phase) during which Ca2+ ions flow in and bring about contraction relatively rapid repolarization (phase- 3) during which membrane Na+K+ pump gets activated and tends to restore ionic distribution to the resting pattern. Resting membrane potential, once attained, does not decay (stable phase-4). ii. Automatic fibres: These are present in the sinoatrial (SA) and atrioventricular (A-V) nodes, and in the His-Purkinje system, i.e. specialized conducting tissue. In addition, patches of automatic tissue are present in the interatrial septum, A-V ring and around openings of the great veins. The most characteristic feature of these fibres is phase-4 or slow diastolic depolarization, i.e. after repolarizing to the maximum value, the membrane potential decays spontaneously. When it reaches a critical threshold value—sudden depolarization occurs automatically. Thus, they are capable of generating their own impulse. The rate of impulse generation by a particular fibre depends on the value of maximal diastolic potential, the slope of phase-4 depolarization and the value of threshold potential. Fig:1.2. (A) Fast channel action potential (AP) in a Purkinje fibre, (B) Slow channel action potential in a SA node fibre Normally, the SA node has the steepest phase- 4 depolarization, undergoes self- excitation and propagates the impulse to rest of the heart. In other words, it acts as the pacemaker. Other automatic fibres which are also undergoing phase-4 depolarization, but at a slower rate, receive the propagated impulse before reaching threshold value and remain as latent pacemakers. The slow channel AP is characterized by: (a) Initiation at a higher threshold (less negative level). (b) Slower depolarization during 0 phase. (c) Less overshoot, low amplitude. (d) Very slow propagation, decremental conduction and a low safety factor for conduction.  (e) Can arise and propagate in fibres too depolarized to support fast channel Responses Slow channel AP in SA node, A-V node, etc. has a shorter duration and phases 1–3 are not clearly demarkated. Slow channel AP can occur in Purkinje fibres (PF) also, but this has a much longer duration with a prominent plateau phase. II. Conduction: The rate of conduction through a fibre is a function of its membrane responsiveness, which is defined by rate of rise of AP (dv/dt) as a function of membrane potential at which activation occurs (Fig. 1.3); a more completely polarized membrane depolarizes faster because more Na+ channels have recovered (voltage dependent reactivation). This type of relationship is seen in atrial, ventricular and Purkinje fibres (fast channel fibres which depolarize by Na+ current), but not in SA and A-V nodal cells which remain refractory for some time even after attainment of maximal resting potential (Ca2+ channel reactivation is time-dependent). Fig: 1.3. Membrane responsiveness curve of a myocardial fibre showing the relationship between membrane polarization & dv/dt of 0 phase. The Na+ channels get progressively inactivated as the resting membrane potential (RMP) drops over the –80 to –60 mV range. Consequently, less negative the RMP at which activation occurs, fewer are the Na+ channels available for activation—slope of ‘0’ phase depolarization, AP amplitude and conduction velocity are reduced. A drug which reduces the slope of 0 phase (at any given resting membrane potential) will shift the membrane responsiveness curve to the right and impede conduction. The reverse occurs with a drug that shifts the curve to the left. Membrane responsiveness curve can also be altered by disease. Small cells at the upper margin of A-V node have very low conduction velocity (20 mm/sec). Normally Purkinje fibres (PFs) have the highest conduction velocity (4000 mm/sec) except near their junction with the ventricular fibres ‘gate region’, or if they change over from fast channel to slow channel response. III. Excitability This property of a fibre is defined by the strength of stimulus required to elicit a response or to produce an AP. Hyperpolarization decreases excitability while small reductions in resting membrane potential increase excitability by respectively increasing and decreasing the gap between it and the threshold potential. Thus, in fast channel fibres excitability is generally super-normal during the end of phase-3. However, when the resting membrane potential is reduced to a value below the threshold potential, the fibre becomes inexcitable. IV. Refractory period: Pharmacologically, the effective refractory period (ERP) which is the minimum interval between two propagating APs, is the most important. It is closely related to the AP duration (APD). An AP can be evoked in fast channel fibres even before complete repolarization, because Na+ channels recover in a voltage-dependent manner above the threshold potential. As such ERP/APD is 1. Most antiarrhythmic drugs increase ERP/APD ratio. Electrocardiogram (ECG): Electrical currents generated in the heart by the propagation of action potential can be detected on the surface of the surface of the body as electrical signals. These changing signals are recorded by an instrument known as electrocardiograph. The recordings obtained are known as electrocardiogram (ECG). Hence, ECG is a composite record of action potential produced by all the muscle fibres of the heart with each heartbeat. Clinically, the recording of ECG in humans is done by positioning four electrodes on the limbs (limb leads) and six electrodes in different positions on the chest (chest leads). These electrodes detect the electrical changes of heart. The electrical signals recorded from each electrode differ slightly from each other due to the difference in its position relative to the heart. Comparison of these records with each other and with each other and with the normal one helps in determining the complications like: i. Any abnormality in the conducting pathway  ii. Any enlargement in the heart iii. Damage to any region of the heart iv. Any type of pain occurring in the chest. In an ECG, there are three clearly recognizable waves. Fig. 1.4: Normal Electrocardiogram (ECG) 1. P wave: This is the first wave spreading from SA node through contractile fibres in both the atria, representing atrial depolarization. It is seen as a small upward deflection on ECG. 2. QRS complex: the second wave results as the action potential spreads through ventricular contractile fibres, representing rapid ventricular depolarization. The QRS complex continues as large, upright, triangular wave, begning as a downward deflection and ending as a downward wave. 3. T wave: the third wave is seen when ventricle is relaxed, indicating ventricular repolarisation. This wave appears as dome shaped upward deflection. Another characteristics feature of T wave is that it is smaller and wider than the QRS complex. This is because of the fact that the process of repolarisation is slower as compared to that of the depolarization. The size of the each wave appearing on the ECG helps in interpreting any abnormality like: i. An enlarged atrium is indicated by larger P waves. ii. Myocardial infarction is the possibility when Q wave is enlarged. iii. Enlarge of ventricles is usually indicated by an enlarged R wave. iv. Interpretation of T wave may be done as: a. In case of insufficient supply of oxygen to the heart nuscle (as in case of coronary artery disease), it happens flatter than the normal. b. In case of high level of K+ in the blood, it is found to be elevated. Analysis of ECG: Intervals or segments are the time spans present between the waves, which helps in analyzing an ECFG. They can be described as follows: I. P-Q Interval: This segment denotes the time taken for the action potential to move from atria to the atrioventricular node, which is travelling through the remaining fibres of the conduction system. It represents the time between the onset of the P wave and that of the QRS complex. In other words, it signifies the time taken for the conduction of wave starting at atrial contraction and ending at ventricular contraction. II. S-T segment: This segment originates when the S wave ends and concludes just as the T wave begins. It represents time when the contracting fibres of the ventricle are depolarised, which take place during the plateau phase of the action potential. III. Q-T Interval: This segment originates at the start of the QRS complex and continues till the T wave ends. It represents the time taken by the electrical signal to move from the beginning point of depolarisation in the ventricles to the point when repolarisation in the ventricle comes to an end. TREATMENT OF CHF There are two distinct goals of drug therapy in CHF: A. Relief of congestive/low output symptoms and restoration of cardiac performance. This can be achieved by: a. Inotropic drugs: Digoxin, dobutamine/dopamine, amrinone/ milrinone b. Diuretics: Furosemide, thiazides c. RAS inhibitors: ACE inhibitors/ARBs d. Vasodilators: hydralazine, nitrate, nitroprusside e. β blocker: Metoprolol, bisoprolol, carvedilol, Nebivolol B. Arrest/reversal of disease progression and prolongation of survival, possible with: a. ACE inhibitors/ ARBs, β blockers b. Aldosterone antagonist: Spironolactone, eplerenone  Fig. 2.1 Fig. 2.1: The vicious cycle in CHF: compensatory mechanisms evoked in response to reduced cardiac output themselves perpetuate failure and contribute to remodeling responsible for disease progression. The parameter which is improved by different therapeutic measures is indicated CARDIAC GLYCOSIDES (Digoxin) These are glycosidic drugs having cardiac inotropic property. They increase myocardial contractility and output in a hypodynamic heart without a proportionate increase in O2 consumption. Thus, efficiency of failing heart is increased. In contrast, ‘cardiac stimulants’ (Adr, theophylline) increase O2 consumption rather disproportionately and tend to decrease myocardial efficiency. Cardiac glycosides are found in several plants and in toad skin (Bufotoxin). Digitalis lanatais the source of Digoxin, the only glycoside that is currently in use. Others like Digitoxin (from Digitalis purpurea) and Ouabain (from Strophanthusgratus), etc. are no longer clinically used or marketed. Chemistry: The cardiac glycosides consist of an aglycone (genin) to which are attached one or more sugar (glucose or digitoxose) moieties. The aglycone consists of a cyclopentanoperhydrophenanthrene (steroid) ring to which is attached a 5 or 6 membered unsaturated lactone ring.  MECHANISM OF ACTION: Digitalis increases force of cardiac contraction by a direct action independent of innervation. It selectively bindsto extracellular face of the membrane associated Na+K+ ATPase of myocardial fibres and inhibitis this enzyme. Inhibition of this cation pump results in progressive accumulation of Na+ intracellularly. This indirectly results inintracellular Ca2+ accumulation. Fig. 2.2 During depolarization Ca2+ ions enter the celldriven by the steep Ca2+ gradient (>1 mM extracellularto < 100 nM cytosolic during diastole)through voltage sensitive L type Ca2+ channels.This triggers release of larger amount of Ca2+stored in sarcoplasmic reticulum (SR) through Ryanodine calcium channel 2 (RYR2) → cytosolic Ca2+ increases transiently to about 500 nM(calcium transients) → triggers contraction by activating troponin C on myofibrils. The sarcoplasmic-endoplasmic reticular Cal. ATPase2 (SERCA2) is then activated which pumps Ca2+back into the SR. A fraction (equal to that which entered from outside during depolarization) is extruded mainly by 3Na+/1Ca2+ exchange transporter (NCX-antiporter) and to a lesser extent by sarcolemmal Ca2+ pump (Ca2+ ATPase). During phase 3 of AP, membrane Na+K+ATPase moves 3 intracellular Na+ ions for 2 extracellular K+ ions. The slight (1–1.5 mM) increase incytosolic Na+ over normal (8–10 mM) due topartial inhibition of Na+K+ATPase by digitalis reduces transmembrane gradient of Na+ which drives the extrusion of Ca2+. The excess Ca2+ remaining in cytosol is taken up into SR which progressively get loaded with more Ca2+→subsequent calcium transients are augmented.  Binding of glycoside to Na+K+ATPase is slow. Moreover, after Na+K+ATPase inhibition, Ca2+ loading occurs gradually. As such, inotropic effect of digitalis takes hours to develop, even after i.v. administration. Pharmacological Actions: All digitalis glycosides have qualitatively similar action. Digoxin is described as prototype. 1. Heart: Digitalis has direct effects on myocardial contractility and electrophysiological properties. In addition, it has vagomimetic action, reflex effects due to alteration in haemodynamics and direct CNS effects altering sympathetic activity. Force of contraction: Digitalis causes a dose dependent increase in force of contraction of heart—a positive inotropic action. This is especially seen in the failing heart which is exquisitely sensitive. There is increased velocity of tension development and higher peak tension can be generated. Systole is shortened, diastole is prolonged. When a normal heart is subjected to increased impedance to outflow, it generates increased tension so that stroke volume is maintained upto considerably higher values of impedance. while the failing heart is not able to do so and the stroke volume progressively decreases. Fig 2.2 The digitalized failing heart regains some of its capacity to contract more forcefully when subjected to increased resistance to ejection. There is more complete emptying off ailing and dilated ventricles—cardiac output is increased and end- diastolic volume is reduced. However, therapeutic doses of digoxin do not increase resting tension (tone) in myocardial fibres. Rate: Heart rate is decreased by digitalis. Bradycardia is more marked in CHF patients because improved circulation (due to positive inotropic action) restores the diminished vagal tone and abolishes sympathetic over activity. In addition, digitalis slows the heart by vagal and extravagal actions. Vagal tone is increased reflexly by sensitization of baroreceptors, as well as by stimulation of vagal centre. Extravagal: A direct depressant action on SA and A-V nodes. This component of bradycardia is not reversed by atropine. Electrophysiological properties: (a) Action potential (AP): Fig 2.3 The resting membrane potential (RMP) is progressively decreased (to less negative values) with increasing doses. Excitability is enhanced at low doses but depressed at toxic doses because Na+ channels are inactivated. The rate of 0 phase depolarization is reduced resulting in slowing of conduction. This action is most marked in A-V node and bundle of His. The slope of phase-4 depolarization is increased in the PFs—ectopic automaticity is enhanced—latent pacemakers become overt at high doses producing extra systoles. High doses of digitalis produce coupled beats by another mechanism: the RMP shows oscillations during phase-4; when their magnitude is sufficient enough, delayed after-depolarizations result. The SA and A-V node automaticity is reduced at therapeutic concentrations by vagal action which hyperpolarizes these cells and reduces their phase-4 slope. Toxic doses markedly reduce RMP of SA nodal cells by direct action and stop impulse generation. The action potential duration (APD) is reduced (primarily at phase-2) and amplitude of AP is diminished. (b) Effective refractory period (ERP): Effective refractory period of Atrium is decreased by vagal action and increased by direct action. Effective refractory period of A-V node and bundle of His increased  by direct, vagomimetic and antiadrenergic actions; the maximum rate at which impulses can be transmitted is reduced. (c) Conduction: A-V conduction is demonstrably slowed by therapeutic doses. At high doses, intraventricular conduction in PFs is also depressed by uncoupling of gap junctions. (d) ECG: Therapeutic doses of digitalis produce changes in the ECG. These are accentuated at high doses—may also produce arrhythmias. Decreased amplitude or inversion of T wave. Increased P-R interval (due to slowing of A-V conduction), A-V block at toxic doses. Shortening of Q-T interval (reflecting shortening of systole). Depression of ST segment (at high doses—due to interference with repolarization). 2. Vasoconstrictor action: Peripheral resistance is increased in normal individuals. However, in CHF patients this is more than compensated by the indirect effect of improvement in circulation, i.e. reflex sympathetic over activity is withdrawn and a net decrease in peripheral resistance occurs.  no prominent effect on BP  systolic BP may increase and diastolic may fall in CHF patients—pulse pressure increases Therapeutic doses of digitalis have no significant effect on coronary circulation. 3. Kidney: Diuresis occurs promptly in CHF patients, secondary to improvement in circulation and renal perfusion. The retained salt and water is gradually excreted. No diuresis occurs in normal individuals or in patients with edema due to other causes. 4. CNS: Digitalis has little apparent CNS effect in therapeutic dose. Higher doses cause CTZ activation → nausea and vomiting. Still higher doses produce hyperapnoea, central sympathetic stimulation, mental confusion, disorientation and visual disturbances. PHARMACOKINETICS: 1. absorption 60–80% 2. Plasma protein binding 25% 3. Time course of action –Onset 15–30 min–Peak 2–5 hr–Duration 2–6 days  4. Plasma t½ 40 hr 5. Therapeutic concn. 0.5–1.4 ng/ml 6. Toxic concn. > 2 ng/ml 7. Daily maintenance dose 0.125–0.5 mg 8. Daily elimination** 35% 9. Route of elimination Renal (predominant) excretion 10. Route of administration Oral, i.v. ADVERSE EFFECTS: Extracardiac: Anorexia, nausea, vomiting and abdominal pain are usually reported first: are due to gastric irritation, mesenteric vasoconstriction and CTZ stimulation. Fatigue, malaise, headache, mental confusion, restlessness, hyperapnoea, disorientation, psychosis and visual disturbances. Cardiac: Almost every type of arrhythmia can be produced by digitalis: pulsus bigeminus, nodal and ventricular extrasystoles, ventricular tachycardia and terminally ventricular fibrillation. Partial to complete A-V block may be the sole cardiac toxicity, or it may accompany other arrhythmias. Severe bradycardia, atrial extrasystoles, AF or AFl have also been noted. In about 2/3 patients showing toxicity, extracardiac symptoms precede cardiac; in the rest serious cardiac arrhythmias are the first manifestation. Digoxin antibody-Fab fragment INTERACTIONS:  Diuretics: cause hypokalemia which increases the risk of digitalis arrhythmias; potassium supplements should be given prophylactically  Calcium: synergises with digitalis → precipitates toxicity  Quinidine: reduces binding of digoxin to tissue proteins as well as its renal and biliary clearance by inhibiting efflux transporter P-glycoprotein → plasma concentration of digoxinis doubled → toxicity can occur. Verapamil, diltiazem, captopril, propafenone and amiodarone also increase plasma concentration of digoxin to variable extents. USES: 1. Congestive heart failure 2. Cardiac arrhythmias a. Atrial fibrillation (AF) b. Atrial flutter (AFI) c. Paroxysmal supraventricular tachycardia(PSVT) DOSAGE: 0.125–0.25 mg/day Contraindications:  Hypokalemia: enhances digitalis toxicity  Elderly, renal or severe hepatic disease: patients are more susceptible to digoxin toxicity.  Myocardial ischaemia: severe arrhythmias are more likely  Thyrotoxicosis: patients are more prone to develop digitalis arrhythmias  Myxoedema: these patients eliminate digoxin more slowly; cumulative toxicity can occur  Ventricular tachycardia: digitalis is contraindicated because it may precipitate ventricular fibrillation  Partial A-V block: may be converted to complete A-V block by digoxin.  Acute myocarditis: Diphtheria, acute rheumatic carditis, toxic carditis—inotropic response to digitalis is poor, more prone to arrhythmias.  Wolff-Parkinson-White syndrome: Digitalis is contraindicated because it decreases the ERP of bypass tract in 1/3 patients. In them rapid atrial impulses may be transmitted to ventricles → VF may occur. Digitalis can increase the chances of reentry by slowing conduction in the normal A-V bundle and accelerating it in the aberrant pathway. Meaning: Frank-Starlingcompensation: Because of lower inotropic state, the failing heart is able to pump much less blood at the normal filling pressure, more blood remains in the ventricles at the end of systole.The venous return is added to it and Frank-Starling compensation is utilized to increase filling pressure: the heart may be able to achieve the required stroke volume, but at a filling pressure which produces congestive symptoms (venous engorgement, edema, enlargement of liver, pulmonary congestion →dyspnoea, renal congestion → oliguria).  Fig 2.4 Laplace equation: A dilated ventricle automatically becomes inefficient according to Laplace equation. Wall tension = Intraventricular pressure ×ventricular radius i.e. to generate the same ejection pressure a dilated ventricle has to develop higher wall tension. By reducing end diastolic volume (due to better emptying), digitalis restores efficiency of translation of cardiac work into cardiac output.That is why O2 consumption does not increase proportionately. Sympathomimetic inotropic drugs; Drugs with β adrenergic and dopaminergic D1 agonistic actions have positive inotropic and (at low doses) vasodilator properties which may be utilized to combat emergency pump failure. Dobutamine (2–8 μg/kg/min) a relatively selective β1 agonist with prominent inotropic action (increase force of contraction & output) is the preferred drug for i.v. infusion in acute heart failure accompanying myocardial infarction (MI), cardiac surgery as well as to tide over crisis in advanced decompensated CHF. Dopamine (3–10 μg/kg/min by i.v. infusion) has been used in cardiogenic shock due to MI and other causes. While dobutamine does not raise (may lower) systemic vascular resistance and is preferred in heart failure, dopamine tends to increase afterload, especially at higher rates of infusion (>5 μg/kg/min) and has limited utility in patients who are not in shock. Low rates of dopamine infusion (~2 μg/kg/min) cause selective renal vasodilatation (D1 agonistic action) which improves renal perfusion and g.f.r. This can restore diuretic response to i.v. furosemide in refractory CHF. These drugs afford additional haemodynamic support over and above vasodilators, digitalis and diuretics, but benefits are short-lasting. Due to development of tolerance and cardiotoxic potential when used regularly, these drugs have no role in the long-term management of CHF. Amrinone (Phosphodiesterase 3 inhibitors) This bipyridine derivative is a selective phosphodiesterase 3 (PDE3) inhibitor. The PDE3 isoenzyme is specific for intracellular degradation of cAMP in heart, blood vessels and bronchial smooth muscles. Amrinone increases myocardial cAMP and transmembrane influx of Ca2+. It does not inhibit Na+K+ATPase, and its action is independent of tissue catecholamines as well as adrenergic receptors. The two most important actions of amrinone are positive inotropy and direct vasodilatation: has been called an ‘inodilator’. Both preload and afterload on the heart is reduced. Compared to dobutamine, proportionately greater decrease in systemic vascular resistance is noted. Adverse effects: Thrombocytopenia is the most prominent and dose related side effect, but is mostly transient and asymptomatic. Nausea, diarrhoea, abdominal pain, liver damage, fever and arrhythmias are the other adverse effects. Dose: 0.5 mg/kg bolus injection followed by 5–10 μg/kg/ min i.v. infusion (max. 10 mg/kg in 24 hours). Uses: It is indicated only for short-term i.v. use in severe and refractory CHF, as an additional drug to conventional therapy with digitalis, diuretics and vasodilators. Diuretics; Almost all cases of symptomatic CHF are treated with a diuretic. High ceiling diuretics (furosemide, bumetanide) are the diuretics of choice for mobilizing edema fluid; later they may be continued in low doses. In advanced CHF after chronic use, resistance may develop to even high ceiling diuretics. Addition of a thiazide/ metolazone/spironolactone to furosemide may overcome the resistance. Thiazide alone has very limited role in CHF. Diuretics work by: (a) Decrease preload and improve ventricular efficiency by reducing circulating volume. (b) Remove peripheral edema and pulmonary congestion. Intravenous furosemide promptly increases systemic venous capacitance and produces rapid symptomatic relief in acute left ventricular failure. It has, in conjunction with vasodilators, virtually obviated the need for i.v. digitalization. Chronic diuretic therapy tends to cause hypokalaemia, alkalosis and carbohydrate intolerance. Aldosterone antagonist (Spironolactone, Eplerenone): In CHF aldosterone is very important contributor to disease progression by direct and indirect effects:  Expansion of e.c.f. volume → increased cardiac preload  Fibroblast proliferation and fibrotic change in myocardium → worsening systolic dysfunction and pathological remodeling.  Hypokalemia and hypomagnesemia → increased risk of ventricular arrhythmias and sudden cardiac death.  Enhancement of cardiotoxic and remodeling effect of sympathetic overactivity.  So, the aldosterone antagonist spironolactone is very beneficial in CHF but found to have a weak diuretic effects by antagonizing the above effects of aldosterone.  It is indicated as add-on therapy to ACE inhibitors + other drugs in moderate- to-severe CHF.  It can retard disease progression, reduce episodes of decompensation and death due to heart failure as well as sudden cardiac deaths, over and above the protection afforded by ACE inhibitors/ARBs ± β blockers.  It may help restoration of diuretic response to furosemide when refractoriness has developed. Renin-angiotensin system (RAS) inhibitors: Since RAS activation is pivotal to development of symptoms and disease progression in CHF, the ACE inhibitors and ARBs are the sheet anchor of drug therapy in CHF. They afford symptomatic as well as disease modifying benefits in CHF by causing vasodilatation, retarding/preventing ventricular hypertrophy, myocardial cell apoptosis, fibrosis intercellular matrix changes and remodeling. In addition to decreasing Ang II production, ACE inhibitors raise the level of kinins which stimulate generation of cardioprotective NO and PGs. Symptomatic and prognostic benefits of ACE inhibitors/ARBs have been established in mild to severe CHF as well as in subjects with asymptomatic systolic dysfunction. They are thus recommended for all grades of CHF, unless contraindicated, or if renal function deteriorates by their use (mainly in those with decresed renal blood flow/renal artery stenosis). ACE inhibitor therapy is generally started at low doses which are gradually increased to obtain maximum benefit or to near the highest recommended doses. (Detaill discussion in antihypertensive drugs). Vasodilators: (Deatails in antianginal drugs) Vasodilators were first used i.v. to treat acute heart failure that occurs in advanced cases or following MI, and serve to tide over crisis. Their use by oral route has been extended to long-term therapy of chronic CHF, but vasodilators other than ACE inhibitors/ARBs have only limited utility. Vasodilators with differing profiles of arteriolar and venodilator action are available. Preload reduction: Nitrates cause pooling of blood in systemic capacitance vessels to reduce ventricular end-diastolic pressure and volume. With reduction in size of ventricles, effectiveness of myocardial fibre shortening in causing ejection of blood during systole improves (Laplace relationship). Controlled i.v. infusion of glyceryl trinitrate affords rapid relief in acute left ventricular failure, particularly that due to myocardial ischaemia/infarction. It is indicated when the central venous pressure (CVP) is raised and in dilated cardiomyopathy. However, lowering of preload (by vasodilators + strong diuretics) beyond a limit may reduce output of a failing heart whose performance is dependent upon elevated filling pressure. Occurrence of nitrate tolerance limits their utility in routine treatment of CHF. Afterload reduction: Hydralazine dilates resistance vessels and reduces aortic impedance so that even weaker ventricular contraction is able to pump more blood; systolic wall stress is reduced. It is effective in forward failure when cardiac index (CI = min output/body surface area) is low (< 2.5 L/min/m2) without a marked increase in CVP (< 18 mm Hg). Marked tachycardia, worsening of myocardial ischaemia and fluid retention limit long-term use of hydralazine monotherapy. Pre- and after load reduction: Sod. Nitroprusside is a high efficacy i.v. dilator with equal action on the two types of vessels. It acts by both the above mechanisms, i.e. reduces ventricular filling pressure as well as systemic vascular resistance. Cardiac output and renal blood flow are increased. The action is very fast and brief. Titrated i.v. infusion of nitroprusside is employed in conjunction with a loop diuretic + i.v. inotropic drug to tideover crisis in severely decompensated patients. For symptomatic treatment of acute heart failure, choice of i.v. vasodilator (glyceryl trinitrate or hydralazine or nitroprusside) depends on the primary haemodynamic abnormality in individual patients. Severe CHF patients already receiving ACE inhibitors + digoxin + diuretic have obtained extra benefit from addition of hydralazine with or without a nitrate. β-Adrenergic blockers: β1 blockers (mainly metoprolol, bisoprolol, nebivolol) and the nonselective β + selective α1 blocker carvedilol are used in mild to moderate CHF treated with ACE inhibitor ± diuretic, digitalis. The benefits appear to be due to antagonism of ventricular wall stress enhancing, apoptosis promoting and pathological remodeling effects of excess sympathetic activity (occurring reflexly) in CHF, as well as due to prevention of sinister arrhythmias. Greatest utility of β blockers has been shown in mild to moderate cases of dilated cardiomyopathy with systolic dysfunction in which they are now routinely coprescribed unless contraindicated. Starting dose should be very low—then titrated upward as tolerated to the target level (carvedilol 50 mg/day, bisoprolol 10 mg/day, metoprolol 200 mg/day) or near it, for maximum protection. A long-acting preparation (e.g. sustained release metoprolol) or 2–3 times daily dosing to produce round-the-clock β blockade should be selected. ANTIHYPERTENSIVE DRUGS These are drugs used to lower BP in hypertension. Hypertension is a very common disorder, particularly past middle age. It is not a disease in itself, but is an important risk factor for cardiovascular mortality and morbidity. The cutoff manometric reading between normotensives and hypertensives is arbitrary. For practical purposes ‘hypertension’ could be that level of BP at or above which long-term antihypertensive treatment will reduce cardiovascular mortality. The JNC 7* (2003) and WHO-ISH@ guidelines (2003) have defined it to be 140 mm Hg systolic and 90 mm Hg diastolic, though risk appears to increase even above 120/80 mm Hg. The hypertension is of two types: i. Essential (primary) hypertension- where causes are not known. Sympathetic and rennin-angiotensin system (RAS) may or may not be overactive. They are not contribute to the tone of the blood vessel and c.o. in hypertensives, as they do in normotensives ii. Secondary hypertension: where causes are known. May be due to sympathetic and renin-angiotensin systems (RAS) imbalance. CLASSIFICATION OF DRUGS: 1. Diuretics: Thiazides: Hydrochlorothiazide, Chlorthalidone, Indapamide High ceiling: Furosemide, etc. K+ Sparing: Spironolactone, Amiloride 2. ACE inhibitors: Captopril, Enalapril, Lisinopril, Perindopril, Ramipril, Fosinopril, etc. 3. Angiotensin (AT1 receptor) blockers: Losartan, Candesartan, Irbesartan, Valsartan, Telmisartan 4. Direct renin inhibitor: Aliskiren 5. Calcium channel blockers: Verapamil, Diltiazem, Nifedipine, Felodipine, Amlodipine, Nitrendipine, Lacidipine, etc. 6. β Adrenergic blockers: Propranolol, Metoprolol, Atenolol, etc. 7. β + α Adrenergic blockers Labetalol, Carvedilol  8. α Adrenergic blockers: Prazosin, Terazosin, Doxazosin, Phentolamine, Phenoxybenzamine 9. Central sympatholytics: Clonidine, Methyldopa 10. Vasodilators Arteriolar: Hydralazine, Minoxidil, Diazoxide Arteriolar + venous: Sodium nitroprusside Diuretics: Diuretics (natriuretics) are drugs which cause a net loss of Na+ and water in urine. However, Na+ balance is soon restored, even with continuing diuretic action, by compensatory homeostatic mechanisms of the body. CLASSIFICATION: 1. High efficacy diuretics (Inhibitors of Na+-K+-2Cl¯ cotransport): (a) Sulphamoyl derivatives: Ex. Furosemide, Bumetanide, Torasemide (b) Phenoxy acetic acid derivatives. Ethacrynic acid 2. Medium efficacy diuretics (Inhibitors of Na+-Cl¯ symport): (a) Benzothiadiazines (thiazides): Hydrochlorothiazide, Benzthiazide,Hydroflumethiazide, Bendroflumethiazide (b) Thiazide like (related heterocyclics): Chlorthalidone, Metolazone, Xipamide,Indapamide, Clopamide HIGH CEILING (LOOP) DIURETICS (Inhibitors of Na+-K+-2Cl¯ Cotransport) Furosemide (Frusemide) Mechanism of Action: The major site of action is the thick AscLH (therefore called loop diuretics) where furosemide inhibits Na+- K+-2Cl¯ cotransporter. A minor component of action on PT. It is secreted in PT by organic anion transport and reaches AscLH where it acts from luminal side of the membrane. The cortico-medullary osmotic gradient is abolished and positive as well as negative free water clearance is blocked. K+ excretion is increased mainly due to high Na+ load reaching DT. A glycoprotein with 12 membrane spanning domains has been found to function as the Na+- K+-2Cl¯ transporterin many epithelia performing secretory/absorbing function, including AscLH. Recently, distinct absorptive and secretory isoforms of Na+-K+-2Cl¯cotransporter have been isolated. The former is exclusively expressed at the luminal membrane of thick AscLH. Furosemide attaches to the Cl¯ binding site of this protein to inhibit its transport function. The secretory form is expressed on the basolateral membrane of most glandular and epithelial cells. Fig. 3.1  Furosemide is a strong diuretic, but the antihypertensive efficacy does not parallel diuretic potency. Furosemide is a weaker antihypertensive than thiazides: fall in BP is entirely dependent on reduction in plasma volume and c.o. The explanation to this paradox may lie in its brief duration of action. The t.p.r. and vascular responsiveness are not reduced. They are indicated in hypertension only when it is complicated by: o Chronic renal failure: thiazides are ineffective, both as diuretic and as antihypertensive. o Coexisting refractory CHF. o Resistance to combination regimens containing a thiazide, or marked fluid retention due to use of potent vasodilators. Desirable properties of thiazide diuretics as antihypertensives are:  Once a day dosing and flat dose-response curve permitting simple standardized regimens  No fluid retention, no tolerance  Low incidence of postural hypotension and relative freedom from side effects, especially CNS, compared to sympatholytics.  Effective in isolated systolic hypertension (ISH)  Lessened risk of hip fracture in the elderly due to hypocalciuric action of thiazides.   Low cost Other Actions: Furosemide has weak CAse inhibitory action; increases HCO3¯ excretion. Causes acute changes in renal and systemic haemodynamics. After 5 min of i.v. injection, renal blood flow is transiently increased and there is redistribution of blood flow from outer to midcortical zone; g.f.r. generally remainsunaltered due to compensatory mechanisms despite increased renal blood flow. Intravenous furosemide causes prompt increase in systemic venous capacitance and decreases left ventricular filling pressure, even before the saluretic response is apparent. This action also appears to be PG mediated and isresponsible for the quick relief it affords in LVF and pulmonary edema. Furosemide increases Ca2+ excretion as well as Mg2+excretion by abolishing transepithelial potential difference in the thick AscLH which drivesre absorption of these divalent cations. It tends toraise blood uric acid level I chronic use by competing with itsproximal tubular secretion as wellby increasing reabsorption in PT which is a consequence of reduced e.c.f. volume. A small rise in blood sugar level may be noted after regular use of furosemide Pharmacokinetics:  Rapidly absorbed orally but bioavailability is about 60%. In severe CHF oral bioavailability may be markedly reduced necessitating parenteral administration. Lipid-solubility is low, and it is highly bound to plasma proteins. It is partly conjugated with glucuronic acid and mainly excreted unchanged by glomerular filtration as well as tubular secretion. Some excretion in bile and directly in intestine also occurs.  Plasma t½ averages 1–2 hour but is prolonged in patients with pulmonary edema, renal and hepatic insufficiency. Dose: Usually 20–80 mg once daily in the morning. In renal insufficiency, upto 200 mg 6 hourly has been given byi.m./i.v. route. In pulmonary edema 40–80 mg may be given i.v. Use of high ceiling diuretics: 1. Edema 2. Acute pulmonary edema (acute LVF,following MI)  3. Cerebral edema 4. Hypertension 5. Along with blood transfusion in severe anaemia, to prevent volume overload. Infused with hypertonic saline, it may be helpful in hyponatraemia. 6. Hypercalcaemia of malignancy THIAZIDE AND RELATED DIURETICS: (Inhibitors of Na+-Cl¯ symport) These are medium efficacy diuretics with primary site of action in the cortical diluting segment or the early DT (Site III). Here they inhibit Na+–Cl¯ symport at the luminal membrane. They do not affect the corticomedullary osmotic gradient indicating lack of action at the medullary thick AscLH. Positive free water clearance is reduced, because tubular fluid is not maximally diluted (very dilute urine cannot be passed in the absence of ADH), but negative free water clearance (in the presence of ADH) is not affected. This strengthens the view that the site of action is in between thick AscLH and late DT. These drugs gain access to their site of action via organic acid secretory pathway in PT and then along the tubular fluid to the early DT, where they bind to specific receptors located on the luminal membrane. The Na+-Cl¯ symporter is also a glycoprotein with12 membrane spanning domains that binds thiazides. Fig. 3.2 Under thiazide action, urinary K+ excretion is increased in parallel to the natriuretic response as the increased amount of Na+ is presented to the distal nephron, more of it exchanges with K+: By their action to reduce blood volume, as well as intrarenal haemodynamic changes, they tend to reduce g.f.r. This is one reason why thiazides are not effective in patients with low g.f.r.  They decrease renal Ca2+ excretion and increase Mg2+ excretion by a direct distal tubular action. Thiazides cause greater reduction in urate excretion than furosemide The extrarenal actions of thiazides consist of a slowly developing fall in BP in hypertensives and elevation of blood sugar in some patients due to decreased insulin release which probably is a consequence of hypokalaemia. Thiazides (like hydrochlorothiazide, chlorthalidone) the diuretic of choice for uncomplicated hypertension. Chlorthalidone is longer acting (~ 48 hours) than hydrochlorothiazide (< 24 hours). Indapamide is also mainly used as antihypertensive, and is equally effective. The antihypertensive action is mainly due to mechanism like: o Initially, the diuresis reduces plasma and e.c.f. volume by 5–15%, and this decreases c.o. o Subsequently, compensatory mechanisms operate to almost regain Na+ balance and plasma volume; c.o. is restored, but the fall in BP is maintained by a slowly developing reduction in t.p.r. o The reduction in t.p.r. is most probably an indirect consequence of a small (~5%) persisting Na+ and volume deficit. Decrease in intracellular Na+ concentration in the vascular smooth muscle may reduce stiffness of vessel wall, increase their compliance and dampen responsiveness to constrictor stimuli (NA, Ang II). A mild slowly developing vasodilator action of thiazides due to opening of smooth muscle K+ATP channels and hyperpolarization has been proposed. Pharmacokinetics: All thiazides and related  drugs are well absorbed orally  Their actionstarts within 1 hour, but the duration varies from6–48 hours TABLE 41.1 Thiazides and related diuretics Drug Daily dose CAse Duration (mg) inhibition of action (Hrs) Hydrochlorothiazide 12.5–100 + 6–12 Chlorthalidone 50–100 ++ 48 Metolazone 5–20 + 12–24 Xipamide 20–40 + 12 Indapamide 2.5–5 – 12–24 Clopamide 10–60 ± 12–18 Uses:  Edema  Hypertension  Diabetes insipidus Thiazides decrease positive free water clearance and are the only drugs effective in nephrogenic diabetes insipidus. However, they reduce urine volume in pituitary origin cases as well.  Hypercalciuria with recurrent calcium stones in the kidney. Thiazides act by reducing Ca2+excretion. Complications of high ceiling and thiazide type diuretic therapy: 1. Hypokalaemia: This is the most significant problem. It is rare at low doses, but may be of grave consequence when brisk diuresis is induced or on prolonged therapy, especially if dietary K+ intake is low. Degree of hypokalaemia appears to be related to the duration of action of the diuretic; longer acting drugs cause more K+ loss. The usual manifestations are weakness, fatigue, muscle cramps; cardiac arrhythmias are the serious complications. Hypokalaemia is less common with standard doses of high ceiling diuretics than with thiazides, possibly because of shorter duration of action of the former which permits intermittent operation of compensatory repletion mechanisms. Hypokalaemia can be prevented and treated by: (a) High dietary K+ intake or (b) Supplements of KCl (24–72 mEq/day) or (c) Concurrent use of K+ sparing diuretics. 2. Acute saline depletion: Over enthusiastic use of diuretics, particularly high ceiling ones, may cause dehydration and marked fall in BP (especially in erect posture). Haemoconcentration increases risk of peripheral venous thrombosis. Serum Na+ and Cl¯ levels remain normal because isotonic saline is lost. It should be treated by saline infusion. 3. Dilutional hyponatraemia:  Occurs in CHF patients when vigorous diuresis is induced with high ceiling agents, rarely with thiazides. Kidney tends to retain water, though it is unable to retain salt due to the diuretic; e.c.f. gets diluted, hyponatraemia occurs and edema persists despite natriuresis. Patients feel very thirsty. Treatment of this distortion of fluid-electrolyte balance is difficult: withhold diuretics, restrict water intake and give glucocorticoid which enhances excretion of water load. If hypokalaemia is present, its correction helps. 4. GIT and CNS disturbances: Nausea, vomiting and diarrhoea may occur with any diuretic Headache, giddiness, weakness, paresthesias, impotence are occasional complaints with thiazides as well as loop diuretics. 5. Hearing loss: Occurs rarely, only with high ceiling diuretics and when these drugs are used in the presence of renal insufficiency. Increased salt content of endolymph and direct toxic action on the hair cells in internal ear appear to be causative. 6. Allergic manifestations: Rashes, photosensitivity occurs, especially in patients hypersensitive to sulfonamides. Blood dyscrasias are rare; any diuretic may be causative. 7. Hyperuricaemia: Long-term use of higherdose thiazides in hypertension has caused rise inblood urate level. This is uncommon now due touse of lower doses. Furosemide produces a lower incidence of hyperuricaemia. This effect can be counteracted by allopurinol. Probenecid is better avoided, because it may interfere with the diuretic response, particularly of loop diuretics. 8. Hyperglycaemia and hyperlipidemia: These are occurred in the use of diuretics as antihypertensive. These metabolic changes are minimal with low dose thiazides now recommended. 9. Hypocalcaemia may occur with high ceiling diuretics when these are administered chronically. Thiazides, on the other hand, tend to raise serum Ca2+; may aggravate hypercalcaemia due to other causes. 10. Magnesium depletion: It may develop after prolonged use of thiazides as well as loop diuretics, and may increase the risk of ventricular arrhythmias, especially after MI or when patients are digitalized. K+ sparing diuretics given concurrently minimise Mg2+ loss. 11. Thiazides have sometimes aggravated renalin sufficiency, probably by reducing g.f.r.  12. Brisk diuresis induced in cirrhotics may precipitate mental disturbances and hepatic coma.It may be due to hypokalaemia, alkalosis and increased blood NH3 levels. 13. Diuretics should be avoided in toxaemia of pregnancy in which blood volume is low despite edema. Diuretics may further compromise placental circulation increasing the risk of miscarriage, foetal death. Interactions: 1. Thiazides and high ceiling diuretics potentiate all other antihypertensives. This interaction is intentionally employed in therapeutics. 2. Hypokalaemia induced by these diuretics: Enhances digitalis toxicity. Increases risk of polymorphic ventricular tachycardia due to drugs which prolong Q-T interval. Reduces sulfonylurea action. 3. High ceiling diuretics and aminoglycoside antibiotics are both ototoxic and nephrotoxic; produce additive toxicity; should be used together cautiously. 4. Cotrimoxazole given with diuretics has caused higher incidence of thrombocytopenia.  Probenecid competitively inhibits tubular secretion of furosemide and thiazides: decreases their action by lowering concentration in the tubular fluid, while diuretics diminish uricosuric action of probenecid. ACE INHIBITORS: The ACE inhibitors are one of the first choice drugs in all grades of essential as well as renovascular hypertension (except those with bilateral renal artery stenosis). Fig. 2.3 Captopril:  It is a sulfhydryl containing dipeptide surrogate of proline which abolishes the pressor action of Ang I but not that of Ang II: does not block AT1 or AT2 receptors.  ACE is a relatively nonspecific enzyme; splits off a dipeptidyl segment from several peptides including bradykinin, substance P, a natural stem cell regulating peptide, etc. in addition to Ang I. As such, captopril increases plasma kinin levels and potentiates the hypotensive action of exogenously administered bradykinin. Pretreatment with B2 kinin receptor antagonist has shown that kinins do contribute to the acute vasodepressor action of ACE inhibitors, but they appear to have little role in the long-term hypotensive effect, probably because, firstly kinins play only a minor role, if at all, in BP regulation, and, secondly another enzyme ‘Kininase I’ (which also degrades bradykinin) is not inhibited by captopril.  Elevated kinins (and PGs whose synthesis is enhanced by kinins) may be responsible for cough and angioedema induced by ACE inhibitors in susceptible individuals.  ACE inhibitors interfere with degradation of substance P also. Rise in the level of stem cell regulator peptide caused by ACE inhibitors could, in part, be responsible for their cardioprotective effect in CHF.  Captopril lowers BP, but in the short-term, magnitude of response is dependent on Na+ status and the level of RAS activity.  In CHF also, the renin level is raised and antihypertensive doses of captopril cause marked fall in BP initially. ACE inhibitor therapy in these situations has to be initiated at much lower doses. A greater fall in BP occurs in renovascular, accelerated and malignant hypertension as well. In essential hypertension it has been found that RAS is overactive in 20%, normal in 60% and hypoactive in the rest. Thus, it contributes to maintenance of vascular tone in over 80% cases and its inhibition results in lowering of BP.  Treatment with ACE inhibitors causes feed back increase in renin release resulting in overproduction of Ang I. Since its conversion to Ang II is blocked, Ang I is diverted to produce more Ang (1-7) which has vasodilator property, and could contribute to the BP lowering action of ACE inhibitors. While the initial fall in BP is dependent on renin and Ang II levels, in the long-term no correlation has been observed between plasma renin activity (PRA) and magnitude of fall in BP due to captopril.   Captopril induced hypotension is a result of decrease in total peripheral resistance. The arterioles dilate and compliance of larger arteries is increased. Both systolic and diastolic BP fall. It has no effect on cardiac output. Pharmacokinetics: About 70% of orally administered captopril is absorbed. Presence of food in stomach reduces its bioavailability. Penetration in brain is poor. It is partly metabolized and partly excreted unchanged in urine. The plasma t½ is ~2 hours, but actions last for 6–12 hours. Adverse effects: The adverse effect profile of all ACE inhibitors is similar. Captopril is well tolerated by most patients, especially if daily dose is kept below 150 mg.  Hypotension: an initial sharp fall in BP occurs especially in diuretic treated and CHF patients; persistent hypotension may be troublesome in MI patients.  Hyperkalaemia: more likely in patients with impaired renal function and in those taking K+ sparing diuretics, NSAIDs or β blockers. In others significant rise in plasma K+ is rare.  Cough:  Rashes, urticaria:  Angioedema:  Dysgeusia:.  Foetopathic: foetal growth retardation, hypoplasia of organs and foetal death may occur if ACE inhibitors are given during later half of pregnancy. A recent report indicates 2.7-fold higher malformation rate in fetuses exposed to ACE inhibitors in the first trimester. ACE inhibitors must be stopped when the woman conceives.  Headache, dizziness, nausea and bowel upset:  Granulocytopenia and proteinuria:  Acute renal failure Interactions:  Diuretics synergise with the hypotensive action of ACE inhibitors by depleting Na+ and raising renin levels. In diuretic treated patients, the starting dose of ACE inhibitors should be low.   Indomethacin (and other NSAIDs) attenuate the hypotensive action by retaining salt and water. Incidents of renal failure have been reported when a NSAID was given to patients (especially elderly) receiving ACE inhibitor + diuretic.  Hyperkalaemia can occur if K+ supplements/ K+ sparing diuretics are given with captopril.  Antacids reduce bioavailability of captopril. Dose: 25 mg BD, increased gradually upto 50 mg TDS according to response.. Enalapril: This is the second ACE inhibitor to be introduced. It is a prodrug, deesterified in the liver to enalaprilat (a tripeptide analogue), which is not used as such orally because of poor absorption, but is marketed as injectable preparation in some countries. Enalapril has the same pharmacological, therapeutic and adverse effect profile as captopril. Dose: 5–20 mg OD or BD. Lisinopril: It is the lysine derivative of enalaprilat; does not require hydrolysis to become active ACE inhibitor. Its oral absorption is slow (making first dose hypotension less likely) and incomplete, but unaffected by food. The duration of action is considerably longer, permitting single daily dose and ensuring uniform hypotensive action round the clock. A reduction in venous return, cardiac contractility and cardiac output has been noted after few weeks of lisinopril use. Dose: 5–20 mg OD or BD. Perindopril; Another long-acting ACE inhibitor with a slow onset of action: less chance of first dose hypotension. Though 66–95% of orally administered perindopril is absorbed, only about 20% is converted to the active metabolite perindoprilat. Extensive metabolism to other inactive products occurs. Efficacy and tolerance of perindopril are similar to other ACE inhibitors. Dose: 2-4 mg/OD Fosinopril: This ACE inhibitor is unique in being a phosphinate compound that is glucuronide conjugated and eliminated both by liver and kidney. The t½ is not altered by renal impairment and the dose remains the same. However, like most others, it is a prodrug suitable for once daily administration. First dose hypotension is more likely. Dose: Initially 10 mg (elderly 5 mg) OD; maximum 40 mg/day. Ramipril: The distinctive feature of this longacting ACE inhibitor is its extensive tissue distribution. Greater inhibition of local RAS has been claimed. However, whether this confers any therapeutic advantage is not known. The plasma t½ of its active metabolite ramiprilat is 8– 18 hours,but terminel t½ is longer due to slow release of tissue bound drug. Quinapril: A prodrug carboxyl ACE inhibitor that is rapidly and completely converted in the liver to the active form Quinaprilat. Like ramiprilat, it is highly bound to the tissue ACE and exhibits a biphasic plasma t½ of 2 hours and 24 hours. Elimination occurs in urine and bile in a ratio of 2:1. Dose: 10–40 mg/day Trandolapril It is a carboxyl prodrug that is 40–60% bioavailable in the active form. Absorption is delayed but not decreased by food. The peak effect occurs at 4–6 hours. It is partly metabolized and eliminated both in urine and faeces. The plasma t½ of active metabolite is biphasic 10–24 hours, suitable for once daily dosing. Dose: 2–4 mg (max 8 mg) OD USES:  Hypertension  CHF  Myocardial infarction (MI)  Prophylaxis in high cardiovascular risk subjects  Diabetic nephropathy  Scleroderma crisis ANGIOTENSIN ANTAGONISTS (Angiotensin receptor blockers or ARBs) Losartan: It is a competitive antagonist and inverse agonist, 10,000 times more selective for AT1 than for AT2 receptor; does not block any other receptor or ion channel, except thromboxane A2 receptor (has some platelet antiaggregatory property).  All overt actions of Ang II, viz. vasoconstriction, reabsorption, central actions like thirst, vasopressin release and growth-promoting actions on heart and blood vessels are blocked.  They result in more complete inhibition of AT1 receptor activation, because responses to Ang II generated via alternative pathways and consequent AT1 receptor activation (which remain intact with ACE inhibitors) are also blocked.   They result in indirect AT2 receptor activation. Due to blockade of AT1 receptor mediated feedback inhibition—more Ang II is produced which acts on AT2 receptors that remain unblocked. ACE inhibitors result in attenuation of both AT1 and AT2 receptor activation.  Losartan causes fall in BP in hypertensive patients which lasts for 24 hours, while HR remains unchanged and cardiovascular reflexes are not interfered. No significant effect on plasma lipid profile, carbohydrate tolerance, insulin sensitivity has been noted. A mild probenecid like uricosuric action is produced. Pharmacokinetics:  Oral absorption of losartan is not affected by food, but bioavailability is only 33% due to first pass metabolism.  It is partially carboxylated in liver to an active metabolite (E3174) which is a 10–30 times more potent noncompetitive AT1 receptor antagonist. After oral ingestion peak plasma levels are attained at 1 hr for losartan and at 3–4 hours for E3174. Both compounds are 98% plasma protein bound, do not enter brain and are excreted by the kidney.  The plasma t½ of losartan is 2 hr, but that of E3174 is 6–9 hr. No dose adjustment is required in renal insufficiency, but dose should be reduced in presence of hepatic dysfunction. Adverse effects:  Hypotension  hyperkalemia,  Angioedema  Headache,  dizziness,  weakness  fetopathic potential like ACE inhibitors—not to be administered during pregnancy. Dose: 50 mg OD, rarely BD; Candesartan: It has the highest affinity for the AT1 receptor and produces largely unsurmountable antagonism, probably due to slow dissociation from the receptors or receptor desensitization. Elimination occurs by both hepatic metabolism and renal excretion with a t½ of 8- 12 hours: action lasts 24 hours. Dose: 8 mg OD (max 8 mg BD), liver/kidney impairment 4 mg OD. Irbesartan: The oral bioavailability is relatively high. It is partly metabolized and excreted mainly in bile. The t½ is ~12 hours. Dose: 150–300 mg OD. Valsartan The AT1 receptor affinity of valsartan is similar to that of losartan. Its oral bioavailability averages 23% and food interferes with its absorption. Elimination occurs mainly by the liver in unchanged form with a t½ of 6–9 hours; action lasts 24 hours. Dose: 80–160 mg OD 1 hour before meal (initial dose in liver disease 40 mg). Telmisartan: The AT1 receptor blocking action of telmisartan is similar to losartan, but it does not produce any active metabolite. After an oral dose, peak action occurs in 3 hours and action lasts > 24 hours. It is largely excreted unchanged in bile; dose reduction is needed in liver disease. Dose: 20–80 mg OD. Uses:  Hypertension  CHF  Myocardial infarction  Diabetic nephropathy DIRECT RENIN INHIBITOR: Aliskiren: Direct renin inhibitors (DRIs) are the latest class of RAS inhibitory drugs, of which only one member Aliskiren has become available for the treatment of cardiovascular and renal diseases in which ACE inhibitors and ARBs are currently used. Aliskiren is a nonpeptide which binds selectively to the catalytic site of renin and competitively blocks the access of angiotensinogen to this site  Ang I is not produced and the chain of RAS is interrupted. While the concentration of renin in plasma is increased by feed back, the plasma renin activity (PRA) is decreased. Ang I and Ang II levels fall. Aliskiren causes fall in BP which is more marked in the Na+ depleted subjects with high basal PRA. Similar to ACE inhibitors, plasma aldosterone levels are lowered accompanied by mild natriuresis and a tendency to K+ retention. The antihypertensive efficacy of aliskiren is equivalent to that of ACE inhibitors or ARBs. Combination of these drugs with aliskiren produces greater fall in BP, at least in the short term. This may be due to blockade of rise in PRA caused by ACE inhibitors/ARBs. The pattern of haemodynamic effect of aliskiren resembles ACE inhibitors; postural hypotension is not a problem Pharmacokinetics Aliskiren is administered orally, but bioavailability is very low due to active extrusion of absorbed drug by P-glycoprotein. The drug is mainly eliminated in faeces; small amount in urine. The plasma t½ is > 24 hours, and its BP lowering effect persists for days after regular intake. Adverse effects Aliskiren produces few and mild side effects—mainly dyspepsia, abdominal pain, loose motions, headache and dizziness. Acute hypotension, hyperkalaemia, cough, angioedema and rashes are much less frequent than with ACE inhibitors. Aliskiren is contraindicated during pregnancy. Dose: 150–300 mg OD CALCIUM CHANNEL BLOCKERS Three important classes of calcium channel blockers are examplified by: Verapamil—a phenyl alkylamine, hydrophilic papaverine congener. Nifedipine—a dihydropyridine (lipophilic). Diltiazem—a hydrophilic benzothiazepine Only the voltage sensitive L-type channels are blocked by the CCBs. The 3 groups of CCBs viz. Phenylalkylamines (verapamil), benzothiazepine (diltiazem) and dihydropyridines (nifedipine) bind to their own specific binding sites on the α1 subunit; all restricting Ca2+ entry, though characteristics of channel blockade differ. Further, different drugs may have differing affinities for various site specific isoforms of the L-channels. This may account for the differences in action exhibited by various CCBs. The vascular smooth muscle has a more depolarized membrane (RMP about –40 mV) than heart. This may contribute to vascular selectivity of certain CCBs. Pharmacological Actions: The common property of all three subclasses of CCBs is to inhibit Ca2+ mediated slow channel component of action potential (AP) in smooth/cardiac muscle cell. The two most important actions of CCBs are: (i) Smooth muscle (especially vascular) relaxation. (ii) Negative chronotropic, inotropic and dromotropic action on heart. Smooth muscle:  Smooth muscles depolarize primarily by inward Ca2+ movement through voltage sensitive channel. These Ca2+ ions trigger release of more Ca2+ from intracellular stores and together bring about excitation-contraction coupling through phosphorylation of myosin light chain as depicted in Fig. The CCBs cause relaxation by decreasing intracellular availability of Ca2+. They markedly relax arterioles but have mild effect on veins. Extravascular smooth muscle (bronchial, biliary, intestinal, vesical, uterine) is also relaxed. The dihydropyridines (DHPs) have the most marked smooth muscle relaxant and vasodilator action; verapamil is somewhat weaker followed by diltiazem. Fig. 2.4 Heart: In the working atrial and ventricular fibres, Ca2+ moves in during the plateau phase of AP and releases more Ca2+ from sarcoplasmic reticulum. This Ca2+ surge causes contraction through binding to troponin—allowing interaction of myosin with actin. The CCBs would thus have negative inotropic action. Fig. 2.5  The 0 phase depolarization in SA and A-V nodes is largely Ca2+ mediated. Automaticity and conductivity of these cells appear to be dependent on the rate of recovery of the Ca2+ channel. The L-type Ca2+ channels activate as well as inactivate at a slow rate. Consequently, Ca2+ depolarized cells (SA and A-V nodal) have a considerably less steep 0 phase and longer refractory period. The recovery process which restores the channel to the state from which it can again be activated by membrane depolarization is delayed by verapamil and diltiazem (resulting in depression of pacemaker activity and conduction), but not by DHPs (they have no negative chronotropic/dromotropic action). Moreover, channel blockade by verapamil is enhanced at higher rates of stimulation, that by nifedipine is independent of frequency, while diltiazem is intermediate. Thus, verapamil slows sinus rate and A-V conduction, but nifedipine does not. Effect of diltiazem on sinus node automaticity and A-V conduction is similar to that of verapamil. Fig 2.6 The relative potencies to block slow channels in smooth muscle do not parallel those in the heart. The DHPs are more selective for smooth muscle L channels. At concentrations which cause vasodilatation they have negligible negative inotropic action which is most prominent in verapamil. Diltiazem causes less depression of contractility than verapamil. Verapamil: It dilates arterioles and has some α adrenergic blocking activity—decreases t.p.r. but BP is only modestly lowered. The pronounced direct cardiodepressant effect is partially offset in vivo by reflex effects of peripheral vasodilatation. The HR generally decreases, A-V conduction is slowed, but c.o. is maintained by reflex sympathetic stimulation and reduction in aortic impedance. However, ventricular contractility may be markedly impaired in CHF patients. Coronary flow is increased. Dose: 40–160 mg TDS oral, 5 mg by slow i.v. injection  Adverse effects: Nausea, Constipation Bradycardia Flushing Headache Ankle edema accentuate conduction defects (contraindicated in 2nd and 3rd degree A- V block) and precipitate CHF in patients with preexisting disease Cardiac arrest has occurred on i.v. injection and when it is given to patients with sick sinus. Interactions: Verapamil should not be given with β blockers—additive sinus depression, conduction defects or asystole may occur. It increases plasma digoxin level by decreasing its excretion: toxicity can develop. Diltiazem: It is somewhat less potent vasodilator than nifedipine and verapamil, and has modest direct negative inotropic action, but direct depression of SA node and A-V conduction are equivalent to verapamil. Usual clinical doses produce consistent fall in BP with little change or decrease in HR. Large dose or i.v. injection decreases t.p.r. markedly which may elicit reflex cardiac effects. Diltiazem dilates coronaries. Dose: 30–60 mg TDS–QID oral Adverse effects: Similar to verapamil. Like verapamil, Diltiazem should not be given to patients with preexisting sinus, A-V nodal or myocardial disease. Only low doses should be given to patients on β blockers.` Nifedipine: It is the prototype DHP with a rapid onset and short duration of action. The overriding action of nifedipine is arteriolar dilatation → t.p.r. decreases, BP falls. The direct depressant action on heart requires much higher dose, but a weak negative inotropic action can be unmasked after β blockade. As described above, it does not depress SA node or A-V conduction. Reflex sympathetic stimulation of heart predominates producing tachycardia, increased contractility and c.o. No decrease in venous return along with lowering of afterload aids increase in c.o. Coronary flow is increased. Dose: 5–20 mg BD–TDS oral Adverse effects Frequent side effects are palpitation, flushing ankle edema hypotension headache drowsiness nausea Felodipine It differs from nifedipine in having greater vascular selectivity, larger tissue distribution and longer t½. The extended release preparation is suitable for once daily administration. Dose: 5–10 mg OD, max. 10 mg BD Amlodipine Pharmacokinetically it is the most distinct DHP and the most popular. Oral absorption is slow, but complete; peak blood level occurs. after 6 to 9 hr—the early vasodilator side effects (palpitation, flushing, headache, postural dizziness) are largely avoided. Because of less extensive and less variable first pass metabolism, its oral bioavailability is higher and more consistent. Volume of distribution and t½ are exceptionally long: diurnal fluctuation in blood level is small and action extends over the next morning. Dose: 5–10 mg OD β-ADRENERGIC BLOCKERS: They are mild antihypertensives; do not significantly lower BP in normotensives. Used alone they suffice in 30–40% patients—mostly stage I cases. Additional BP lowering may be obtained when combined with other drugs. The hypotensive response to β blockers develops over 1–3 weeks and is then well sustained. Despite short and differing plasma half lives, the antihypertensive action of most β blockers is maintained over 24 hr with a single daily dose. All β blockers, irrespective of associated properties, exert similar antihypertensive effect. Drugs with intrinsic sympathomimetic activity (ISA) cause less/no reduction of HR and c.o. but lower vascular resistance by β2 agonism. Nebivolol reduces t.p.r. by generating NO. The nonselective β blockers slightly reduce renal blood flow and g.f.r., but this is minimal in the β1 selective blockers and in those with ISA. There are several contraindications to β blockers, including cardiac, pulmonary and peripheral vascular disease. The nonselective β blockers have an unfavourable effect on lipid profile (raise triglyceride level and LDL/HDL ratio). They have also fared less well on quality of life parameters like decreased work capacity, fatigue, loss of libido and subtle cognitive effects (forgetfulness, low drive), nightmares and increased incidence of antidepressant use. Many of these drawbacks are minimized in the β1 selective agents and in those which penetrate brain poorly. Patient’s acceptability of a β1 selective hydrophilic drug like atenolol is better than that of propranolol. β blockers retain their place among the first choice drugs recommended by JNC 7 and WHO-ISH, especially for relatively young non-obese hypertensives, those prone to psychological stress or those with ischaemic heart disease. β blockers and ACE inhibitors are the most effective drugs for preventing sudden cardiac death in postinfarction patients. However, they are less effective for primary prophylaxis of MI and for preventing left ventricular hypertrophy. All cause mortality has been lowered in long-term trials by β blockers. Hypertensives with stable heart failure should be treated with one of the selected β blockers (metoprolol/ bisoprolol/carvedilol/nebivolol) along with an ACE inhibitor. Rebound hypertension has occurred on sudden discontinuation of β blockers; myocardial ischaemia may be aggravated and angina or MI may be precipitated. (Details in adrenergic blockers) β+α ADRENERGIC BLOCKERS: Labetalol: It is a combined α and β blocker; reduces t.p.r. and acts faster than pure β blockers. It has been used i.v. for rapid BP reduction in hyperadrenergic states, cheese reaction, clonidine withdrawal, eclampsia, etc. Oral labetalol therapy is restricted to moderately severe hypertension not responding to a pure β blocker, because side effects of both α blocker and β blocker occur with it. Carvedilol: This nonselective β + weak selective α1 blocker produces vasodilatation and has additional antioxidant/free radical scavenging properties. Whether these ancilliary properties confer any superiority is not known. Carvedilol is a frequently selected drug for long-term treatment of CHF, and is approved as an antihypertensive as well. Side effects are similar to labetalol; liver enzymes may rise in some. α-ADRENERGIC BLOCKERS: Prazosin: This prototype selective α1 antagonist dilates both resistance and capacitance vessels; effect on the former predominating. The haemodynamic effects, viz reduction in t.p.r. and mean BP accompanied by minor decrease in venous return and c.o. are similar to that produced by a direct acting vasodilator. However, unlike hydralazine, there is little reflex cardiac stimulation and rennin release during long-term therapy. Tachycardia does not compensate for the fall in BP, because release inhibitory α2 (presynaptic) receptors are not blocked: autoregulation of NA release remains intact. It probably decreases central sympathetic tone also. Adverse effects Prazosin is generally well tolerated at low doses. Apart from postural hypotension related symptoms (particularly in the beginning), other side effects are headache, drowsiness, dry mouth, weakness, palpitation, nasal blockade, blurred vision and rash. Ejaculation may be impaired in males: especially with higher doses. Fluid retention attending prazosin monotherapy may precipitate CHF. (Details in adrenergic blockers) Use:  moderately potent antihypertensive CENTRAL SYMPATHOLYTICS: Clonidine: It is an imidazoline derivative having complex actions. Clonidine is a partial agonist with high affinity and high intrinsic activity at α2 receptors, especially α2A subtype in brainstem. The major haemodynamic effects result from stimulation of α2A receptors present mainly postjunctionally in medulla (vasomotor centre). This decreases sympathetic out flow → fall in BP and bradycardia. Enhanced vagal tone contributes to bradycardia. Plasma NA declines. Though clonidine is capable of reducing NA release from peripheral adrenergic nerve endings (release inhibitory prejunctional α2 action), this is not manifest at clinically used doses. Clonidine is a moderately potent antihypertensive. Clonidine also activates Imidazoline receptors which are distinct from α2 receptors and are present in the brain as well as periphery. Activation of medullary imidazoline receptors also causes decreased sympathetic outflow and fall in BP. On chronic administration of clonidine decrease in c.o. contributes more to the fall in BP than decrease in t.p.r. Cardiovascular reflexes are affected little. Decreased sympathetic flow to the kidney results in reduced renin release. Plasma lipid levels are not altered. Pharmacokinetics:  Clonidine is well absorbed orally; peak occurs in 2–4 hours; 1/2 to 2/3 of an oral dose is excreted unchanged in urine, the rest as metabolites. Plasma t½ is 8–12 hours. Effect of a single dose lasts for 6–24 hours. Dose: Start with 100 μg OD or BD, max. 300 μg TDS, orally or i.m. Adverse effects: Sedation, mental depression, disturbed sleep; dryness of mouth, nose and eyes (secretion is decreased by central action), constipation (antisecretory effect on the intestines Impotence, salt and water retention, bradycardia Postural hypotension occurs, but is mostly asymptomatic. Alarming rise in BP, in excess of pretreatment level, with tachycardia, restlessness, anxiety, sweating, headache, nausea and vomiting occur in some patients when doses of clonidine are missed for 1–2 days. Use:  Antihypertensive  Opioid withdrawal  vasomotor symptoms of menopausal syndrome  control loose motions due to diabetic neuropathy. Methyldopa: This α-methyl analogue of dopa, the precursor of dopamine (DA) and NA is one of the first rationally designed antihypertensives. The α methyl-NA (a selective α2 agonist) formed in the brain from methyldopa acts on central α2 receptors to decrease efferent sympathetic activity. Because methyldopa decreases t.p.r. more than HR or c.o., it may be acting on a different population of neurones in the vasomotor centre than clonidine. In large doses, methyldopa inhibits the enzyme dopa decarboxylase in brain and periphery → reduces NA synthesis and forms the false transmitter methyl-NA in periphery as well. These mechanisms were considered to be responsible for the antihypertensive effect; but it was demonstrated that neither responses to stimulation of sympathetic nerves nor their NA content was reduced at clinically used antihypertensive doses. Moreover, α methyl NA is as potent vasoconstrictor as NA. The primary central site of action of methyldopa has been confirmed. Methyldopa is a moderate efficacy antihypertensive. Circulating levels of NA and rennin tend to fall due to reduction in sympathetic tone. Inhibition of postural reflexes is mild. Pharmacokinetics  Though methyldopa is transported actively by intestinal amino acid carrier, less than 1/3 of an oral dose is absorbed. It is partly metabolized and partly excreted unchanged in urine. Antihypertensive effect develops over 4–6 hours and lasts for 12–24 hours. Dose: 0.25–0.5 g BD–QID oral. Adverse effects Sedation, lethargy and reduced mental capacity are common side effects. Cognitive impairment may develop. Dryness of mouth, nasal stuffiness, headache, fluid retention, weight gain, impotence are the other side effects. Postural hypotension is generally mild. Positive Coomb’s test occurs in 1/6 patients, few develop haemolytic anaemia. Fever, rash, hepatitis, ‘flu’ like illness, thrombocytopenia and rarely lupus syndrome occur. Rebound hypertension on sudden withdrawal of methyldopa is mild and less common. Interactions Tricyclic antidepressants reverse its action by blocking its active transport into the adrenergic neurones. Use: Methyldopa was a widely used antihypertensive, especially in combination with a diuretic. However, it is infrequently used now, except to treat hypertension during pregnancy wherein it has a long track record of safety, both for the mother as well as the foetus. VASODILATORS: Hydralazine/Dihydralazine: It is a directly acting arteriolar vasodilator with little action on venous capacitance vessels; reduces t.p.r. and causes greater decrease in diastolic than in systolic BP. Reflex compensatory mechanisms are evoked which cause tachycardia, increase in c.o. and renin release → increased aldosterone → Na+ and water retention. The disproportionate cardiac stimulation appears to involve direct augmentation of NA release and myocardial contractility as well. Thus, a hyperdynamic circulatory state is induced—angina may be precipitated due to increased cardiac work as well as steal phenomenon. There is no reduction in renal blood flow despite fall in BP. However, fluid retention and edema may occur by the above mechanism. Tolerance to the hypotensive action develops unless diuretics or β blockers or both are given together to block the compensatory mechanisms. The mechanism of vascular smooth muscle relaxant action of hydralazine is not clearly known. Interference with Ca2+ release, opening of certain K+ channels and/or NO generation may be involved. Pharmacokinetics Hydralazine is well absorbed orally, and is subjected to first pass metabolism in liver. The chief metabolic pathway is acetylation which exhibits a bimodal distribution in the population: there are slow and fast acetylators. Bioavailability is higher in slow acetylators, but these patients are more prone to develop the lupus syndrome. Hydralazine is completely metabolized both in liver and plasma; the metabolites are excreted in urine, t½ 1–2 hours. However, hypotensive effect lasts longer (12 hours), probably because of its persistence in the vessel wall. Dose: 25–50 mg OD–TDS Adverse effects are frequent and mainly due to vasodilatation. Facial flushing, conjunctival injection, throbbing headache, dizziness, palpitation, nasal stuffiness, fluid retention, edema, CHF. Angina and MI may be precipitated in patients with coronary artery disease. Postural hypotension is not prominent because of little action on veins: venous return and c.o. are not reduced. Paresthesias, tremor, muscle cramps, rarely peripheral neuritis. Lupus erythematosus or rheumatoid arthritis like symptoms develop on prolonged use of doses above 100 mg/day. This is more common in women and in slow acetylators. Use:  Preferred antihypertensives during pregnancy, especially preeclampsia  Management of CHF particularly in combination with isosorbide dinitrate Minoxidil: It is a powerful vasodilator, the pattern of action resembling hydralazine, i.e. direct relaxation of arteriolar smooth muscle with little effect on venous capacitance. Marked vasodilatation elicits strong compensatory reflexes: increased renin release and proximal tubular Na+ reabsorption → marked Na+ and water retention → edema and CHF may occur. Reflex sympathetic activity causes palpitation and increased c.o. To offset these, it has to be used along with a loop diuretic and a β blocker. Minoxidil is a prodrug—converted to an active metabolite (by sulfate conjugation) which is an opener of ATP operated K+ channels; acts by hyperpolarizing smooth muscle. Use in alopecia Oral minoxidil increases growth of body hair. Applied topically (2% twice daily) it promotes hair growth in male pattern baldness and alopecia areata. The response is slow (takes 2–6 months) and incomplete, but upto 60% subjects derive some benefit, albeit for short periods. Baldness recurs when therapy is discontinued. The mechanism of increased hair growth is not known; may involve: (a) Opening of K+ channels and enhanced microcirculation around hair follicles. (b) Direct stimulation of resting hair follicles.  (c) Alteration of androgen effect on genetically programmed hair follicles. Local irritation, itching and burning sensation are frequent. Dermatological reaction and systemic side effects (headache, dizziness, palpitation) occur in 1–3% cases. Diazoxide: This K+ channel opener arteriolar dilator was used in the past for rapid reduction of BP in hypertensive emergencies. Administered by rapid i.v. injection it can be employed in place of nitroprusside, when regulated i.v. infusion or close monitoring is not possible. Sodium nitroprusside: It is a rapidly (within seconds) and consistently acting vasodilator; has brief duration of action (2–5 min) so that vascular tone can be titrated with the rate of i.v. infusion. It relaxes both resistance and capacitance vessels: reduces t.p.r. as well as c.o. (by decreasing venous return). Myocardial work is reduced—ischaemia is not accentuated, as occurs with selective arteriolar dilators (hydralazine). Little reflex tachycardia is produced in supine posture. Plasma rennin is increased. In patients with heart failure and ventricular dilatation, nitroprusside improves ventricular function and c.o. mainly by reducing aortic impedance (afterload), but also by lowering atrial filling pressure (preload). Endothelial cells, RBCs (and may be other cells) split nitroprusside to generate NO which relaxes vascular smooth muscle. This occurs both enzymatically and nonenzymatically. The enzymes involved are different from those that produce NO from glyceryl trinitrate. Nonenzymatically it is converted to NO (and CN) by glutathione. This may be responsible for the different pattern of vasodilator action compared to nitrates, as well as for the fact that no nitrate like tolerance develops to nitroprusside action. Nitroprusside has gained popularity in the management of hypertensive emergencies; 50 mg is added to a 500 ml bottle of saline/glucose solution. The infusion is started at 0.02 mg/min and titrated upward with the response: 0.1–0.3 mg/min is often needed. It decomposes at alkaline pH and on exposure to light: the infusion bottle should be covered with black paper. Nitroprusside is split to release cyanide. The latter is converted in liver to thiocyanate which is excreted slowly. If larger doses are infused for more than 1–2 days, excess thiocyanate may accumulate and produce toxicity, including psychosis. Side effects mainly due to vasodilatation are— palpitation, nervousness, vomiting, perspiration, pain in abdomen, weakness, disorientation, and lactic acidosis (caused by the released cyanide). Nitroprusside has also been used to produce controlled hypotension, in refractory CHF, pump failure accompanying MI and in acute mitral regurgitation. ADRENERGIC NEURONE BLOCKERS: Reserpine:  It is an alkaloid from the roots of Rauwolfia serpentina (sarpgandha) indigenous to India which has been used in ‘Ayurvedic’ medicine for centuries. The pure alkaloid was isolated in 1955 and later found to act by causing CA and 5-HT depletion. It was a popular antihypertensive of the late 1950s and early 1960s, but is now used only as a pharmacological tool. Reserpine acts at the membrane of intraneuronal vesicles which store monoamines (NA, DA, 5-HT) and irreversibly inhibits the vesicular monoamine transporter (VMAT2). The monoamines are gradually depleted and degraded by MAO. The effects last long after the drug is eliminated (hit and run drug) because tissue CA stores are restored only gradually. Higher doses deplete CAs and 5-HT in the brain as well; cause sedation and mental depression. Antipsychotic effect (mild) and extrapyramidal symptoms are produced due to DA depletion. Dose; 0.25 mg tab; 1 mg/ml inj Guanethidine It is a polar guanidine compound which is taken up into the adrenergic nerve endings by active amine transport, and has three important facets of action: (a) Displaces NA from storage granules stoichiometrically. (b) Inhibits nerve impulse coupled release of NA. (c) Engages and blocks NA uptake mechanism at the axonal membrane. ANTIANGINAL DRUGS Antianginal drugs are those that prevent, abortor terminate attacks of angina pectoris. Angina pectoris is a pain syndrome due to induction of an adverse oxygen supply/demand situation in a portion of the myocardium Threeprincipal forms are recognized: a. Stable/ classical angina b. Variant/Prinzmetal/Vasospastic angina(uncommon form) c. Unstable angina a. Stable/ classical angina:  Classical angina (common form) Attacksare predictably provoked (stable angina) byexercise, emotion, eating or coitus and subside when the increased energy demand is withdrawn.The underlying pathology is—severe arteriosclerotic affliction of larger coronary arteries(conducting vessels) which run epicardially andsend perforating branches to supply the deeper tissue. The coronary obstruction is‘fixed’; blood flow fails to increase during increased demand despite local factors mediated dilatation of resistance vessels and ischaemic pain is felt. Due to inadequacy of ischaemic left ventricle, the end diastolic left ventricular pressure rises from 5 to about 25 mmHg—produces subendocardial ‘crunch’ during diastole (blood flow to the subendocardial region.occurs only during diastole) and aggravates the ischaemia in this region. Thus, a form of acutely developing and rapidly reversible left ventricular failure results which is relieved by taking restand reducing the myocardial workload. Fig 4.1 b. Variant/ Prinzmetal/ Vasospastic angina (uncommon form) Attacks occur at rest or during sleep and are unpredictable. They are due to recurrent localized (occasionally diffuse) coronary vasospasm which may be superimposed on arteriosclerotic coronary artery disease. Abnormally reactive and hypertrophied segments in the coronary arteries have been demonstrated. Drugs are aimed at preventing and relieving the coronary vasospasm. c. Unstable angina Unstable angina (UA) with rapid increase in duration and severity of attacks is mostly due to rupture of an atheromatous plaque attracting platelet deposition and progressive occlusion of the coronary artery; occasionally with associated coronary vasospasm. CLASSIFICATION 1. Nitrates (a) Short acting: Glyceryltrinitrate (GTN, Nitroglycerine) (b) Long acting: Isosorbide dinitrate (short acting by sublingual route), Isosorbide mononitrate,  Erythrityl tetranitrate, Pentaerythritol tetranitrate 2. β Blockers: Propranolol, Metoprolol, Atenolol and others. 3. Calcium channel blockers: (a) Phenyl alkylamine: Verapamil (b) Benzothiazepine: Diltiazem (c) Dihydropyridines: Nifedipine, Felodipine, Amlodipine, Nitrendipine, Nimodipine, Lacidipine, Lercanidipine, Benidipine 4. Potassium channel opener: Nicorandil 5. Others: Dipyridamole, Trimetazidine, Ranolazine, Ivabradine, Oxyphedrine NITRATES: Mechanism of action: Organic nitrates are rapidly denitrated enzymatically in the smooth muscle cell to release the reactive free radical nitric oxide (NO) which activates cytosolic guanylylcyclase which converts Guanosine triphosphate (GTP) to Cyclic guanosine monophosphate (cGMP), that causes dephosphorylation of myosin light chain kinase (MLCK) through a cGMP dependent protein kinase. Reduced availability of phosphorylated (active) MLCK interferes with activation of myosin, which fails to interact (Coupling) with actin need for contraction. Consequently relaxation occurs. Raised intracellular cGMP may also reduce Ca2+ entry which is also contributing to relaxation.  Fig. 4.2 Pharmacological effect: Preload reduction: The most prominent action is exerted on vascular smooth muscle. Nitrates dilate veins more than arteries Peripheral pooling of blood Decreased venous return, i.e. preload on heart is reduced End diastolic size and pressure are reduced These results decreased cardiac work according to Laplace relationship—which describes the effectivenes