Cardiac Anatomy and Physiology

Introduction

• Heart disease is often diagnosed in veterinary medicine due to increased pet longevity
• Acquired heart disease is most prevalent in geriatric patients
• Congenital heart disease presents more often in young animals
• Treatment of heart disease is very specific to both the patient and its disease stage
• Cardiovascular drugs have a small margin of safety and requires careful monitoring

Cardiac Anatomy/Physiology

• The body delivers deoxygenated blood into to the right atrium via the cranial and caudal vena cava
• Blood passes though the right atrioventricular/tricuspid valve into to the right ventricle
• During Systole the right AV valve closes
• The right ventricle pushes deoxygenated blood out the pulmonic valve to the pulmonary arteries and then to the alveoli to become oxygenated
• Blood returns to the heart via pulmonary veins leading to the left atrium
• The blood then flows from the left atrium through the left atrioventricular/mitral valve into the left ventricle
• During systole the mitral valve closes
• The left ventricle then contracts pushing blood through the aortic valve into the coronary arteries, the aorta and arteries of the body

Cardiac Cycle

• Each complete contraction and relaxation is termed the cardiac cycle
• Systole is when the heart muscle contracts pushing blood to atria/ventricles
• Diastole is when the heart muscle relaxes and refills with blood
• The sinoatrial node (SA) node emits heartbeat impulses
• The SA node is a specialized cardiac muscle cell are that automatically generates the electrical impulses that trigger the heart to beat
• The SA node generates electrical current by moving cations (Na+, K+, Ca++) and anions (Cl-)
• Cations pumped across the cell membrane is polarization
• Opening of the gates on cell membrane causes cations flow out which equalizes charge as the cell depolarizes
• Action of depolarization generates the electrical current that tells cardiac muscles to contract
• Electrical current from the SA node takes two routes from heart base to its apex
• One is fast and goes via specific cardiac cells of the SA node to the AV node and Purkinje fibers
• The action of the other slower route travels locally through cardiac muscle fibers
• Cardiac cells can transmit electrical impulses from cell to cell
• Skeletal muscles needs electrical message which requires a nerve
• Electrical impulse when exiting SA node will quickly travel to the AV node
• There is slightly slower conduction through the AV node
• The AV node conducts of electrical impulse from the atria to the ventricles
• AV node delay allows the atria can complete their systolic contraction before ventricular systole begins
• Simultaneous atrial and ventricular systole generates high pressure which stops blood flow from the atria into the ventricles
• Electrical impulse from the AV node travels via of the Purkinje fibers/bundle of His through ventricles
• Bundle of His fibers run down interventricular septum to apex of the right and left ventricles
• Purkinje fibers bring impulses from the bundle of His into ventricular myocardium
• Ventricular contraction will start at the heart's apex for effective ejection of blood even if the electrical impulse starts at the heart's base
• During ventricular systole, the atria are in diastole while filling with blood for the next heart contraction
• Spontaneous cardiac muscle depolarization, or an abnormal conduction system may cause arrhythmia

Cardiac Receptor Physiology

• Sympathetic nervous system physiology:
• Beta-1 receptors generate positive inotropic action (increases contractility)
• Beta-1 receptors also generate positive chronotropic effects (increases heart rate)
• Parasympathetic nervous system physiology utilizes Cholinergic receptors which produce negative chronotropic effects
• Alpha 1 receptors cause arterioles to constrict when stimulated
• Beta 2 receptors cause arterioles to dilate when stimulated

Cardiac Compensatory Mechanisms

• The system has reserves to increase output during times of need- like athletic performance
• Increased heart rate can compensate for cardiac disease up to the point where ventricular filling goes too fast
• Increasing stroke volume via increased force of contraction can increase the amount of pumped blood to a limit
• Enhanced efficiency of the heart muscle and physiologic heart enlargement are other compensatory mechanisms
• Cardiac muscle can respond to the amount of work by increasing in size and becoming stronger

Basic Cardiac Diseases

• Valvular disease is a common acquired heart disorder
• Ascites may occur if the right AV/tricuspid valve has been affected
• Pulmonary edema occurs is left AV/mitral valve has been affected
• Valvular disease can be caused by stenosis or progressive endocarditis
• Valvular disease can be associated with a murmur
• Cardiac arrhythmias can be caused by cardiac muscle depolarizing the SA node
• Rapid heart rate is defined as Tachyarrhythmia
• Slow heart rate is defined as Bradyarrhythmia
• Supraventricular arrhythmia is arrhythmia that starts in the atria
• Ventricular arrhythmia is arrhythmia that starts ventricles
• Some cardiac drugs can predispose the heart to arrhythmia

Myocardial Disease

• Cardiomyopathy refers to any disease of the cardiac muscle
• Dilative muscle becomes thin and ineffective at pumping
• Great Danes and Boxer breeds are prone to Dilative, AKA boxer cardiomyopathy
• Hypertrophic heart muscle is thickened restricting ventricular filling
• More common in cats and may be associated with hyperthyroidism
• Congenital defects, abnormal cardiac innervation, hypertension, heartworm disease can cause cardiomyopathy
• Congestive heart failure (CHF) is an end stage for many diseases
• Sodium and water end up being retained by the kidneys because the heart fails impairing its pumping ability to compensate for inadequate cardiac output
• Signs of CHF:
• Exercise intolerance
• Pulmonary edema
• Ascites
• Cyanosis
• Cardiac enlargement

Objectives for Treatment

• Controlling rhythm disturbances
• Increasing cardiac output by:
• Increasing strength of contraction
• Decreasing afterload using an Arteriolar dilator
• Decreasing preload with Venodilator
• Relieving fluid accumulation by using Diuretics and dietary salt restriction
• Increasing oxygenation of blood with Bronchodilation
• Ancillary treatment may include Narcotics/sedatives and oxygen

Positive Inotropic Drugs

• Cardiac glycosides like Digitalis
• Catecholamines like Isoproterenol, Dopamine and Dobutamine
• Bipyridine derivatives
• Inotropic/mixed dilator such as Pimobendan

Cardiac Glycoside- Digitalis

• Digoxin and digitoxin are derived from Digitalis purpurea
• Cardiac contractility is improved
• Includes increased calcium ions available for contraction
• Heart rate decreases and slows atrioventricular conduction time increasing parasympathetic stimulation
• It also has antiarrhythmic effects and decreases signs of dyspnea
• Digitalis is used in treatment of CHF, atrial fibrillation, and supraventricular tachycardia
• Adverse effects include anorexia, vomiting, diarrhea, and arrhythmia
• These drugs are narrow with their therapeutic range and drug levels affect many drugs, needing compatibility checks

Catecholamines

• Sympathomimetic compounds
• Muscle contraction force and rate increases
• Elevates blood glucose and constricts peripheral vessels to elevate blood pressure
• Acts by stimulating the beta 1 receptors
• They are short-acting and can only be used for short term management

Epinephrine

• The preferred drug for cardiac arrest, AKA adrenalin
• Not useful for chronic heart failure management
• Can use IV, intratracheal, intravenous route
• Concentration is typically 1:10,000= 0.1mg/ml or 1:1000= 1mg/ml
• The drug is light sensitive and most formulations are stored in refrigerator

Isoproterenol

• Atropine-resistant bradycardia is an indication

Dopamine

• A synthetic precursor of norepinephrine
• Dopaminergic receptors are stimulated in the coronary, renal and cerebral vascular beds
• Stimulates alpha and beta adrenergic receptors
• Heart contractility, rate, and blood pressure increases
• Dopamine is a treatment for heart failure with anesthetic emergencies, acute heart failure, oliguric renal failure, pharmacologic support of blood pressure
• Supportive shock treatment
• Adverse effects: Vomiting, tachycardia, dyspnea and variations in BP

Dobutamine

• A synthetic inotrope related to dopamine
• Cardiac contractility increases
• No dilation of select vascular beds occur and it is a Direct beta 1 adrenergic agonist
• Increases cardiac output without increased HR or arrhythmia
• Administered as an IV infusion
• Given to many anesthetized horses to support blood pressure

Bipyridine Derivatives

• Amrinone and milrinone are a new class of positive inotrope that increases cellular calcium but not often used in veterinary medicine

Inotropic, Mixed Dilator “Inolator”= Pimobendan

• Approved for use in 2007
• Positive inotrope
• Increases calcium sensitivity
• Inhibits phosphodiesterase
• The Vetmedin version is becoming a popular treatment for heart failure
• Labeled for treatment for arterioventricular insufficiency and dilated cardiomyopathy and is available as chewable tablets
• Adverse effects includes anorexia, lethargy, diarrhea
• Hypertrophic cardiomyopathy is a contraindication although has been used successfully in cats
• Aortic stenosis is a contraindication

Antiarrhythmic Drugs

• Arrhythmia that is a variation from normal rhythm may arise from an ectopic focus of cells being outside of the sinoatrial node
• Trained auscultation can find abnormality, but ECG may be required
• Potential factors predisposing heart to arrhythmia include hypoxemia, electrolyte imbalances, increased levels or sensitivity to catecholamines, drugs and/or cardiac trauma.
• May be tachyarrhythmias or bradyarrhythmias

Antiarrhythmic Drug Categories

• Drugs in Class 1A depress myocardial excitability, prolong the refractory period and treat atrial/ventricular arrhythmias
• Can be administered orally long term
• Quinidine is made from cinchona plants or quinine
• Used for ventricular arrhythmias and atrial fibrillation
• May cause anorexia, vomiting, diarrhea, weakness, and laminitis
• Primary treatment for equine A-fib
• Procainamide is chemically related to procaine
• Prescribed to treat ventricular premature contractions (VPCs)
• Maybe related to anorexia, vomiting, diarrhea, hypotension
• Drugs in Class 1B stabilize cell membranes by blocking the influx of sodium into the cell to prevent depolarization
• Lidocaine is a local anesthetic and antiarrhythmic, only available for Injectable, and is commonly recommended for emergency treatment of VPCs and V-tach in 1 and 2 % solutions
• Do not combine lidocaine with epinephrine for arrhythmia treatment
• Tocainide is available as a oral form with a similar use to lidocaine
• Mexiletine is available as a oral form with a similar use to lidocaine

Other Classes of Antiarrhythmic Drugs

• Class 1C drugs are not used in veterinary medicine
• Available meds include Flecainide and Encainide
• Class II blocks beta 1 and/or beta-2 adrenergic receptors, used in treatments of atrial/ventricular arrhythmias
• Beta blockers reduce cardiac conduction, cardiac output, and blood pressure
• Propanolol blocks beta1 and beta 2
• Decreases myocardial oxygen demand and decreases HR, BP, and cardiac output
• Hypertrophic cardiomyopathy in animals can benefit
• Can use in cats for hypertension
• May cause May cause bradycardia, hypotension, bronchospasm, or depression
• Sinus bradycardia and overt heart failure is a Contraindications
• Do not end administration of meds rapidly because hypertension and tachycardia can occur
• Atenolol is a selective beta-1 blocker and can be safer in animals prone to bronchospasm
• Is known to decreases HR, BP, CO, and myocardial oxygen demand
• Can be used for supraventricular tachyarrhythmia, VPC, hypertension, and cardiomyopathy
• May cause bradycardia, syncope, lethargy, heart failure and/or depression in older animals
• Class III is not commonly prescribed, amiodarone only in CPR for emergency treatment of ventricular arrhythmias

Calcium Channel Blockers

• Class IV meds block channels that allow calcium ions to pass trans cardiac cell membrane
• This depresses cardiac and smooth muscle contraction which depresses impulse transmission
• Verapamil has limited use in the vet field
• Used for supraventricular tachycardia, atrial fibrillation or atrial flutter
• Amlodipine is a calcium channel blocker
• Used for hypertension
• Commonly prescribed to cats who have hypertension from hyperthyroidism and chronic renal disease
• Diltiazem is a calcium channel blocker
• Given for dogs/cats battling supraventricular tachyarrhythmia
• Is commonly given for cats battling hypertrophic cardiomyopathy

Vasodilator Drugs

• Heart failure may result in decreased cardiac output, low blood pressure and overall poor perfusion
• Body responds to this outcome via stimulating the sympathetic nervous system
• Heart rate increases
• Small arteries constrict to increase blood pressure
• Renin is released by poorly perfused kidneys which activates the renin-angiotensin system
• Releases signals which convert angiotensin to angiotensin I
• Angiotensin I is converted to angiotensin-converting enzyme (ACE) which turns into angiotensin II
• Angiotensin II initiates further vasoconstriction and aldosterone release
• Aldosterone directs kidneys to reabsorb sodium and osmotic water
• Increases tissue perfusion expanding total blood volume containing water and sodium
• In the short term this is beneficial, but long term the heart has to push through constricted vessels and the increased blood which is increasingly harder
• Vasodilators dilate arteries, veins, or both
• Angiotensin I drugs may act via direct action on smooth muscles or block how Angiotensin II converts
• Improves cardiac output, preload, and dilates vessels

Vasodilator Drugs List

• Hydralazine is a arterial dilator that effects smooth muscle via calcium movement interference and contraction inhibition
• Reduces afterload and improves cardiac output in CHF and CHF especially due to mitral valve insufficiency
• Common adverse effects include vomiting, tachycardia, hypotension, diarrhea, sodium and water retention.
• Nitroglycerin Ointment is venodilator that reduces preload via lessening venous return to the heart

Angiotensin-Converting Enzyme Inhibitors (ACE Inhibitors)

• Vasodilators that prevent formation of vasoconstrictor angiotensin II
• Prevent the conversion of angiotensin I to angiotensin II
• May combine with diuretics, cardiac glycosides and pimobendan
• Treats the renin-angiotensin cycle, these meds are frequently administered
• Options includes Captopril, Enalapril, Lisinopril and Benazepril
• When combined risk of hypotension can occur
• Recommended that kidney health is re-tested along with electrolytes a few weeks after medication starts

Angiotensin II Antagonist/Angiotensin Receptor Blocker

• Can be used for animals with proteinuria
• Telmisartan is a New hypertension drug specifically prescribed to cats, and especially those that have renal secondary hypertension
• Preserves beneficial effects of angiotensin II
• Monitor for anemia at the initial stage of treatment
• Monitor for weight loss when start of treatment
• Cannot be handled by pregnant women during pregnancy due to risk associated with fetal morbidity and death
• Cannot combine with hepatic disease, safety in animals is not yet tested

Diuretics

• Prescribed in the treatment of cardiac diseases due to their ability to reduce preload through diuresis
• Reduce reabsorption of sodium and water in the tubules
• Retaining sodium means water stays within the tubules and is urinated

Loop Diuretics

• Furosemide is the most effective and common in veterinary medicine
• Assists with removing edema in CHF patients, and is commonly prescribed for rapid pulmonary edema/CHF
• Can administer (PO, IM, IV, SQ) and the sodium reabsorption happens in the loop of Henle
• Adverse effects may include weakness and also ototoxicity
• Hypokalemia may increase the risk of digitalis toxicity requiring that potassium has supplements
• Animals need open access to drink water to stay hydrated and urinate 20-30 mins post administration

Thiazides and Spironolactone

• Thiazides loop the distal tubules
• Is not commonly used, the ingredient is chlorothiazide
• Spironolactone is a potassium diuretic
• Has aldosterone antagonist- sodium reabsorption is reduced from kidneys
• May prescribe in combinations of diuretics
• Used in veterinary medicines in situations with high hyperkalemia

Helpful Care Aspects for Pets with Heart Failure

• Dietary Management is prescribed for reducing sodium, controlling weight and support digestible diets
• Have pets follow low sodium diet and avoid treats
• Owners can give oxygen
• Decreases both hypoxia and cyanosis that stem from pulmonary from edema with oxygen from face masks
• Pets prescribed anti-clotting formation is prescribed to those with hypertrophic cardiomyopathy
• Reductions of hypoventilation occur with reduce anxiety, and sedation

Thoracocentesis and Pleurocentesis

• A lifesaving method of cardiac medications assists to improve function via removal of pulmonary treatments
• Ascites removals reduces diaphragm pressure improves ventilation