Cardiac Muscle Properties - Gulf Medical University - PDF

Cardiac muscle properties Dr. Ghada Elgarawany Assistant professor of Medical Physiology www.gmu.ac.ae COLLEGE OF MEDICINE Describe the automaticity and conducting system of the heart Describe the sequence of excitation of the heart Describe the electrical potentials of pacemaker tissues Describe th...

Cardiac muscle properties Dr. Ghada Elgarawany Assistant professor of Medical Physiology www.gmu.ac.ae COLLEGE OF MEDICINE Describe the automaticity and conducting system of the heart Describe the sequence of excitation of the heart Describe the electrical potentials of pacemaker tissues Describe the electrical potentials of cardiac muscle fiber List the factors affecting the Rhythmicity Discuss the cardiac muscle contractility and factor affecting Cardiac muscle properties Excitability Rhythmicity Conductivity Contractility Rhythmicity Definition: Ability of the heart to beat regularly without nerve supply. Rhythmicity= automaticity Chronotropism = Rhythmicity “ +ve chronotropic means increase heart rate , - ve chronotropic means decrease heart rate” Cardiac rhythmicity is myogenic or neurogenic???? Evidence??? It's initiated by the ‘pacemaker’ of the heart, the SA- node. SA node initiate from 90- 110 impulse/minute. Why SAN is the pacemaker of the heart (Mechanism of automaticity)? 1. Resting membrane potential (RMP) =-55 to -60 mv 2. RMP is unstable?? (slow diastolic depolarization, spontaneous depolarization, prepotential or pacemaker potential, slow response action potential due to opening of slow Na+ channels) 3. SAN has higher rhythm & higher slope What are the cardiac muscle fibers that have automatic rhythm? 1. SAN (initiates heart beats from 90-110 beats/min) 2. AVN (initiates heart beats from 50-60 beats/min) 3. Bundle branches (initiates heart beats, 45 beats/min) 4. Purkinje fiber (initiates heart beats from 15-35 beats/min) 5. Ventricles (initiates heart beats , 25 beats/min) Conductivity Conducting fibers Atrioventricular bundle (bundle of His) Right and left bundle Purkinje fibers Conductivity Definition: The ability of the cardiac muscle to conduct cardiac impulse from one part to another Dromotropism = Conductivity “ +ve dromotropic means increase conduction, - ve dromotropic means decrease conduction” SAN action potential transmitted to all cardiac tissues Conduction of Impulse 1. Pacemaker of the heart (SAN) 2. Internodal and interatrial tract Connect SA Node and AV Node Faster rate of conduction than atrial muscles By Three Tracts: 1- anterior 2- intermediate 3- posterior interatrial tract (Bachmann’s bundle) conducts impulse from SAN to left atrium 3- Atrioventricular node (AV Node) conducting pathway between atria and ventricles Slow Velocity of conduction- 0.05m/sec  Here, there is 0.1 sec Nodal delay (decremental conduction) Nodal delay is due to 1- fewer gap junctions 2- Small size fibers Significance of delay: 1- To allow the atria to completely depolarize, contract and empty its blood before the ventricular depolarisation and contraction. 2- To limit the passage of the electrical signals that can activate the ventricles, to allow sufficient time for ventricles to relax and to be filled with blood before they contract. 4-Bundle of His It begins from AV Node, passes downwards in the intraventricular septum for 5-15mm Divides into right and left bundle branches Conduction speed is 1-2m/sec 5- Purkinje fibers Fastest conducting velocity (4-5m/Sec) It is the largest conducting fiber , More gap junction. Passes impulses to ventricular myocytes in the ventricular walls →depolarization of ventricular myocytes and ventricular contraction. Significance of fast conduction: to allow the 2 ventricles to contract at the same time simultaneously The conduction velocities The slowest conduction velocity in AV-node The fastest Conduction velocity in Purkinje fibers www.gmu.ac.ae COLLEGE OF MEDICINE Excitability Definition: Ability of cardiac muscle to respond to stimulation Respond by action potential → contraction and relaxation Bathmotropism= Excitability Resting Membrane potential of cardiac muscle cell is – 85-90 mv ?? S A Nodal potential vs Action potential of cardiac muscle Revise Excitability changes during action potential Refractory period: period following the action potential during which the cardiac muscle doesn't respond to a stimulus. 1. Absolute refractory period (ARP): No impulse can excite the cardiac muscle occurs during rapid depolarization and plateau. Duration: 0.25 to 0.30 second (250 -300msec) 2. Relative Refractory period (RRP) Strong impulse can excite the cardiac muscle occurs during rapid repolarization Duration: 0.05 second (50msec) What is the significance of the refractory period It prevents stimulation of the ventricle by another impulse → gives the ventricle sufficient time to empty their contents and refill before the next cardiac contraction Mechanical response (contraction & relaxation) Contraction of cardiac muscle (systole) reach maximus by the end of plateau. (This means that the cardiac muscle contraction occurs in the absolute refractory period) Relaxation of cardiac muscle (diastole) starts with rapid phase of repolarization. Factors affecting rhythmicity of cardiac muscle Nervous Physical Chemical Factors affecting rhythmicity of cardiac muscle Nervous factors 1. Parasympathetic nervous system has -ve chronotropic →↓ heart rate. 2. Sympathetic nervous system: has +ve chronotropic →↑ heart rate Factors affecting rhythmicity of cardiac muscle Physical factors 1. Warming: Fever, has +ve chronotropic →↑ heart rate “ 1˚C increases H.R by 10-13 beats” 2. Cooling: , has -ve chronotropic →↓heart rate. Chemical factors 1. 2. 3. 4. Ions: increase of Na+, K+, & Ca++ →↓heart rate. Drugs and hormones: Thyroxin, Catecholamines(↑) while , digitalis (↓) Blood gases: Mild hypoxia ↑, alkalosis ↑, acidosis ↓ Toxins: Typhoid and diphtheria → ↓heart rate.  Marked hyperkalemia stop the heart in diastole  Marked hypercalcemia stop the heart in systole Digitalis used for treatment of heart failure. It increase contractility, thereby improving cardiac output, digitalis reduces rhythmicity and conduction through the AV node and thus, used to treat atrial fibrillation and atrial flutter. Contractility Definition: The Ability of the cardiac muscle to convert electrical and chemical changes into mechanical work ( contraction) that pumps into circulation. Inotropism = Contractility ( -ve inotropic means decrease contractility while +ve inotropic means increase contractility) Mechanism of contraction depends on?? (Excitation contraction coupling) Contractility Revise Mechanism of cardiac muscle contraction Mechanism of cardiac muscle relaxation Factors affecting cardiac contractility Intrinsic factors Preload Afterload Extrinsic factors Healthy cardiac muscle Frequency of stimulation Nervous Physical Chemical  End diastolic volume is the volume of blood in each ventricle at the end of diastole. (End diastolic volume= 120ml )  End systolic volume is the volume of blood that remains in each ventricle at the end of systole.(End systolic volume= 50ml)  Stroke Volume is the amount of blood ejected by each ventricle per stroke (Stroke volume = 70 ml) Preload Preload is the End diastolic volume which determine the initial length of the cardiac muscle. 3. Starling̛̛s Law of the heart (Frank-Starlying’s Law ) Within limit, the greater the initial length of cardiac muscle fiber (End diastolic volume, EDV), the greater the force of contraction.  What is the relation of venous return and EDV???? Afterload Afterload is the resistance in front of the blood after starting ventricular contraction and ejection like Aortic pressure, arterial wall rigidity and blood viscosity. Force Velocity Curve: The initial velocity of shortening of cardiac muscle is inversely proportionate to the magnitude of afterload Afterload increased by 1- Increased Arterial blood pressure. 2- Stenosed valves Healthy cardiac muscle Contractility depends on healthy cardiac muscle fibers. Contractility means the intrinsic ability of the intact heart to expel blood into the outflow channels independent of preload or afterload. Frequency of stimulation (heart rate)  Staircase (Treppe) phenomenon (Force frequency relationship) Gradual increases in strength of cardiac muscle contraction due to repeated stimulation (Increase heart rate). Mechanism?? produce thermal, chemical and ionic changes → increase force of contraction Extrinsic factors Nervous Sympathetic : +ve inotropic effect. Parasympathetic : -ve inotropic effect Physical Fever : +ve inotropic effect. Cooling: -ve inotropic effect Chemical Drugs,Thyroxin,Catechola mines, glucagon, Digitalis, caffeine : +ve inotropic effect. Ions: Ca +ve inotropic while K -ve inotropic effect N.B: Any factor increases Ca+2 permeability has +ve inotropic effect (↑contraction) while decreases Ca+2 has –ve inotropic effect (↓contraction). Cardiac muscle properties Excitability (Bathmotropism) Rhythmicity (Chronotropism) Conductivity (Dromotropism) Contractility (Inotropism) Review Questions The “pacemaker” current (If) represents which of the following cellular activities? A. Efflux of potassium from the cell B. Efflux of sodium from the cell C. Influx of calcium into the cell D. Influx of potassium into the cell E. Influx of sodium into the cell https://usmle-rx.scholarrx.com/rx-bricks/brick/CP_CAR0012 Which of the following describes the normal order of cardiac conduction in healthy individuals? A. B. C. D. E. AV node, bundle branches, bundle of His, SA node, Purkinje fibers Bundle branches, SA node, AV node, Purkinje fibers, bundle of His Purkinje fibers, bundle branches, AV node, SA node, bundle of His SA node, AV node, bundle of His, bundle branches, Purkinje fibers SA node, Purkinje fibers, bundle of His, bundle branches, AV node https://exchange.scholarrx.com/brick/cardiac-conduction-system Summary Describe the automaticity and conducting system of the heart Describe the sequence of excitation of the heart Describe the electrical potentials of pacemaker tissues Describe the electrical potentials of cardiac muscle fibre List the factors affecting the Rhythmicity Discuss the cardiac muscle contractility and factors affecting Learning Resources  Hall JE, Hall ME. Guyton and Hall textbook of medical physiology e-Book. 14ed, Elsevier Health Sciences; 2020. Chapter 10, 127-133 https://www-clinicalkeycom.gmulibrary.com/#!/content/book/3-s2.0-B9780323597128000102  https://exchange.scholarrx.com/brick/cardiac-conduction-system  https://exchange.scholarrx.com/brick/cardiac-muscle-contraction  Power-point presentation in the Moodle. www.gmu.ac.ae COLLEGE OF MEDICINE Thank you