PHM 480 Cardiac Electrophysiology_Students.pdf

CARDIAC ELECTROPHYSIOLOGY [PHM 480C] Physiology, Pathophysiology and Drug Targets Cardiology Module Spring 2024 Denise Kelley, PharmD, BCPS, FASHP Clinical Assistant Professor / Internal Medicine Clinical Pharmacy Specialist [email protected] 1 Learning Objectives ❑ Trace the spread of...

CARDIAC ELECTROPHYSIOLOGY [PHM 480C] Physiology, Pathophysiology and Drug Targets Cardiology Module Spring 2024 Denise Kelley, PharmD, BCPS, FASHP Clinical Assistant Professor / Internal Medicine Clinical Pharmacy Specialist [email protected] 1 Learning Objectives ❑ Trace the spread of cardiac excitation, including purpose of brief delay at the AV node ❑ Explain the characteristics and major underlying current of action potential phases of the SA nodal cells and ventricular myocytes ❑ Differentiate depolarization and repolarization on action potential and electrocardiogram (ECG) ❑ Label phases of SA node and ventricular action potentials ❑ Classify cardiac cells as slow or fast tissue ❑ Describe the significance of the refractory period of the heart ❑ Identify the components of an ECG reading and the corresponding physiologic excitation of the myocardium 2 ELECTROPHYSIOLOGY 3 Autorhythmicity Heart beats rhythmically as a result of action potentials it generates by itself LET’S CHAT: What is the purpose of the heart? 4 Cardiac Physiology 5 5 Two Types of Cardiac Cells Contractile Cells 99% of cardiac muscle cells Perform mechanical work of pumping Normally do not initiate action potentials Autorhythmic Cells Do not contract Initiate and conduct the action potentials responsible that prompt working cells to contract 6 Autorhythmic Cells Sinoatrial node (SA node) Atrioventricular node (AV node) Bundle of His (atrioventricular bundle) Purkinje fibers 7 Autorhythmic Cell Locations Sinoatrial (SA) node Located in right atrial wall, near opening of superior vena cava Pacemaker of the heart Atrioventricular (AV) node Located at base of right atrium near septum 8 Autorhythmic Cell Locations Bundle of His Originate at AV node and enter septum between ventricles Divide to form right and left bundle branches that travel down septum. Curve around tip of ventricular chambers and travel back toward atria along outer walls Purkinje fibers Small, terminal fibers that extend from Bundle of His and spread throughout ventricular myocardium 9 Electrical Path Cardiac impulse originates at SA node Action potential spreads throughout right and left atria via interatrial pathway Impulse passes from atria into ventricles through AV node (ONLY point of electrical contact between chambers) Action potential briefly delayed at AV node – WHY? 10 Electrical Path Impulse travels rapidly down interventricular septum by means of Bundle of His Impulse rapidly disperses throughout myocardium by means of Purkinje fibers Rest of ventricular cells activated by cell-to-cell spread of impulse through gap junctions 11 Spread of Cardiac Excitation SA Node AV Node Bundle of His Purkinje Fibers Interatrial Pthwy Internodal Pthway 12 Next steps.. After cardiac excitation, myocyte contraction occurs Membrane currents generated through ions shifting inward or outward through ion channels creates action potential/myocyte contraction + Na 2+ Ca + K INa and If channels ICa-L and ICa-T channels IK channels 13 14 Figure 14-1 (Katzung) Stages of Action Potential Resting state (polarized) Depolarization/Upstroke Repolarization Hyperpolarization Refractory period – Absolute – Relative 15 Membrane Electrical States Depolarization (Upstroke) Increased inward or decreased outward current; upward trace Repolarization Membrane returns to resting state, decreased inward or increased outward current; downward trace Hyperpolarization Membrane becomes more polarized than when at resting potential 16 Stages of Action Potential Absolute Refractory Period No additional action potentials can be induced during this time Relative Refractory Period Additional action potentials can occur during this time to activate channels which have recovered from inactivation 17 Cardiac Cell Action Potentials Figure 23-1 (Golan) Figure 23-1 (Golan) 18 Ventricular Myocyte Action Potential – Initial Phases Rapid Depolarization Sodium channels open Influx of sodium (Na+) into Phase 0 cells via INa channels Overshoots electrical potential Action Potential (AP) for cardiac myocyte Brief Repolarization Inactivation of open Na+ channels/decrease in Na+ Phase 1 influx Efflux of potassium (K+) begins via IK channels 19 Ventricular Myocyte Action Potential – Later Phases Plateau Phase Phase 2 Influx of Calcium (Ca2+) into cell via ICa-L and ICa-T Outward potassium (K+) efflux continues via IK channels Balance in influx/efflux Rapid Repolarization Action Potential (AP) for cardiac myocyte Phase 3 Decrease in inward Ca2+ current Large increase in K+ efflux out of cells Restores membrane to resting potential 20 Ventricular Myocyte Action Potential – Return to Rest Resting Potential Phase 4 Inward and outward currents are equal Via Na+/K+ ATPase pump Action Potential (AP) for cardiac myocyte 21 SA Node Action Potential Figure 23-1 (Golan) Table 23-1 (Golan) 22 Slow and Fast Tissue SLOW Responses: Normal pacemakers SA, AV nodal tissue Phase 4 depolarization due to If FAST Responses: Bundle of His/Purkinje fibers and all non pacemaker cell myocytes Phase 0 depolarization (INa) Phase 2 plateau (ICa) Phase 3 repolarization (IKdr) Richard Morrisett, Ph.D. 23 Electrical Activity of Heart Ca2+ entry into the cardiac cell triggers larger release of Ca2+ from sarcoplasmic reticulum Ca2+ induced Ca2+ release leads to cross-bridge cycling and contraction Because the long refractory period occurs in conjunction with the prolonged plateau phase, summation and tetanus of cardiac muscle is impossible Ensures alternate periods of contraction and relaxation which are essential for pumping blood Chapter 9 Cardiac Physiology Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning 24 Cardiac AP Phases for Ventricular Myocyte 0= 1= 2= 3= 4= Bank of Options: Plateau Repolarization Rapid depolarization Early phase of repolarization Resting potential 25 Electrocardiogram (ECG) Different parts of the ECG record can be correlated to specific cardiac events Depolarization = systole Repolarization = diastole Chapter 9 Cardiac Physiology Human Physiology by Lauralee Sherwood ©2010 Brooks/Cole, Cengage Learning 26 Figure 29-4 (G&G) 27 Electrocardiogram (ECG) Measure the overall electrical activity of the heart Body surface manifestation of de- and re-polarization waves of the heart Culmination of individual APs Figure 23-4 (Golan) P wave - QRS complex - T wave - PR interval - QT interval - ST segment 28 Tie it All Together Action Potential, Myocyte Contraction Cardiac Excitation ECG (culmination of action potentials) 29 CARDIAC ELECTROPHYSIOLOGY [PHM 480C] Physiology, Pathophysiology and Drug Targets Cardiology Module Spring 2024 Denise Kelley, PharmD, BCPS, FASHP Clinical Assistant Professor / Internal Medicine Clinical Pharmacy Specialist [email protected] 30