Cardiac Electrophysiology: Week 4 Lecture Notes

Introduction

• Week 4 Overview: Focus on cardiac electrophysiology, which involves electrical events driving mechanical events in the heart.
• Week 3 Recap: Covered coronary artery disease, cardiac blood flow, blood pressure regulation, valvular disease, and venous disease.
• Importance: Understanding electrolyte anomalies and their impact on cardiac function.
• Topics to be Covered: Objective: Role of ion movement, electromechanical association, comparison between fast and slow response cells, and practical applications in patient management.

Key Concepts

Electromechanical Association

• Definition: Electrical events occur in the heart to drive mechanical contraction.
• Electrical and Mechanical Dynamics: Key Electrolytes:
  • Sodium (Na+): Rapidly depolarizes cell membranes.
  • Potassium (K+): Dominant in resting states, leaks through channels maintaining negative membrane potential.
  • Calcium (Ca2+): Central to muscle contraction, enters cell following depolarization to facilitate contraction.
  • Chloride (Cl-): Attempts to follow K+/Na+ but remains bound, maintaining membrane potential.

The Action Potential

• Phases Explained:
  1. Phase 4 (Resting State/Diastole): K+ channels open, K+ moves out, membrane potential around -60 mV.
  2. Phase 0 (Depolarization): Na+ channels open, Na+ rushes in, membrane potential rises.
  3. Phase 1: Brief repolarization, K+ movement.
  4. Phase 2 (Plateau): Ca2+ channels open, Ca2+ enters extending depolarization.
  5. Phase 3 (Repolarization): K+ efflux, closure of Ca2+, return to resting membrane potential.

Types of Cells

Fast Response Cells

• Location: Contractile tissue - Atria and Ventricles
• Function: Generate force for blood movement.
• Phases: Complex 5-phase action potential (phases 0-4).
• Depolarization: Na+ driven rapid phase 0.

Slow Response Cells

• Location: Rate-setting tissues - SA node, AV node.
• Function: Control heart rate.
• Phases: Simplified 3-phase action potential (phases 4, 0, 3).
• Depolarization: Ca2+ T-type channels in phase 4,

Pathophysiology of Cardiac Conduction

Normal Electrical Pathways

• SA Node: Initiates impulse, right atrium.
• Atrial Conduction Pathways: Ensure synchronous contraction of atria.
• AV Node: Delays impulse, allowing ventricular filling.
• His Bundle & Bundle Branches: Rapid conduction to ventricles.
• Purkinje Fibers: Ensure uniform contraction of ventricles.

Coordinated Function of Electrical Pathways

• Excitability: Determined by resting potential. Excitable cells fire action potential at threshold voltage.
• Refractory Periods: Protect heart from premature stimulation.
  • Absolute Refractory Period: No stimulus can trigger another action potential.
  • Relative Refractory Period: Stronger-than-normal stimulus needed to trigger action potential.

Sympathetic and Parasympathetic Influence

Sympathetic Stimulation

• Effect: Lowers threshold for depolarization.
• Outcome: Faster heart rate, narrower time between beats.
• Mechanism: Beta-1 receptors, epinephrine release.

Parasympathetic Stimulation

• Effect: Increases starting potential (hyperpolarizes membrane potential), raises depolarization threshold.
• Outcome: Slower heart rate, increased time between beats.
• Mechanism: Acetylcholine, muscarinic receptors opening K+ channels.

Electromechanical Coupling: Contraction

Sarcomere Function

• Composition: Actin (thin filaments), Myosin (thick filaments), Troponin, Tropomyosin.
• Calcium Role: Key to binding actin and myosin, leading to contraction.
• Process:
  • Action potential increases membrane potential.
  • Calcium released from sarcoplasmic reticulum into sarcomere.
  • Calcium binds troponin, displacing tropomyosin exposing actin-binding sites.
  • Actin and Myosin cross-bridge formation, causing contraction.

ATP and Muscle Relaxation

• ATP Role: Detachment of actin and myosin, resetting muscle for next contraction cycle.
• Calcium Reuptake: Ends contraction, sequestered back into sarcoplasmic reticulum.

Clinical Implications

• Healthy Heart Function: Coordinated electrical impulses and contractions ensure efficient blood movement.
• Cardiac Diseases: Disruption in ion balance or electrical pathways leads to arrhythmias and reduced cardiac output.
• Medication Management: Understanding of electrophysiology critical for safe prescription of drugs affecting heart's electrical activity.
• Conditions Discussed: Pericarditis, Endocarditis, Cardiomyopathy (to be covered in next session).