Understanding Cardiac Electrophysiology

Aug 19, 2024

Electrophysiology of the Cardiovascular System

Introduction

  • Electrophysiology involves the electrical activity that controls the mechanical pumping of the heart.
  • The heart contracts in response to an electrical stimulus leading to chemical changes in myocardial cells.

Depolarization and Repolarization

  • Depolarization: Discharge of resting cardiac muscle fibers via an electric impulse causing contraction.
    • Polarized cell: Net charge of -90 mV.
    • Sodium (Na+) and Calcium (Ca2+) influx leads to depolarization.
  • Repolarization: Restoration of the resting state.
    • Potassium (K+) ions exit, restoring negative charge.
    • Sodium-potassium pump restores ion balance.

Cardiac Action Potential Phases

  1. Phase 0: Rapid Na+ influx causes depolarization.
    • Represented by the QRS complex on an ECG.
  2. Phase 1: Initial repolarization, inward Na+ channels close.
  3. Phase 2: Plateau phase, prolonged depolarization.
    • Corresponds to the ST segment.
  4. Phase 3: Final repolarization as Ca2+ channels close.
    • Represented by the T wave.
  5. Phase 4: Resting membrane potential restored.

Refractory Periods

  • Absolute Refractory Period: Ventricles cannot depolarize.
  • Relative Refractory Period: Some cells can depolarize again.

Cardiac Conduction System

  • Conducts electrical impulses to heart muscles.
  • SA Node: Dominant pacemaker located at the junction of the superior vena cava and right atrium.
  • AV Node: Delays impulse allowing atria to fill ventricles.
  • AV Junction & Bundle of His: Conduct impulses to bundle branches.
  • Purkinje Fibers: Distribute impulses across ventricles.
  • Secondary Pacemakers: AV junction or Purkinje fibers can take over if SA node fails.

Accessory Conduction Pathways

  • Bypass tracts like James fibers, Mahaim fibers, and Bundle of Kent.
  • Can cause tachydysrhythmias requiring medical intervention.

Autonomic Nervous System

  • Sympathetic Nervous System: Increases heart rate to prevent lactic acid build-up.
    • Norepinephrine affects SA node, AV node, and ventricles.
  • Parasympathetic Nervous System: Decreases heart rate via the vagus nerve.
    • Acetylcholine released to slow heart rate.

ECG and Monitoring

  • ECG provides a graphic record of heart depolarization and repolarization.
  • Electrode Placement: Consistent placement is crucial for accurate readings.
  • Types of leads: Limb leads and precordial leads.
  • Limb Leads: I, II, III, AVR, AVL, AVF.
  • Precordial Leads: V1-V6.

ECG Components

  • P Wave: Atrial depolarization.
  • PR Interval: Time from atrial depolarization to ventricular depolarization.
  • QRS Complex: Ventricular depolarization.
  • ST Segment: Beginning of ventricular repolarization.
  • T Wave: Ventricular repolarization.
  • U Wave: Final stage of ventricular repolarization.
  • QT Interval: Total time of ventricular activity.
  • RR Interval: Time between two ventricular depolarizations.

Conclusion

  • Understanding cardiac electrophysiology is essential for diagnosing and treating cardiovascular emergencies.
  • Next lecture will cover dysrhythmia interpretation and treatment.