Cardiac Physiology

CARDIAC PHYSIOLOGY SUMMARY

Cardiac Muscle Action Potential

  • Cardiac action potential phases:

    • Resting state, depolarization (Na+ influx), plateau (Ca2+ influx), repolarization (K+ efflux)

    • Long duration (~300 ms) with a stable resting membrane potential (RMP = -90 mV)

Cardiac Conduction System

  • Controls depolarization and contraction coordination via specialized cells (nodal cells)

  • Sinoatrial (SA) node: Primary pacemaker, ~75 bpm

  • Atrioventricular (AV) node: Delays impulses, secondary pacemaker (40 bpm if SA is damaged)

  • AV bundle and branches transmit impulses to ventricles through Purkinje fibers

Electrocardiogram (ECG/EKG)

  • Measures electrical activity during cardiac cycle

  • Key intervals and waves:

    • P wave: atrial depolarization, QRS complex: ventricular depolarization, T wave: ventricular repolarization

    • PR interval: atrial to ventricular excitation time, QT interval: ventricular depolarization and repolarization

Arrhythmias

  • Abnormal heart rhythms:

    • Bradycardia: < 60 bpm, Tachycardia: > 100 bpm, Atrial flutter/fibrillation, heart block types

Cardiac Output (CO)

  • CO = Heart Rate (HR) x Stroke Volume (SV)

  • Normal SV ~70 ml; CO can rise significantly during activity

Regulation of CO

  • Controlled by:

    • Heart Rate (autonomic nervous system influence)

    • Stroke Volume (intrinsic factors like preload and contractility)

  • Hormonal effects via epinephrine and thyroxine

Cardiac Cycle Stages

  1. Passive Ventricular Filling

  2. Atrial Contraction

  3. Isovolumetric Contraction

  4. Ventricular Ejection

  5. Isovolumetric Relaxation

Key Concepts

  • Afterload: Resistance blood must overcome for ventricular ejection

  • Frank-Starling Law: Greater venous return increases stroke volume.

  • Ion effects (Ca2+, K+, Na+) and temperature impact contraction rates.

  • Defibrillation used for re-coordination in arrhythmias.