Understanding Heart Conduction and Physiology

May 20, 2025

Lecture Notes: Dr. Mike on Heart Conduction and Cardiac Physiology

Introduction to Heartbeats

  • Heart beats about 5000 times per hour, totaling approximately 3.5 billion times in a lifetime.
  • Heart contraction is driven by electrical stimulus, known as neural conduction.

Anatomy of the Heart

  • Chambers: Right atrium, left atrium, right ventricle, left ventricle.
  • Myocardium: Heart muscle that contracts to pump blood.

Spontaneous Heart Contraction

  • The heart can beat independently due to specialized myocardial tissue.
  • SA Node (Sinoatrial Node):
    • Starts the heart rhythm, termed sinus rhythm (60-100 beats/min).
    • Located in the right atrium.

Neural Conduction of the Heart

  1. SA Node fires and sends electrical signals through atrial myocardium.
  2. Signal is funneled to AV Node (Atrioventricular Node):
    • Causes a 0.4-second delay, creating a pause between heartbeats.
    • Can act as a secondary pacemaker (40-60 beats/min if SA Node fails).
  3. Bundle of His conducts the signal down interventricular septum.
  4. Purkinje Fibers distribute the impulse to ventricular myocardium.

Electrical Properties of SA and AV Nodes

  • Cells spontaneously depolarize, leading to action potentials.
  • Sodium-Potassium Pump: Creates charge differences, crucial for depolarization.
  • Calcium and Sodium Channels: Lead to increased positive charges inside cells.
  • Depolarization occurs when reaching a threshold, followed by Repolarization.

Autonomic Nervous System Control

  • Sympathetic System: Increases heart rate and contractility (fight or flight response).
  • Parasympathetic System: Decreases heart rate (rest and digest).
  • Neurotransmitters:
    • Sympathetic: Noradrenaline increases heart rate by allowing sodium/calcium influx.
    • Parasympathetic: Acetylcholine increases potassium efflux, slowing heart rate.

ECG (Electrocardiogram) Basics

  • Purpose: Measures heart's electrical activity.
  • 12-lead ECG: Provides comprehensive views with 10 electrodes.
  • Electrode Placement:
    • Limb electrodes (right arm, left arm, left leg, and right leg as ground).
    • Precordial (chest) electrodes V1-V6.
  • Leads: Measure electrical activity direction and provide ECG readings.

ECG Interpretation

  • P Wave: Atrial depolarization.
  • QRS Complex: Ventricular depolarization.
  • T Wave: Ventricular repolarization.
  • Lead Directions: Affects ECG waveforms based on position relative to depolarization/repolarization.

Cardiac Output

  • Definition: Volume of blood pumped by the heart per minute (~5 liters/min).
  • Components:
    • Heart Rate: Beats per minute.
    • Stroke Volume: Blood ejected per beat.
  • Influences on Stroke Volume:
    • Contractility: Force of heart muscle contraction.
    • Preload: Stretch of ventricles before contraction; high preload increases stroke volume.
    • Afterload: Resistance ventricles must overcome; high afterload reduces stroke volume.

Preload and Afterload

  • Preload: Related to venous return; increased preload enhances cardiac output.
  • Afterload: Resistance against which the heart pumps; excessive afterload can lead to heart failure.
  • Drugs: Beta blockers and calcium channel blockers can modify afterload and cardiac workload.

These notes cover the key points from Dr. Mike's lecture on the heart's electrical conduction system, ECG interpretation, and factors affecting cardiac output. Understanding these concepts is essential for comprehending how the heart functions and how various physiological factors impact cardiac performance.

Full transcript