Cardiac Physiology and Electrocardiogram

Cardiac Conduction System

  • Heart Pacemaker Nodes

    • Sinoatrial (SA) Node: Primary pacemaker, located in the right atrium.
    • Atrioventricular (AV) Node: Secondary pacemaker, situated where the atria meet the ventricles. It conducts impulses from the SA node to the ventricles.
    • Purkinje Fibers: Tertiary pacemaker that spreads the electrical impulses through the ventricles.

  • Action Potential and Threshold Point (TP)

    • Electrical impulses from the SA node reach the AV node and then travel to Purkinje fibers, causing ventricular muscle contractions.
    • Threshold Point refers to the millivolt level that triggers the influx of sodium and potassium ions, leading to heart muscle contraction.

  • Hierarchy of Pacemakers

    • If the SA node fails, the AV node can assume control but at a slower rhythm.
    • If both SA and AV nodes fail, Purkinje fibers take over, resulting in even slower heart rhythms.

Cardiac Control Mechanisms

  • Sympathetic Nervous System

    • Responsible for increasing heart rate in response to stress or activity (fight or flight response).
    • Accelerates the SA node, moving the threshold point to facilitate faster heart rate.

  • Parasympathetic Nervous System

    • Calms the body and reduces heart rate during restful states.
    • Delays the signal from the SA node causing the heart to take longer to reach threshold.

Electrocardiogram (ECG or EKG)

  • Three Standard Leads:

    • Lead I: Between right arm and left arm.
    • Lead II: From right arm to left leg.
    • Lead III: Between left arm and left leg.

  • Einthoven's Triangle: A conceptual triangle used for placing electrodes to assess the heart’s electrical activity.

  • Understanding ECG Readings:

    • P Wave: Represents atrial depolarization, initiated by the SA node.
    • QRS Complex: Corresponds to ventricular depolarization, major part of the heartbeat signal.
    • T Wave: Indicates ventricular repolarization, the process of the ventricles relaxing.
    • Atrial Repolarization: Typically not visible on the ECG; occurs simultaneously with the QRS complex, which masks it.

Contextual Anecdotes

  • Mention of a student with an unusual heart orientation that went undetected until an EKG was performed. This demonstrates the importance of monitoring cardiac health.