Electrophysiology of Normal Cardiac Rhythm

The SA node, located in the right atrium, is the heart's primary pacemaker made of specialized noncontractile cells. It initiates electrical impulses that trigger each cardiac cycle, transmitting signals through the AV node and specialized conducting structures to coordinate ventricular contraction and blood circulation throughout the body.

Voltage-sensitive membrane channels regulate ion movement across cardiac muscle cell membranes. Sodium channels open during Phase 0 for rapid depolarization, calcium channels create the plateau in Phase 2, and potassium channels open during Phase 3 for repolarization. This coordinated ion flow generates the five-phase action potential essential for heart contraction.

Phase 1 involves partial repolarization following sodium channel inactivation. Phase 3 represents rapid repolarization as calcium channels inactivate and potassium ions flow outward. Phase 4 restores the resting membrane potential through the sodium-potassium pump and background ion channels, preparing the cell for the next electrical impulse.

Cardiac muscle cells possess automaticity, allowing them to generate electrical impulses; excitability, enabling them to respond to stimuli; and conductivity, permitting impulse propagation. These unique properties work together with the specialized conducting system to ensure stable, coordinated heartbeats and efficient blood circulation.

After the SA node initiates an impulse, it propagates through the AV node, bundle of His, and Purkinje fibers. This specialized conducting system ensures rapid and orderly spread of excitation throughout the heart, coordinating atrial and ventricular contractions for efficient blood circulation.

Heart disease such as ischemia or myocardial infarction can impair the electrical conduction system. Additionally, certain drugs and circulating hormones modulate ion channel function, altering the action potential's trajectory and duration. These disruptions can lead to abnormal cardiac rhythms and compromised blood circulation.

The electrical waves produced by the heart's coordinated contractions are transmitted throughout the body and detected by an electrocardiogram (ECG). This non-invasive tool records the heart's electrical activity, allowing clinicians to assess cardiac rhythm and identify abnormalities in the conduction system.