Cardiac action potentials are essential for proper heart function, enabling the rhythmic contractions needed for adequate blood circulation. Nodal cells and Purkinje fibers, specialized for electrical conduction, generate these action potentials.

The cardiac action potential process involves a series of phases characterized by the movement of ions across the cardiac cell membranes, leading to the depolarization and repolarization of the cardiac myocytes.

Ionic Basis of Cardiac Action Potentials

In the resting state, cardiac cells exhibit a polarized membrane potential with a higher concentration of sodium ions extracellularly and potassium ions intracellularly, resulting in a negative charge inside the cell compared to the outside.

Upon stimulation, sodium ions rapidly influx through fast sodium channels, while calcium ions enter more slowly via calcium channels. This ion movement leads to depolarization, where the intracellular space becomes positively charged.

As a result, potassium ions exit the cell, initiating repolarization, which restores the cell to its resting state. This cyclical exchange of ions constitutes the cardiac action potential, which can be divided into five distinct phases.

Phases of the Cardiac Action Potential

Phase 0 (Depolarization): This phase is marked by a rapid influx of sodium ions through fast channels in atrial and ventricular myocytes, leading to rapid depolarization. In contrast, the sinoatrial (SA) and atrioventricular (AV) nodes depolarize more slowly due to calcium influx through slow channels, combined with the closure of potassium channels and a slow inward leak of sodium ions.

Phase 1 (Initial Repolarization): Early repolarization occurs as potassium ions begin to exit the cell, causing a slight decline in membrane potential.

Phase 2 (Plateau Phase): The plateau phase is characterized by a balance between calcium influx and potassium efflux, resulting in a sustained depolarized state. This phase is crucial for prolonging the contraction of the heart muscle, allowing for efficient blood ejection.

Phase 3 (Final Repolarization): Repolarization completes as potassium efflux predominates, returning the membrane potential to its resting state.

Phase 4 (Resting Phase): The cell remains in a polarized resting state until the next depolarization cycle.

Refractory Periods

During the cardiac action potential, myocardial cells undergo refractory periods when they cannot be restimulated. The effective refractory period spans from the start of phase 0 to the middle of phase 3, during which the cell is completely unresponsive to new stimuli.

It is followed by the relative refractory period in the latter part of phase 3, where a stronger-than-normal stimulus can provoke an early depolarization. Premature contractions during this period can lead to arrhythmias, with potential risks including ventricular tachycardia and fibrillation, particularly in the context of myocardial ischemia.

Electrocardiogram (ECG)

An electrocardiogram (ECG) records the heart's electrical activity, tracing the sequential depolarization and repolarization of the atria and ventricles. Key components include the P wave (atrial depolarization), the QRS complex (ventricular depolarization), and the T wave (ventricular repolarization).

Anomalies in these waveforms and intervals can indicate underlying cardiac pathologies, necessitating further clinical evaluation.