During which phase is the cardiac absolute refractory period observed?

The cardiac absolute refractory period is observed from the onset of the QRS complex to the peak of the T wave. This period is critical because it represents the time during which no new cardiac action potentials can be initiated, regardless of the strength of the stimulus. The absolute refractory period corresponds to the time when the cardiac muscle cells are already depolarized and in the process of repolarization.

During this phase, the sodium channels are inactivated and cannot be reopened until they return to their resting state. This protects the heart from undergoing tetany, which is a state of sustained contraction, and ensures that the heart has enough time to fill with blood before the next contraction. This regulatory mechanism is essential for maintaining an effective cardiac cycle and proper hemodynamic function.

Other time frames mentioned do not capture the precise phase of the cardiac action potential where new impulses cannot be generated. The QRS complex marks the rapid depolarization of the ventricles, and the T wave represents the beginning of ventricular repolarization, creating the context for the absolute refractory period.