Which of the following components is NOT part of the cardiac cycle as depicted on an ECG?

The R wave, while a significant part of the QRS complex, is not referred to as a distinct component of the cardiac cycle when looking at the ECG as a whole. The cardiac cycle encompasses several important phases, which are represented on an ECG by specific waveforms.

The P wave reflects atrial depolarization, indicating the electrical activity associated with atrial contraction. The QRS complex represents ventricular depolarization, which is a critical part of the cardiac cycle as it signifies the contraction of the ventricles. The T wave follows the QRS complex and signifies ventricular repolarization, marking the period in which the ventricles recover electrically and prepare for the next cycle.

The R wave specifically refers to the upward deflection within the QRS complex and, while it is an important part of interpreting the QRS complex itself, it does not stand alone in the context of the cardiac cycle as depicted on an ECG. Thus, it is categorized differently compared to the other components that represent the entire cycle of electrical activity in the heart.