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Ventricular depolarization begins during which phase of the cardiac cycle?
A. Ventricular relaxation X
B. Ejection X
C. Isovolumic contraction ✔
Image source – CC Lic 2.5 Daniel Wiggers.
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Ventricular depolarization, corresponding to the QRS complex on the ECG, coincides with the onset of isovolumic contraction which is the first phase of systole. During about the middle of ejection (the second phase of systole), ventricular repolarization occurs (T-wave on the ECG). Repolarization is complete by the end of Isovolumic relaxation, the first phase of diastole.
The coordination of electrical and mechanical events in the cardiac cycle has practical implications. As an example, it is important for the functioning of intraaortic balloon pumps (IABPs). The goal of IABPs is to rely on counterpulsation, or reversed flow, generated by blood displacement with balloon filling to augment coronary perfusion and systemic perfusion. The device is programmed to recognize the peak of the R wave, which is isovolumic contraction before the aortic valve opens, to time deflation of the balloon. The balloon then recognizes the peak of the T wave as the start of ventricular relaxation and cardiac diastole and reinflates.
Ventricular depolarization, corresponding to the QRS complex on the ECG, coincides with the onset of isovolumic contraction which is the first phase of systole. During about the middle of ejection (the second phase of systole), ventricular repolarization occurs (T-wave on the ECG). Repolarization is complete by the end of Isovolumic relaxation, the first phase of diastole. Systole begins with isovolumic contraction of the ventricles followed by ejection.
Pagel, P. S. (2014). Cardiac physiology. In J. A. Kaplan, D. L. Reich, & J. S. Savino (Eds.), Kaplan’s cardiac anesthesia: The echo era (pp. 98–131). Elsevier.
CA 1 Lecture 13.1 Review of Basic Cardiac Physiology
CV Online Module 13 Intra-Aortic Balloon Pump
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Date of last update: May 19, 2025
