isovolumetric relaxation
Summary
Isovolumetric relaxation is the phase of the cardiac cycle occurring immediately after ventricular systole, during which the ventricles relax and pressure drops sharply while ventricular volume remains constant. It occurs after the aortic and pulmonic valves close and before the mitral and tricuspid valves open. This phase corresponds to the period between S2 (aortic/pulmonic valve closure) and the opening of the AV valves.
Detail
Isovolumetric relaxation begins right after the semilunar valves (aortic and pulmonic) close, marking the end of ventricular ejection. At this point, all four heart valves are closed, so no blood enters or leaves the ventricles, meaning ventricular volume stays constant (isovolumetric) even as the ventricular myocardium relaxes and intraventricular pressure falls rapidly. This rapid pressure decline continues until ventricular pressure drops below atrial pressure, at which point the mitral and tricuspid valves open, initiating the rapid filling phase of diastole.
On the Wiggers diagram, this phase corresponds to the period right after the dicrotic notch on the aortic pressure curve and is associated with the S2 heart sound. The rate of pressure decline during this phase is quantified by the time constant of relaxation (tau), which is a key index of diastolic function; a prolonged tau indicates impaired ventricular relaxation, seen in conditions like diastolic heart failure (HFpEF), myocardial ischemia, and hypertrophic cardiomyopathy.
Clinically, isovolumetric relaxation is important because impaired relaxation (a form of diastolic dysfunction) can occur even when systolic function (ejection fraction) is preserved. This is often assessed using echocardiography, where the isovolumetric relaxation time (IVRT) is measured using Doppler imaging - a prolonged IVRT suggests diastolic dysfunction. Energy for relaxation is ATP-dependent, involving calcium reuptake into the sarcoplasmic reticulum via SERCA2a, regulated by phospholamban. This makes ventricular relaxation an active, energy-requiring process, not merely passive recoil, and dysfunction in calcium handling (as in ischemia or heart failure) directly impairs this phase.
Sources
- Guyton and Hall Textbook of Medical Physiology
- Costanzo Physiology
- First Aid for the USMLE Step 1
- Braunwald's Heart Disease: A Textbook of Cardiovascular Medicine
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