1.9.6 Isovolumetric Relaxation Definition
Isovolumetric relaxation is the ventricular phase where relaxation occurs without volume change, between valve closures.
Isovolumetric Relaxation Definition is the phase of the cardiac cycle occurring at the very beginning of diastole, during which the ventricular myocardium relaxes and intraventricular pressure falls rapidly while the volume of blood within the ventricle remains unchanged. This phase begins the instant the semilunar valves close, marking the end of ventricular ejection, and ends when the atrioventricular (AV) valves open, meaning that throughout this brief interval all four cardiac valves are closed and no blood enters or leaves the ventricle.
Mechanical Basis
Isovolumetric relaxation mirrors isovolumetric contraction in principle: a sealed, fluid-filled chamber cannot change volume even as the surrounding muscle changes its mechanical state.
Valve Closure
At the end of ventricular ejection, falling ventricular pressure drops below the pressure in the aorta and pulmonary artery, causing the semilunar valves to snap shut. This closure produces the second heart sound (S2) and marks the onset of isovolumetric relaxation.
Constant Volume
With the semilunar valves closed and ventricular pressure still exceeding atrial pressure, the AV valves also remain shut. Because every valve is closed, the ventricle is once again a sealed chamber, so despite active myocardial relaxation and falling internal pressure, the enclosed blood volume, the end-systolic volume, cannot change.
Rapid Pressure Decline
Active relaxation of the ventricular myocardium during this phase produces one of the steepest pressure declines observed in the cardiac cycle, occurring entirely at constant volume.
Termination of the Phase
Isovolumetric relaxation ends once ventricular pressure falls below the pressure in the corresponding atrium.
Mitral Valve Opening
In the left heart, the phase ends when left ventricular pressure falls below left atrial pressure, causing the mitral valve to open and initiating rapid ventricular filling.
Tricuspid Valve Opening
In the right heart, the phase ends when right ventricular pressure falls below right atrial pressure, causing the tricuspid valve to open.
Pressure–Volume Correlation
On a ventricular pressure–volume loop, isovolumetric relaxation appears as a vertical line at the end-systolic volume, since pressure falls steeply while volume remains fixed.
Clinical and Physiological Relevance
The rate at which ventricular pressure declines during isovolumetric relaxation, often quantified by the time constant of relaxation, is used as an index of diastolic function, reflecting how efficiently the myocardium relaxes independent of loading conditions. Prolongation of isovolumetric relaxation is a hallmark of impaired diastolic function, as seen in conditions such as myocardial ischemia, ventricular hypertrophy, and diastolic heart failure.