Isovolumetric Relaxation Phase

Isovolumetric Relaxation Phase is the initial period of ventricular diastole during which the ventricular myocardium actively relaxes and intraventricular pressure falls steeply while chamber volume remains constant, occurring between the closure of the semilunar valves and the opening of the atrioventricular valves. It represents a phase of purely pressure-declining relaxation, during which the ventricle remains a sealed chamber, with both its inflow and outflow valves closed, allowing no blood to enter or leave despite the active, rapid myocardial relaxation taking place.

Position Within the Cardiac Cycle

Onset Following Semilunar Valve Closure

The isovolumetric relaxation phase begins at the instant declining ventricular pressure falls below arterial pressure, causing the semilunar valves to close and sealing the outflow tract of the ventricle, marking the transition from the end systolic state into the earliest stage of ventricular diastole.

Termination at Atrioventricular Valve Opening

The phase concludes when falling ventricular pressure drops below atrial pressure, causing the atrioventricular valves to open and permitting the onset of rapid ventricular filling, at which point the chamber is no longer sealed and volume begins to change as blood enters from the atria.

Mechanistic Basis

Sealed Chamber Condition

Because both the semilunar and atrioventricular valves are closed throughout this phase, the ventricle behaves as a fixed, sealed compartment. The blood volume contained within cannot leave, and no additional blood can enter, meaning the ongoing relaxation of the myocardium is expressed entirely as a fall in pressure rather than a change in volume.

Active Myocardial Relaxation

The fall in pressure during this phase results from active, energy-dependent relaxation of the ventricular myocardium, involving the sequestration of intracellular calcium and the dissociation of actin-myosin cross-bridges, a process termed lusitropy that determines the speed and completeness of the resulting pressure decline.

Relationship to Electrical Events

Timing Relative to the T Wave

Isovolumetric relaxation occurs in close temporal association with the later portion of ventricular repolarization, represented electrocardiographically by the T wave, since the decline in intracellular calcium concentration underlying mechanical relaxation follows the electrical repolarization of the myocyte membrane.

Dependence on Relaxation Rate

The rate at which pressure falls during this phase reflects the intrinsic relaxation properties of the ventricular myocardium, since more efficient, rapid relaxation produces a steeper pressure decline, reaching the threshold required to open the atrioventricular valves more quickly.

Pressure-Volume Loop Representation

Vertical Segment of the Loop

On the ventricular pressure-volume loop, isovolumetric relaxation is represented by a vertical line descending from the end systolic pressure-volume point, reflecting the characteristic fall in pressure occurring at a fixed volume before filling begins.

Duration Relative to Relaxation Rate

The vertical extent and duration of this segment on the pressure-volume loop vary with the speed of ventricular relaxation, so that a more efficiently relaxing ventricle traverses the isovolumetric segment more rapidly, reaching atrial pressure sooner and shortening the overall duration of this phase.

Auscultatory Timing

Position After the Second Heart Sound

The isovolumetric relaxation phase begins immediately following the generation of the second heart sound, which is produced by the closure of the semilunar valves marking the onset of this phase, and continues until the atrioventricular valves open, an event that is typically silent under normal conditions.

Valvular State Throughout the Phase

Both Valve Sets Closed

Throughout isovolumetric relaxation, the semilunar valves remain closed, having sealed the outflow tract at the onset of the phase, while the atrioventricular valves remain closed as well, since ventricular pressure has not yet fallen sufficiently below atrial pressure, together maintaining the fully sealed condition that defines this phase.

Functional Significance of the Representation

Generation of the Pressure Threshold for Filling

Isovolumetric relaxation functions to rapidly reduce ventricular pressure from its end systolic level to a value below atrial pressure, establishing the necessary condition for the atrioventricular valves to open and rapid diastolic filling to commence.

Reflection of Myocardial Relaxation Capacity

Because the rate of pressure fall during this phase depends directly on the intrinsic relaxation capacity of the ventricular myocardium, the isovolumetric relaxation phase serves as a representation of ventricular lusitropic function, providing insight into diastolic performance independent of the loading conditions that influence the subsequent filling phase.