Isovolumetric relaxation is a brief transition phase in the cardiac cycle that marks the beginning of ventricular diastole, or the heart’s relaxation period. This phase is defined by the ventricles relaxing without a change in the volume of blood inside them. Its purpose is to rapidly drop the pressure within the ventricles following a powerful contraction, setting the stage for the next phase of blood intake.
Where It Fits in the Heartbeat
This period immediately follows ventricular ejection. Ejection, which is part of systole (contraction), ends when the pressure in the ventricles drops below the pressure in the great arteries, causing the semilunar valves to snap shut. The closure of the aortic and pulmonary valves marks the beginning of isovolumetric relaxation and is associated with the second heart sound (S2).
Isovolumetric relaxation is the first phase of ventricular diastole, the full period of ventricular relaxation and filling. The ventricles are relaxing after their intense contraction but are not yet taking in blood. This phase is relatively short, typically lasting only about 0.04 seconds, which is a small percentage of the total cardiac cycle time. It acts as a bridge between the high-pressure ejection phase and the low-pressure filling phase that follows.
Mechanical Actions and Pressure Changes
The term “isovolumetric” means the volume of blood inside the ventricles remains constant during this phase. This constant volume occurs because all four major heart valves are closed simultaneously. The semilunar valves closed at the start of this phase, preventing blood from flowing back from the arteries.
The atrioventricular (AV) valves, including the mitral and tricuspid valves, also remain closed. They stay closed because the ventricular pressure, though rapidly falling, is still momentarily higher than the pressure in the atria. Since all entry and exit points are sealed, no blood can enter or leave the ventricles, maintaining the volume at the end-systolic volume.
The mechanical action driving this phase is the rapid relaxation of the ventricular muscle fibers. This relaxation is an active, energy-requiring process known as lusitropy, regulated by the sarcoplasmic reticulum’s rapid re-sequestration of calcium ions. As the muscle relaxes, the tension within the ventricular walls decreases dramatically, causing a steep decline in internal pressure. The pressure drops from its high systolic value, around 120 mmHg in the left ventricle, toward a much lower pressure near zero.
This rapid pressure drop happens without any change in volume, which is the defining characteristic of this phase. The rate of this pressure decline measures the heart muscle’s ability to relax. An impaired ability to relax can hinder the subsequent filling phase, even if the heart’s ability to contract remains normal.
Transition to Ventricular Filling
Isovolumetric relaxation must resolve to allow the heart to refill. The phase ends the moment the pressure within the relaxing ventricles drops below the pressure within the atria. Throughout this phase, the atria have been steadily filling with blood returning from the body and lungs, causing their pressure to slowly rise.
Once the ventricular pressure falls below the atrial pressure, a pressure gradient is established across the atrioventricular (AV) valves. This gradient forces the mitral and tricuspid valves to open. The opening of the AV valves terminates the isovolumetric period because blood can now rush into the ventricles.
The opening of these valves initiates the next major phase of the cardiac cycle, which is rapid ventricular filling. The speed of this initial filling is augmented by the low pressure, almost suction-like state, created in the ventricles during the relaxation period. This allows the heart to maximize the amount of blood it takes in during diastole, preparing for the next forceful contraction.