During which phase of the cardiac cycle is the chamber pressure in all four chambers decreasing while maintaining constant volume?

The correct phase of the cardiac cycle during which the chamber pressure in all four chambers is decreasing while maintaining a constant volume is classified as isovolumetric relaxation time (IVRT). This phase occurs immediately after ejection of blood from the ventricles, following systole. During IVRT, the ventricles are in a state where both the aortic and pulmonary valves are closed, resulting in no change in volume even though the pressures inside the chambers continue to drop as the heart muscle relaxes.

In this state, the myocardial fibers lengthen and the ventricular pressures fall more rapidly than the atrial pressures. Since both valves are closed, there is no inflow or outflow of blood, allowing for the pressure to decrease without a change in volume, which is an essential aspect of this phase.

This is distinct from other phases, such as isovolumetric contraction time (IVCT), where pressure increases without a change in volume, or systole, when the heart is actively contracting and ejecting blood, leading to a dynamic change in chamber volume. Diastole involves filling of the chambers with blood, which clearly results in a change in volume. Thus, the unique conditions of IVRT make it the phase where the chamber pressure decreases