Final answer:
The phase with little or no change in atrial or ventricular pressures during relaxation is called the isovolumic ventricular relaxation phase of the cardiac cycle.
Step-by-step explanation:
The phase of the cardiac cycle where there is little or no change in atrial or ventricular pressures as both ventricle and atria are relaxed is called the isovolumic ventricular relaxation phase, also known as the isovolumetric ventricular relaxation phase. This occurs at the initial phase of the ventricular diastole when the pressure in the ventricles drops below the pressure in the pulmonary trunk and aorta.
At this point, the semilunar valves close to prevent blood from flowing back into the heart, and the atrioventricular valves remain closed, so there is no change in the volume of blood in the ventricles. This phase proceeds the atrial diastole where both the atria and ventricles are in the state of relaxation, and the atrioventricular valves are open, allowing for passive filling of the ventricles.