In normal patients, when does most ventricular filling occur?

Most ventricular filling occurs during rapid ventricular filling in normal patients. This phase takes place just after the mitral and tricuspid valves open, allowing blood to flow from the atria into the ventricles due to the pressure gradient created by the decreased ventricular pressure. This rapid filling phase accounts for the majority of the ventricular volume before the atria contract.

Atrial contraction does contribute to ventricular filling, particularly at the end of diastole, but its contribution is relatively small compared to the volume of blood that flows into the ventricles passively during the rapid filling phase. In fact, the atrial contraction provides the final "topping off" of blood in the ventricles.

During isovolumetric contraction, both the atrioventricular and semilunar valves are closed, which means no filling occurs during this phase; rather, this is the moment when the ventricles begin to contract and generate pressure in preparation for ejection of blood into the aorta and pulmonary artery.

Ventricular systole is when the ventricles contract to pump blood out of the heart, and during this stage, no filling occurs, as the focus is on ejecting blood rather than receiving new blood.

Thus, the correct understanding of the timing and phases