Which event in the cardiac cycle is responsible for producing the S4 heart sound?

The S4 heart sound, also known as the "atrial gallop," is produced during atrial systole, which is the phase of the cardiac cycle when the atria contract to push blood into the ventricles. This sound is generated by the vibrations of the ventricular wall as it fills with blood that is actively being pushed in from the atria.

In healthy individuals, the S4 sound is often inaudible, but it can become more prominent in conditions where the ventricle is stiff or not fully compliant, such as in left ventricular hypertrophy or in cases where the ventricle is facing increased resistance to filling. Because the S4 is associated with atrial contraction and the last phase of ventricular filling, it reflects the ability of the ventricle to accommodate the additional volume brought in by atrial contraction.

Other events in the cardiac cycle, such as ventricular contraction, isovolumetric relaxation, and ventricular filling, do not lead to the S4 sound. Instead, they are associated with different phases and sounds in the cardiac cycle (like S1 or S2 heart sounds), which occur at different points of the cycle. Understanding this unique relationship between atrial systole and the generation of the S4 sound