Final answer:
The "a" dip on M-mode echo represents the downward deflection of the pulmonary valve leaflets following atrial contraction, which corresponds with atrial systole.
Step-by-step explanation:
On M-mode echo, the abrupt downward motion of the pulmonary valve leaflet following atrial contraction is called the "a" dip. This motion corresponds with atrial sytole, which occurs after atrial depolarization. During this phase, the atria contract to push blood into the ventricles, and this is seen on the M-mode as a downward deflection of the pulmonary valve leaflets due to the increased atrial pressure.
Ventricular systole immediately follows atrial systole and represents the contraction phase of the cardiac cycle, during which the ventricles are pumping blood into the arteries. It is associated with the closing of the atrioventricular valves.