What heart sound is accompanied by the beginning of systole?

The heart sound that signifies the beginning of systole is S1. This sound occurs when the atrioventricular (AV) valves (the mitral and tricuspid valves) close as the ventricles contract. The closing of these valves marks the transition from the diastolic phase, where the heart fills with blood, to the systolic phase, where blood is pumped out of the heart.

S1 is often described as a "lub" sound and is crucial for determining the start of ventricular contraction. Understanding the timing of heart sounds and the mechanical events associated with them is fundamental in cardiac sonography, as it helps in the assessment of cardiac function and valve integrity.

Other sounds such as S2, S3, and S4 correspond to different phases of the cardiac cycle. S2 marks the closure of the aortic and pulmonary valves at the end of systole, while S3 and S4 are associated with the filling phases and can indicate different cardiac conditions but do not coincide with the start of systole.