The cardiac cycle is a sequence of events cyclically repeated in the heart. This involves contraction of the heart i.e. systole and relaxation of the heart i.e. diastole of both atria and ventricles. Two prominent heart sounds are heard during each cardiac cycle. These are Lub-Dub and can be easily heard with the help of a stethoscope. The first heart sound is Lub or S1 and this is produced due to the closure of atrioventricular valves i.e. bicuspid and tricuspid valves and is heard at the beginning of ventricular systole. The second heart sound is Dub or S2 and this is due to the closure of semilunar valves and is heard at the end of ventricular systole.