Under physiological circumstances two heart sounds can always be heard: the first and second heart sounds. These are high-frequency valve closure sounds. The third and fourth heart sounds are only present in some cases. These are extra sounds and have a low frequency.
First heart sound
The first heart sound is caused by the closure of the atrio-ventricular valves closely after each other at the start of systole: the mitral valve first, followed by the tricuspid valve immediately after. The first heart sound is also referred to as S1 (S for sound) and consists of M1 (mitral component of the 1st sound) and T1 (tricuspid component of the 1st sound).
Second heart sound
The second heart sound is caused by the closure of the semilunar (half-moon shaped) valves one after the other at the start of diastole: the aortic valve first, followed by the pulmonary valve immediately after. The second heart sound is referred to as S2 and consists of the A2 (aortic component of the 2nd sound) and P2 (pulmonary component of the 2nd sound).
Third heart sound
The third heart sound is caused by a vibration of the left or right ventricular wall as a result of increased rapid filling of one of or both ventricles. In children, young adults and pregnant women, a third sound can be found. It also occurs in the event of increased volume load (e.g. as a consequence of anaemia or fever) or in the case of pathology of the heart (e.g. reduced compliance of the ventricular wall as a result of hypertension or infarction).
A combination of S1, S2 and S3 is referred to as a protodiastolic gallop or ventricular gallop.
Fourth heart sound
The fourth heart sound is caused by an increased intensity of left and/or right atrial contraction at the end of diastole. The presence of a 4th sound is always indicative of cardiac pathology (e.g. increased rigidity which impairs left ventricular filling).
A combination of S1, S2 and S4 is referred to as a pre-systolic gallop or atrial gallop.
A combination of S1, S2, S3 and S4 is termed a summation gallop.