Heart sounds are the audible sounds produced by the mechanical activity of the heart, traditionally heard through a stethoscope placed on the chest wall. In a healthy heart, two main sounds are heard with each cardiac cycle: the first heart sound (S1) and the second heart sound (S2).
S1 is produced by the closure of the two atrioventricular valves (the mitral and tricuspid valves) at the beginning of ventricular systole, as the ventricles start to contract. It is described as a low-pitched ‘lub’. S2 is produced by the closure of the two semilunar (arterial) valves (the aortic and pulmonary valves) at the end of systole, as ventricular pressure falls and blood would otherwise flow back. It is described as a shorter, slightly higher-pitched ‘dub’. Together they form the familiar ‘lub-dub’ of the heartbeat. S2 normally splits into its two components during inspiration, as the pulmonary valve closes slightly later than the aortic valve due to increased right heart filling.
Two additional heart sounds may be abnormal or physiological depending on context. S3 is a low-pitched sound occurring early in diastole, caused by rapid ventricular filling. In young people it can be a normal finding (physiological S3), but in adults over 40 it usually indicates elevated filling pressures and is an important sign of heart failure or volume overload. S4 occurs late in diastole, just before S1, caused by the atria contracting forcefully to push blood into a stiff, non-compliant ventricle. S4 suggests left ventricular hypertrophy or diastolic dysfunction.
Heart murmurs are additional sounds caused by turbulent blood flow, which can result from valve stenosis, valve regurgitation, septal defects, or increased flow states. Murmurs are characterised by their timing (systolic or diastolic), location, loudness (graded 1 to 6), and radiation pattern. Echocardiography provides definitive assessment of the valve or structural abnormality responsible for a murmur.
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