first heart sound
The first heart sound is the lub in the familiar lub-dub of a heartbeat. It is the deeper, slightly longer sound you hear at the start of each beat through a stethoscope.
It is produced when the two valves between the atria and ventricles — the mitral and tricuspid valves — snap shut at the onset of systole. As the ventricles begin to contract, rising pressure forces these inlet valves closed, and the sudden tensing sets up the vibration we hear.
S1 marks the beginning of ventricular systole, so it is a timing landmark for the whole exam: a murmur after S1 is systolic, a sound before it relates to filling. Its loudness varies with how forcefully the valves close, which depends on contractility, valve condition, and the PR interval on the ECG.
Changes in S1 carry clues. A loud S1 can occur in conditions like mitral stenosis or a fast, vigorous heart; a soft S1 may suggest a poorly closing valve or weak contraction. It can also be split when the two valves close slightly out of step.
Placing a stethoscope over the apex of the heart, a clinician hears S1 loudest there because it captures mitral valve closure at the start of each contraction.
S1 is heard best at the apex, reflecting mitral valve closure.
S1 comes from mitral and tricuspid (atrioventricular) valve closure and marks systole's start; S2 comes from aortic and pulmonary valve closure and marks its end. Together they bracket systole.