Heart sounds are visualized on a phonocardiograph.
The first heart sound (“lub”), indicated as S1, is produced when the mitral and tricuspid close at the beginning of ventricular systole; the mitral valve closes slightly before the tricuspid valve (S1 is best heard at heart apex).
The second heart sound (“dub”), indicated as S2, is produced when the aortic and pulmonary valves close at the end of ventricular systole (S2 is best heard at the left sternal edge).
Audible physiological splitting of S2 occurs because the aortic valve closes before the pulmonary valve (the left ventricle contracts slightly before the right ventricle); splitting is most apparent at end-inspiration, because increased venous filling causes a delay in pulmonary valve closure (sounds like “lub d/dub” at inspiration).
The third heart sound, S3, can be heard in children during rapid ventricular filling; it may also be present some very fit adults, but, in unhealthy adults, may signify volume overload and congestive heart failure.
S4, the fourth heart sound, may be heard during the aortic “kick” in individuals with noncompliant left ventricles, and, therefore, is heard in aortic stenosis, systemic hypertension, hypertrophic cardiomyopathy, and ischemia. If present, it is loudest at the heart apex.
