The third heart sound is a low pitched diastolic sound usually best heard over the apex. It is probably caused by tautening of the mitral and tricuspid capillary muscles at the end of rapid diastolic filling. It usually indicates left ventricular dysfunction.
A fourth heart sound is a late diastolic sound, more high pitched than the third. It is never physiological, and usually reflects poor ventricular function (e.g. aortic stenosis, systemic hypertension, HOCM).
Characteristic sounds occur with some lesions, such as the opening snap of mitral stenosis, heard over the left sternal border in the fourth interspace (figure 56).