3.4.4 The third heart sound
The third heart sound (S3) is a transient sound which is heard in some horses. It is low in frequency, with a short, thudding quality. It is best heard over the apex beat area just ventral to the point where S1 is loudest. It is most often heard in fit athletic animals and is a less common finding in older horses and ponies. S3 is also loud in animals with significant left ventricular (LV) volume overload as a result of moderate to severe mitral regurgitation. The reason that the sound is loud in these animals is that it is created by the rapid deceleration of blood in the LV at the end of passive early diastolic filling. When the volume of blood filling the ventricles in early diastole is large, the deceleration of blood as the ventricles reach their elastic limit is marked. Supportive evidence for the source of the sound can be produced by recording a simultaneous phonocardiogram and Mechocardiogram. S3 occurs at the end of early diastolic filling, when the early diastolic dip of the septum seen on echocardiography indicates rebound of the ventricular walls.
The third heart sound varies in intensity at different heart rates. It may sum-mate with the fourth heart sound at high heart rates to produce a gallop sound.