3.4.3 The second heart sound
The second heart sound (S2) is a relatively high frequency sound, usually described as a 'dup' or 'dub'. S2 is shorter in duration than S1. It marks the end of systole and usually occurs shortly after the T wave of the ECG (the T wave is labile and therefore does not have a fixed relationship with mechanical events). The sound is principally caused by deceleration of blood in the great arteries just after the closure of the semilunar valves, at the incisura.
The intensity of S2 is reduced when the stroke volume is reduced, either as a result of a premature supraventricular or ventricular contraction or due to myocardial disease. This is because the deceleration of blood at the end of systole is reduced.
In the normal horse, S2 is frequently split. This can be best appreciated if the stethoscope is placed cranially over the heart base, corresponding to the pulvalve area. The aortic component of S2 (A2) precedes the pulmonary component (P2) in most horses, although P2 can precede A2 in normal animals. Respiration increases the degree of splitting because the pulmonary valve closes later during inspiration due to increased right ventricular stroke volume and increased ejection time. Pulmonary valve closure may be delayed in animals with a significant left to right shunt such as a large ventricular septal defect for the same reason. Pulmonary hypertension may result in an increase in the intensity of P2, making S2 sound louder than normal.