3.4.2 The first heart sound
The first heart sound (S1) is a relatively low frequency sound usually described as a 'lub'. It marks the beginning of mechanical systole and therefore starts some time shortly after the beginning of the QRS complex of the ECG. The sound is caused by a number of mechanical events including opening of the semi-lunar valves, motion of the ventricular walls and acceleration of blood into the great arteries. In some cases, closure of the atrioventricular valves also contributes to the sound, but in normal animals, at normal heart rates, the AV valves have already closed at the time of the onset of the pressure rise within the ventricles at the beginning of systole.
The first heart sound is loudest over the apex beat area on the left side of the chest and in the corresponding area on the right. It may be accentuated in cases when the ventricles begin to contract while the AV valves are open. This may occur with premature beats, atrial fibrillation or AV dissociation. The intensity of S1 may be reduced when there is poor myocardial contractility, or poor transof sound.
A split S1 is an unusual finding, and may or may not be associated with any pathological condition. It is important that the fourth heart sound (S4, or atrial contraction sound) is not mistaken for a split S1, because identification of S4 is an important part of the assessment of the cardiac rhythm as it identifies atrial contraction (see below). The finding of a split S1 is not usually significant, except when it is mistaken for S4 in an animal with atrial fibrillation and an incorrect diagnosis is made. The characteristic irregularly irregular rhythm would be another important clinical finding in this situation. Where doubt exists it is a wise precaution to record an ECG.