3.4.5 The fourth heart sound or atrial sound
The fourth heart sound (S4) is a transient sound which is heard immediately prior to S1. The sound is associated with the movement of blood cause by atrial contraction. Because of its timing, the sound has been called the atrial sound, atrial contraction sound, or 'A' sound. In the author's view, this is a less confusing terminology than S4, because at normal heart rates it is perceived as the first of the transient sounds in a group of four. The author uses the term A sound rather than atrial contraction sound because the sound may not be caused entirely by contraction of the atria. Phonocardiographic studies show the sound to have two components, S4 and S4'. The first of these is associated with the acceleration of blood at atrial contraction, the second with closure of the AV valves after the contraction is over. The second of the sounds is the louder, but the PR interval needs to be greater than 0.30 seconds for it to be heard. It is most clearly heard during a period of 20AVB.
The A sound is best heard just dorsal to the apical beat areas. Sometimes it is more easily heard on the right side of the chest. It is best heard at slow heart rates and may be difficult to distinguish from S1. In some horses the interval between the A sound and S1 may be occupied by a presystolic murmur, but it can be difficult to distinguish a presystolic murmur and a loud A sound in some cases. This may be because they are both caused by the sound of AV valvular leakage which occurs just as the valve leaflets close. The jet of blood associated with this is easily detected on pulsed wave Doppler examination and is most apparent in animals with loud A sounds and presystolic murmurs. No pathological abnormality seems to be associated with a loud A sound or a presystolic murmur.