3.4.1 The generation of the heart sounds
Four transient heart sounds, associated with vibrations occurring during the normal cardiac cycle, are heard in the horse. Two, three or four of these sounds may be heard in a normal animal. The two sounds which mark the beginning and end of systole, and which are heard in all normal animals, are the first and second heart sounds. In many animals, the third heart sound will be heard shortly after the second. In most, the fourth heart sound will be heard immediately before the first heart sound. The heart sounds are described as transient sounds, in order to distinguish them from murmurs. Murmurs can be defined as pronoises heard during a normally silent period of the cardiac cycle. Because the transient sounds are related to the electrical and mechanical events of the cardiac cycle, it is useful to identify them. They provide information on the timing of the cycle and the presence or absence of some of these events. Usually the mechanical events on the left and right sides of the heart occur almost simultaneously. Often the interval is so short that it could only be detected using phonocardiography, however, there are circumstances under which the interval is long enough for it to be detected on auscultation. If the interval between the events on the left and right sides is sufficient (>30 msec), the sound can be 'split' into two elements.
The intensity of the transient sounds varies in different animals. They are best heard in fit thin horses and in foals, but may be very quiet in fat, barrel-chested ponies. They are louder in an excited animal, or during tachycardia. Sounds may be muffled by fluid in the thoracic or pericardial cavities. Solid tissue such as tumours, abscesses or consolidated lung, which displace the heart or are present between the heart and the chest wall, may result in the sounds being reduced or increased in intensity, depending on the position of the heart valves in relation to the body surface.