Cardiac sounds
Cardiac sounds are the distinct sounds produced during systole (S1) and diastole (S2) of the cardiac cycle, which can be recorded by the phonocardiogram.
The first heart sound “lub”, is systolic sound indicates the firm closure of AV valves, occur during ventricular systole. It is associated with the vibration of the valves (valvular factor) and also by the vibrations of the chordae tendinae and contracting ventricular muscle fibres (muscle factors). This sound is louder, low pitched but longer duration.
The second heart sound “dub” is the diastolic sound developed during the closure of the aortic and pulmonary valves at protodiastole phase of the cardiac cycle.
In addition to the above two normal sounds, occasional sounds like third heart sound (S3) is produced early in diastole at the end of rapid ventricular filling due to sudden tension of chordae tendinae. The S4 sound also referred as atrial sound, is due to the contraction of the atrial muscles which causes tension of the A.V. valves. These two sounds are very common in horses but pathological in dogs and cats. Usually in other animals the S3 sound indicates congestive heart failure whereas S4 indicates some degree of A.V. bundle block.
Cardiac Murmurs
Prolonged sounds of the heart occurring during normally silent intervals of cardiac cycle are referred as murmurs or adventitious heart sounds, which is due to valvular insufficiency or stenosis (narrowing) of the orifices. There are two types of cardiac murmurs.
Systolic murmurs
Systolic murmurs occurs due to improper or defective closure of the mitral or tricuspid valves or due to stenosis of pulmonary or aortic valves.
Diastolic murmurs
Diastolic murmurs is caused by insufficiency of pulmonary or aortic valves or by stenosis of AV valves.
Endocarditis and erysepalus infection (in hogs) cause valvular insufficiency, which is very similar to that of rheumatic fever in man.