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The first heart sound (S1) occurs at the onset of ventricular systole. There are two components of the first heart sound which reflect the vibrations that occur in the valve cusps, chordae, papillary muscles, and ventricular walls when the mitral and tricuspid valves close. In small animals, it is very difficult to auscult the physiologic splitting of S1. The ear can only differentiate two distinct sounds if the distinct sounds are approximately 30 msec apart. Due to the rapid heart rate in dogs and cats, it is rare to detect the splitting of S1. The second heart sound (S2) occurs from the vibrations of the cardiac hemodynamic structures (mentioned above) as the aortic and pulmonic valves close. The third heart sound (S3) is generated by rapid ventricular filling and is of low frequency. It occurs 15-20 msec after S2. It is not ausculted in human. If heard it is often associated with decompensated heart failure. (dilated cardiomyopathy, severe chronic valve disease) The fourth heart sound (S4) is produced from the vibrations of the cardiac structures that occur with atrial systole Not heard in normal small animals and if ausculted is usually associated with a stiff or hypertrophied ventricle (hypertrophic cardiomyopathy)
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