Phonocardiography in the Horse: 2. The Relationship of the External Phonocardiogram to Intracardiac Pressure and Sound

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Abstract
The effects of individual cardiac cycles on the changing pressures and sounds recorded from the various chambers of the heart and aorta were related to phonocardiograms from an external microphone hand-held over the cardiac area. The majority of the 1st sound appears to arise from the left side of the heart after the AV [atrioventricular] valves have closed and the 2nd sound is a result of sudden deceleration of blood against already closed semilunar valves. Systolic murmurs at the aortic root may be benign from increased flow and turbulence at the peak of ejection but these may not be recorded externally.