Relation Between Amplitude of First Heart Sound and Duration of P-Q Interval in Dogs With Atrioventricular Block

Curves that relate the amplitude of the first heart sound to the P-Q interval have been obtained in 13 dogs with complete atrioventricular (AV) heart block. The amplitude was highest at a P-Q interval of approximately 0.05 second, became lowest at a P-Q interval of 0.13 second and continued low for intervals up to 0.25 second or longer and then increased again to some extent with further prolongation of the P-Q interval. Cardiac failure or infusions into peripheral or pulmonary veins caused an earlier and higher rise of the amplitude with very long P-Q intervals, whereas exsanguination had the opposite effect. These factors probably alter reopening of the AV valves during atrial diastole and thereby change the intensity of the first heart sound.