Siecke, Helmut

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.

Measurements of the left atrial and ventricular pressure pulse in dogs with complete atrioventricular (AV) heart block revealed positive and negative differences in pressure across the mitral valve which were associated with high and low amplitudes of the first heart sound, respectively. Congestive heart failure, infusions and bleeding altered the duration of the negative difference in pressure and the amplitude of the first heart sound during atrial diastole. High and low amplitudes of the first heart sound are coincident with the opened and closed AV valves, respectively. The position of the valve leaflets at the onset of ventricular excitation seems to affect the rise of the ventricular pressure.

Eight dogs with complete atrioventricular heart block showed a definitive relation between the P-Q interval and the systolic left ventricular pressure if the P-Q interval increased or decreased slowly at a regular rate. The ventricular pressure reached a ‘plateau-like’ maximum at a P-Q interval between 0.1 and 0.2 second, and decreased only slightly with somewhat longer P-Q intervals. The lack of abrupt decrease in pressure suggests that the mitral valve closes passively during early atrial diastole and that this prevents rapid regurgitation of blood into the left atrium.