Effect of Altering P-R Interval on the Amplitude of the First Heart Sound in the Anesthetized Dog

ABSTRACTTo determine the effect of changing the P-R interval on the intensity of the mitral component of the first heart sound (M1 amplitude), fixed-rate sequential atrioventricular pacing was used to vary the P-R interval in a steady hemodynamic state in six anesthetized dogs. Left ventricular sound and pressure events were monitored with a catheter-tip micromanometer, and the rate of rise of left ventricular pressure (dP/dt) was recorded using an R/C differentiator. The P-R interval was varied from the longest interval at which continuous ventricular capture was achieved (indifferent P-R interval) to 0.00 msec in 10-msec increments. The mean increase in M1 amplitude at the short P-R intervals (30 to 50 msec) was 119% compared to the amplitude at the indifferent P-R interval; there was no significant change in maximum dP/dt. No significant changes in left ventricular ejection time and left ventricular peak pressure occurred over the range of P-R intervals. At the indifferent P-R interval, a norepinephrine infusion increased M1 amplitude 22% and maximum dP/dt 50%. At short P-R intervals in the new inotropic state, the M1 amplitude was further augmented.These data indicate that both the timing of atrial and ventricular systoles and the force of left ventricular contraction are major, independent determinants of M1 amplitude. These findings are consistent with the concept that the abrupt deceleration of blood by the mitral valve shortly after the onset of ventricular systole sets the cardiohemic system into vibration producing the M1 sound. The intensity of the first heart sound is proportional to the force produced in this manner.