Haemodynamic explanation of why the murmur of mitral regurgitation in independent of cycle length.

In each of 9 patients with mitral regurgitation and one with a ventricular septal defect, a contraction following a normal or short RR interval (control beat) was compared with the contraction following a prenature ventricular beat or a long RR interval (long cycle beat). At end-diastole long cycle volumes exceeded control volumes (I90 ml vs. I64 ml P< O ooi). By io6 msec after end-diastole the control and long cycle volumes were virtually identical. In a dog with mitral regurgitation forward flow was augmented throughout systole after a premature ventricular contraction, but between the time of volume equalization and end-systole, regurgitant flow was unchanged among long and control cycles. These results suggest that, after a premature ventricular beat, reduced impedance to left ventricular ejection and augmented left ventricular contraction favour forward flow, and lead to volume equalization early during ejection when control and long cycles are compared. This mechanism explains the constancy of the murmur of mitral regurgitation in the beat after a premature ventricular contraction.