Effects of changes in afterload impedance on left ventricular ejection in isolated canine hearts: dissociation of end ejection from end systole

To examine how end systole differs from end ejection and also whether the slope of the end systolic pressure-volume relation can be approximated to that of the end ejection pressure-volume relation, nine isolated, perfused, paced canine hearts ejecting into a hydraulic loading system that simulated the aortic input impedance of a dog's arterial tree were studied. To measure left ventricular volume changes the heart was placed in a plethysmograph. Peripheral resistance (Rp) and arterial compliance (C) were independently varied from 1.9 (Rp=1.9) to 3.3, 6.4, and 9.6 × 108 Pa·m−3·s (Rp run) with a constant value of compliance 1.3 × 10−9 Pa−1·m3 (C=1.3), and from C=0.4 to C=0.8, C=1.3 and C=2.3 (C run) with a constant value of resistance (Rp=6.4). Five pressure-volume loops were obtained by changing the end diastolic volume at each value of compliance and peripheral resistance. It was clearly shown that ventricular ejection continued after end systole and the time duration between end systole and end ejection became longer with increasing arterial compliance (24(4) at C=0.4 vs 49(4) ms at C=2.3, p<0.001), while the time duration between end diastole and end systole was constant regardless of afterload impedance change. Regarding the left ventricular pressure-volume relation the end systolic relation was almost linear (r≥0.98) and the slope was not significantly affected by change in any afterload impedance tested. End ejection pressure-volume relation was also linear (r≥0.97) and the slopes in the peripheral resistance and compliance runs were lower than those of the end systolic pressure-volume relation in each corresponding run. The former slopes decreased at smaller values of Rp or larger values of C — namely, 4.4(0.6) at Rp=9.6 vs 3.6(0.6) at Rp=1.9, p<0.05; 4.8(0.6) at C=0.4 vs 3.1(0.5) mm·Hg·ml−1 at C=2.3, p<0.001. Thus it is concluded that end ejection is usually different from end systole and the time difference between them is affected by changes in arterial compliance. In addition, the slope of end ejection pressure-volume relation was dependent on the changes in afterload impedance and cannot be approximated to that of the end systolic pressure-volume relation.