Ejecting beats exerted positive effects on ventricular end-systolic pressure, exceeding isovolumic pressure by up to 18 mm Hg with small stroke volumes.
Does ejection affect end-systolic pressure compared to isovolumic contraction in isolated canine hearts?
Ejection exerts both positive and negative effects on ventricular end-systolic pressure, likely due to length-dependent activation.
We studied the effect of ejection on end-systolic pressure in isolated heart preparations. Ejecting beats were compared with isovolumic beats having the same volume as at end systole. While holding end-systolic volume constant, various stroke volumes, including negative stroke volumes (volume injected during systole), were imposed using a predetermined volume command. After switching contraction mode between ejecting and isovolumic, we measured the immediate and steady changes in end-systolic pressure. In the first isovolumic beat after switching from steady-state ejecting beats, the change in end-systolic pressure was variable, depending on the stroke volume. The end-systolic pressure of the ejecting beat exceeded that of the isovolumic beat on average by up to 18 mm Hg with small stroke volume, but the ejecting end-systolic pressure became lower than isovolumic with either large stroke volume (stroke volume/end-systolic volume less than 0.96) or with negative stroke volume. During the transient phase following a switch from ejecting to isovolumic, the end-systolic pressure gradually decreased to a steady state. Consequently, even in steady state, ejecting end-systolic pressure exceeded isovolumic pressure over a significant range of stroke volume (stroke volume/end-systolic volume less than 1.18). After returning contraction mode from isovolumic back to ejecting, we observed responses that were a mirror image. These results indicated that in addition to negative uncoupling effect, ejection exerts positive effects on ventricular end-systolic pressure that are manifest both quickly and gradually. We hypothesized that the mechanism responsible for the positive effect is length-dependent activation via the larger volume (both at the initiation of contraction and averaged over a cardiac cycle) of a beat that ejects compared to one held isovolumic at end-systolic volume. The results with volume injection were consonant with this concept.
Sugiura et al. (1989) studied Isolated canine heart preparations. Ejecting beats vs. Isovolumic beats having the same volume as at end systole was evaluated on End-systolic pressure. Ejecting beats exerted positive effects on ventricular end-systolic pressure, exceeding isovolumic pressure by up to 18 mm Hg with small stroke volumes.