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Long-term versus intrabeat history of ejection as determinants of canine ventricular end-systolic pressure
S Sugiura1, W C Hunter, K Sagawa
1Department of Biomedical Engineering, Johns Hopkins Medical School, Baltimore, Maryland 21205.
Insights
Ejection positively impacts ventricular end-systolic pressure, exceeding isovolumic pressure in isolated heart preparations. This effect, influenced by stroke volume, is linked to length-dependent activation during ejection beats.
Area of Science:
- Cardiovascular Physiology
- Cardiac Mechanics
- Heart Function Studies
Background:
- End-systolic pressure is a critical determinant of cardiac performance.
- The influence of ejection on end-systolic pressure is complex and not fully understood.
- Isolated heart preparations allow controlled investigation of cardiac mechanics.
Purpose of the Study:
- To investigate the effect of ejection on end-systolic pressure in isolated heart preparations.
- To compare end-systolic pressure during ejecting versus isovolumic beats.
- To elucidate the mechanisms underlying ejection's impact on end-systolic pressure.
Main Methods:
- Isolated heart preparations were utilized.
- Ejecting and isovolumic beats were compared under controlled end-systolic volumes.
- Variable stroke volumes, including negative stroke volumes, were imposed.
- Immediate and steady-state changes in end-systolic pressure were measured.
Main Results:
- Ejecting beats showed higher end-systolic pressure than isovolumic beats for small stroke volumes (up to 18 mm Hg).
- Ejecting end-systolic pressure was lower with large or negative stroke volumes.
- A gradual decrease in end-systolic pressure was observed during the transient phase from ejecting to isovolumic mode.
- Ejection exerted both immediate and gradual positive effects on ventricular end-systolic pressure.
Conclusions:
- Ejection has a significant positive effect on ventricular end-systolic pressure, beyond negative uncoupling.
- Length-dependent activation, due to larger volumes during ejection, is hypothesized as the underlying mechanism.
- These findings advance the understanding of cardiac mechanics and pressure regulation during ejection.
Abstract:
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.
