Mechanical determinants of coronary blood flow during dynamic alterations in myocardial contractility
Mulligan, L.J.; Escobedo, D.; Freeman, G.L.
American Journal of Physiology 265(4 Pt 2): H1112-H1118
1993
ISSN/ISBN: 0002-9513 PMID: 8238397 Document Number: 419087
Recently it has been proposed that the decrease in coronary blood flow (CBF) resulting from cardiac contraction referred to as systolic flow impediment (SFI) is dependent on the level of left ventricular elastance (E-es). The average rate of LV relaxation (R-avg) has been shown to be major determinant of diastolic flow development (DFD). We tested these hypotheses using the unique hemodynamic condition of pulses alternans (PA) where end-systolic LV pressure and instantaneous E-es vary on beat-to-beat basis. In six mongrel dogs instrumented with LV and aortic manometers, ultrasonic dimension crystals, and Doppler coronary flow probes we measured phasic CBF and E-es during PA and control conditions. Maximal pressure development over time (dP/dt-max) and SFI were significantly different between weak (WB) and strong beats (SB) as were R-avg and DFD. Minimum CBF (Q-min) was not different between SB and WB; however, Q-min and peak E-es occurred nearly simultaneously in the WB. Q-min occurred much earlier than peak E-es in the strong and control beats. Plots of instantaneous LV elastance and CBF showed that for control beats and for the strong beats of PA CBF was similar during systole and diastole, suggesting elastance is a unique determinant of CBF. This was quantified as CBF at the time in either systole or diastole when elastance was half-maximal for that beat (E-50). During the WB of PA, however, CBF at E-50 was significantly higher during systole than during diastole. We conclude that while SFI and DFD are highly dependent on the dP/dt and R-avg, E-es is not a unique determinant of CBF under all conditions.