MECHANICAL DETERMINANTS OF CORONARY BLOOD-FLOW DURING DYNAMIC ALTERATIONS IN MYOCARDIAL-CONTRACTILITY

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 elasta nce (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 pulsus alt ernans (PA) where end-systolic LV pressure and instantaneous E(es) var y 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 co nditions. Maximal pressure development over time (dP/dt(max)) and SFI were significantly different between weak (WB) and strong beats (SB) a s 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 simultaneous ly in the WB. Q(min) occurred much earlier than peak E(es) in the stro ng and control beats. Plots of instantaneous LV elastance and CBF show ed that for control beats and for the strong beats of PA CBF was simil ar during systole and diastole, suggesting elastance is a unique deter minant of CBF. This was quantified as CBF at the time in either systol e or diastole when elastance was half-maximal for that beat (E50). Dur ing the WB of PA, however, CBF at E50 was significantly higher during systole than during diastole. We conclude that while SFI and DFD are h ighly dependent on the dP/dt and R(avg), E(es) is not a unique determi nant of CBF under all conditions.