Preload-induced alterations in capacitance-free diastolic pressure-flow relationships

We have studied the influence of left ventricular diastolic pressure (LVDP) on diastolic coronary pressure-flow relationships independently of capacitance flow in an open-chest heart-blocked canine preparation in which the left circumflex (LC) bed was vasodilated with adenosine and perfused with a programmable servo valve pressure source. At the onset of long diastoles produced by cessation of ventricular pacing, LVDP was adjusted to, and maintained at, a preselected level using a blood-filled reservoir. Right atrial pressure was kept constant at ~8 mmHg. LC pressure (P(LC)) was then made to decline and rise sequentially at a constant rate (2-40 mmHg/s), with LC inflow reaching zero at the nadir of the declining pressure ramp. The capacitance-free diastolic pressure-flow relationship was considered to lie midway between the instantaneous relationships derived from each down and up ramp pair. All capacitance-free relationships were curvilinear, and the degree of curvilinearity was accentuated with increasing preload. Pressure-axis intercepts (P(f=0)) increased from 14 ± 1.1 (SE) to 23 ± 1.4 mmHg as preload was raised from 6-10 to 31-35 mmHg. Coronary conductance, taken as the slope of the pressure-flow relationship at any given (P(LC), fell progressively as preload rose, with the fall being more marked at higher levels of preload and lower values of P(LC). Diastolic coronary flow also decreased as a function of preload, reflecting the increases in P(f=0) and decreases in conductance. We conclude that LVDP can importantly influence diastolic coronary perfusion by effects on both P(f=0) and conductance.