Can coronary systolic-diastolic flow differences be predicted by left ventricular pressure or time-varying intramyocardial elastance?

In six isolated rabbit hearts perfused with a pressure source and Krebs-Henseleit as the perfusion medium, the effect of left ventricular pressure on coronary inflow in the maximally vasodilated bed was studied. This effect was determined from isovolumic beats, low afterloaded isobaric beats (afterload maintained at values below 10 mm Hg) and during cardiac arrest. For isovolumic beats end-diastolic left ventricular pressure was varied by means of an intraventricular balloon between 0-40 mm Hg and systolic left ventricular pressure varied between 90-130 mm Hg. In these ranges diastolic inflow decreased significantly 18 +/- 6% (mean +/- SD) with increasing pressure and systolic inflow could not be shown to depend on pressure (n = 6). For isobaric beats, diastolic and systolic inflow remained at values similar to those found for the isovolumic beats (n = 6). In the arrested heart inflow diminished 8 +/- 2% when the pressure in the left ventricle was increased from 0 to 40 mm Hg (n = 3). We conclude that systolic coronary inflow is hardly affected by left ventricular pressure. Systolic inflow decreased by the same amount in the isovolumically and isobarically beating heart, when cardiac contractility was enhanced by epinephrine infusion. We suggest the results can be explained on the basis of the time-varying elastance concept: systolic elastance is the same for isovolumic and isobaric beats but depends on contractility. Models that relate coronary inflow impediment to left ventricular pressure should therefore be reevaluated.