Changes in coronary pressure-flow relation after transition from blood to Tyrode perfusion

In six isolated, diastolic-arrested, maximally vasodilated cat hearts, we studied changes in coronary pressure-flow relations (zero-flow pressure intercept, resistance) during the first 25 min, after a change of perfusate, from blood to Tyrode. The apparent intercept (zero-flow) pressure changed from 2.0 +/- 0.94 (+/- SD) kPa during blood perfusion to 2.5 +/- 0.55, 2.6 +/- 0.68, 2.5 +/- 0.94, and 2.7 +/- 1.34 kPa during Tyrode perfusion for 2:15, 5:30, 10:30, and 25:00 min, respectively. Intercept pressures during Tyrode perfusion were significantly different from the intercept pressure during blood perfusion, except for the one measured after 25 min of Tyrode perfusion (P less than 0.05). Resistance (defined as the ratio of perfusion pressure and flow at 10 kPa perfusion pressure) steadily rose to approximately 170% of the value during blood perfusion. The observation that the apparent intercept pressure is maintained, when a particle-free (Newtonian) isotonic perfusate is used, may indicate that this intercept is not a result of blood rheology alone. The increase rather than decrease in resistance suggests an effect of edema, which increases interstitial volume at the expense of intravascular volume.