Canine coronary microvascular resistance at vasodilation depends on coronary perfusion pressure

Abstract

ObjectiveWe tested the premise of a constant microvascular resistance at full vasodilation (Rm) that underlies the concept of fractional flow reserve.MethodsIn 7 anesthetized dogs, we measured aortic (Pa) and venous (Pv) pressure, flow in the left anterior descending (LAD) artery, distal pressure (Pd) in the left circumflex (LCX) artery. During LCX occlusion, wedge pressure (Pw) was measured and μspheres infused into the LAD at full vasodilation. In one dog, the RCA contribution was determined by left atrial injection of differently colored μsph. Hearts were analyzed using a high‐resolution 3D imaging cryomicrotome. The mass of LCX and LAD perfusion territories was obtained. Assuming equal resistance per gram tissue for LAD and LCX, the weight‐normalized Rm of LCX was determined at Pd and Pw. Collateral flow was calculated as the ratio of μsph in LCX and LAD territories multiplied by LAD flow.ResultsAccounting for the added 30%RCA contribution to the collateral flow obtained from LAD injection, Rm was 2.3 ± 1.9 times higher at low (Pw= 15.7+4.8 mmHg) compared to normal pressure (Pd=69.3+12.4 mmHg) (p<0.05). Collateral flow from LAD to LCX was 8.6% + 5.3% of LAD hyperemic flow.ConclusionsOur data confirm the pressure‐dependence of coronary Rm at vasodilation, which suggests a reason for pressure‐based overestimation of collateral flow. Supported by the Netherlands Heart Foundation (2006B226)