Transmural myocardial flow distribution during hypothermia: Effects of coronary inflow restriction

Abstract

Hypothermic coronary perfusion and blood cardioplegia have been used clinically to minimize intraoperative myocardial damage. However, pressure-flow characteristics in regions supplied by inflow-limiting collateral coronary arteries have not been investigated during hypothermic conditions. In this study tracer microspheres determined transmural myocardial blood flow distribution during cardiopulmonary bypass in normothermic empty, beating dog hearts (EBH), during hypothermic sanguineous perfusion at 15 degrees C (HP), and after hemodilution of cooled (15 degrees C) hearts to a hematocrit value of 20 vol% (HDL). Animals in Group I (N = 8) had normal hearts. Group II dogs (N = 9) had one region supplied predominantly by narrow collateral vessels (CR) and another nourished by normal coronary arteries (NR). Retrograde circumflex pressures were measured continuously for Group II as an additional index of CR perfusion. Flow characteristics in Group I hearts were always similar to the NR of Group II dogs. With HP, endocardial blood flow in the NR decreased from approximately 0.80 to 0.50 ml/min/gm. Subsequently, following HDL this flow increased to approximately 1.70 ml/min/gm, or over twice control levels. In comparison, flow to CR endocardium decreased even more during HP (0.12 ml/min/gm). Even though control flow levels were reestablished in CR endocardium by adding HDL, an unfavorable endocardial/epicardial ratio persisted. With both HP and HDL, retrograde circumflex pressure never changed from EBH values. These data suggest that a significant endocardial flow defect exists during periods of hypothermic sanguineous perfusion and may become more prevalent in regions subserved by inflow-limiting coronary vessels. Similar flow maldistributions may occur in patients if blood-containing cardioplegic solutions are used and during systemic hypothermia. Significant hemodilution helps minimize these imbalances and permits salutary effects of hypothermia to be delivered more evenly across the ventricular wall.