Relationship between perfusion pressure and myocardial microcirculation in the beating empty or spontaneously fibrillating heart

The influence of graded perfusion pressure (30, 60, and 90 mmHg) at cardiopulmonary bypass were studied on beating empty hearts (BEH) or spontaneously fibrillating hearts (SFH) in the normothermic state. The adequacy and distribution of coronary flow and the myocardial oxygen consumption (MVO₂) were examined using the tracer microsphere technique in twelve mongrel dogs. In the SFH, the left ventricular (LV) endocardium (ENDO)/epicardium (EPI) flow ratio indicated significant decrease at 30 and 60 mmHg (0.83±0.05 and 0.86±0.06, p<0.005, respectively), but was recovered to control value at 90 mmHg (1.01±0.13). In the BEH, these low perfusion pressures did not result in an abnormal flow distribution in the LV (1.03±0.03 at 30 mmHg). The flow distribution to the right ventricle (RV) relatively increased in both the BEH and the SFH (p<0.001). The ENDO/EPI ratio of the RV did not decrease at 30 mmHg in both groups (1.11±0.03 in BEH and 1.16±0.08 in SFH). Coronary blood flow and MVO₂ were significantly higher in the SFH than in the BEH. Coronary blood flow increased significantly with increase in the perfusion pressure, in both groups. The MVO₂ was constant in the BEH, regardless of the perfusion pressure, but increased in the SFH at increasing pressure. These results show that in the SFH, subendocardial underperfusion of the LV is induced at the perfusion pressure of 30 and 60 mmHg.