EFFECTS OF ISOFLURANE ON REGIONAL CORONARY BLOOD-FLOW AND MYOCARDIAL TISSUE PRESSURE IN CHRONICALLY INSTRUMENTED DOGS

Background: The effects of isoflurane on distribution of transmural bl ood flow and transmural intramyocardial tissue pressure (IMP) were stu died in chronically instrumented dogs, to address following aims: (1) to evaluate the direct effects of isoflurane on transmural blood flow distribution in the absence of compounding effects of baseline anesthe tics, acute surgery, and indirect effects caused by changes in systemi c blood pressures and heart rate-factors that were not well controlled in the past studies; (2) to examine the relation between transmural m yocardial perfusion pressure and concurrent changes in transmural bloo d flow distribution during isoflurane anesthesia; and (3) to evaluate the effects of isoflurane on transmural myocardial oxygen supply-deman d relation. Methods: Dogs were allowed to recover at least 1 week afte r surgery for instrumentation. Blood flow of the left anterior descend ing coronary artery and subendocardial and subepicardial blood flows, regional IMPs, regional segmental dimension, heart rate, aortic pressu re and left ventricular pressure were measured while dogs were awake a nd during 1.3% isoflurane anesthesia, with and without correction of h eart rate and aortic pressure. Concurrently regional myocardial perfus ion pressure, regional myocardial stroke work, and systolic pressure t ime index were calculated, based on direct measurements of IMP in sube ndocardium and subepicardium. Results: Without correction of aortic pr essure, neither left anterior descending coronary artery flow nor tran smural blood flow distribution was altered with isoflurane. When aorti c pressure and heart rate were corrected to the awake values, left ant erior descending coronary artery now increased (37 +/- 2%) and the inc rease was preferentially distributed to subendocardium, resulting in a shift in transmural blood now. The subendocardial/subepicardial blood flow ratio increased from 1.2 +/- 0.3 to 1.4 +/- 0.4 (p, 0.05). The t ransmural blood flow changes were closely related to changes in region al myocardial perfusion pressure ratio between subendocardium and sube picardium (r = 0.76, P < 0.001). Concurrent with marked increases in b lood flow (55 +/- 4% increase), regional myocardial stroke work and sy stolic pressure time index of subendocardium were decreased more than 50% with isoflurane, resulting in a favorable subendocardial oxygen su pply-demand balance. Conclusions: Isoflurane is a coronary vasodilator and redistributes blood now in favor of subendocardium and depresses subendocardial work when heart rate and aortic pressure are controlled . These changes in regional myocardial blood flow, regional myocardial stroke work, and systolic pressure time index appear to be a result o f changes in regional IMP.