Effect of pacing on oxygen supply-to-consumption ratio in ischemic myocardium

This study was performed to determine if the partially occluded myocardium could maintain its O₂ supply-to-consumption ratio during atrial pacing. In 14 open-chest anesthetized dogs, the left anterior descending coronary artery was occluded to 50% of base-line flow, and in half of the dogs the hearts were paced to 50% above base-line heart rate. Blood flows were determined by use of microspheres before and after occlusion, and O₂ supply-consumption variables were determined by use of microspectrophotometry. After occlusion in control and paced groups, blood flow was depressed in the ischemic region compared with the nonischemic region. In paced animals, blood flows in all regions were significantly higher compared with their respective values in nonpaced animals. After pacing, mean O₂ consumption was 34% higher (53% in the occluded region) compared with control animals. The O₂ supply-to-consumpton ratio was similar in the occluded region of paced animals compared with the same region in controls. This ratio was lower in ischemic regions of both groups compared with their respective nonischemic regions. This indicates that, despite a reduced O₂ supply-to-consumption ratio in the ischemic area, an unutilized reserve of flow exists.