Myocardial blood flow and function during gradual coronary occlusion in awake dog

The purpose of this study was to correlate dimensions and transmural blood flow in a segment of myocardium supplied by a coronary artery undergoing gradual closure. Nine adult dogs were instrumented with an electromagnetic flow probe, pneumatic occluder, and an ameroid constrictor on the circumflex coronary artery. Ultrasonic crystals were implanted 10 mm from the external surface of the left ventricle in a segment perfused by the circumflex artery. Regional blood flow was determined with 7- to 10-microns radiolabeled microspheres. Data were collected in the awake state at rest before closure began (control), during partial closure, and immediately after total closure. Seven of the nine animals were studied after occlusion during treadmill exercise. During both partial and total closure at rest the rate and extent of systolic shortening as well as the transmural blood flow were unchanged from control. During treadmill exercise mean flow increased. However, flow was redistributed away from the inner two layers causing deterioration in both the rate and extent of shortening of this segment. These data suggest that, although regional myocardial function and flow can be maintained at rest by the immature canine collateral circulation, these parameters are impaired markedly during augmented flow with exercise.