Heterogeneous changes in epimyocardial microvascular size during graded coronary stenosis. Evidence of the microvascular site for autoregulation.

The purpose of this study was to determine the coronary microvascular sites of autoregulation. The epimyocardial coronary microcirculation was observed through an intravital microscope by stroboscopic epi-illumination in anesthetized open-chest dogs (n = 20). Aortic pressure and heart rate were held constant by an aortic snare and atrial pacing, respectively. Distal pressure of the left anterior descending coronary artery was controlled by a screw occluder on the proximal left anterior descending coronary artery and monitored with a 24-gauge plastic cannula inserted into the branch or distal portion of the left anterior descending coronary artery. Distal pressure of the left anterior descending coronary artery was stepwisely reduced to 59 +/- 1 mm Hg (mild stenosis, n = 20) and 38 +/- 1 mm Hg (severe stenosis, n = 16). In the left circumflex coronary artery area, myocardial blood flow measured with radioactive microspheres to subepicardium, midmyocardium, and subendocardium did not change with the mild an...