Magnitude of dilatation of large and small coronary arteries of nitroglycerin.

Vasodilatory responses of segments of large epicardial left coronary artery (CA), small intramyocardial CAs (0.3-1.0 mm), coronary stenoses and CAs filled by collaterals were determined in 34 patients. Measurements were made before and after nitroglycerin (0.4 mg, sublingual) by means of quantitative magnification coronary angiography using photospot film and a calibrated 6-power viewing device. The left main CA, proximal, middle and distal anterior descending and circumflex segments, and small CAs showed dilatation that varied in magnitude. When magnitude of dilatation was compared with control diameter of the vessel and its location, control diameter proved to be the significant independent variable. CAs with the smallest control diameter showed the greatest magnitude of vasodilatation. CA branches filled by collaterals had vasodilatation similar in magnitude to that of CAs of comparable control diameter. Although coronary stenoses dilated, the magnitude of dilatation was less than that observed in nonstenosed arterial segments of similar control diameter. When areas of stenosis were excluded, however, results were similar regardless of whether the patient had CA disease. These data indicate that a principal determinant of the CA vasodilatory response to nitroglycerin is the size of the artery before nitroglycerin.