Nitrous oxide constricts epicardial coronary arteries without effect on coronary arterioles

The authors sought to determine the effects of nitrous oxide on both epicardial coronary artery dimensions and intramyocardial coronary arteriolar tone. Nine dogs were anesthetized with fentanyl-pentobarbital-oxygen. High resolution angiograms of the left coronary system were obtained, and cross-sectional areas of the proximal, mid, and distal left anterior descending and proximal circumflex coronary arteries were quantitated using a computerized analysis system. Measurements were made at three coronary perfusion pressures before nitrous oxide administration, and then repeated following the addition of both 30% and 60% nitrous oxide. At the same time, coronary arteriolar tone was assessed by measuring coronary blood flow using 133Xenon washout. Sixty percent nitrous oxide was accompanied by constriction of the epicardial coronary arteries. Thirty percent nitrous oxide had a less marked effect. At 70 mmHg coronary perfusion pressure, following 60% nitrous oxide, mid left anterior descending cross-sectional area decreased from 4.6 +/- 0.3 mm2 (mean +/- SD) to 3.5 +/- 0.3 mm2. At 90 mmHg, area decreased from 4.57 +/- 0.3 mm2 to 3.69 +/- 0.4 mm2, and, at 110 mmHg, from 4.7 +/- 0.4 mm2 to 3.9 +/- 0.4 mm2 (P less than 0.01). Nitrous oxide had no effect on the relationship between coronary blood flow and myocardial oxygen consumption, indicating an absence of effect on coronary arteriolar tone. It is concluded that intramyocardial coronary arterioles and epicardial coronary arteries are dissimilar in their response to nitrous oxide. In the intact, anesthetized, normal dog, nitrous oxide does not affect coronary arteriolar tone. Sixty percent nitrous oxide produces constriction of epicardial coronary arteries.