Effects of nitrous oxide on coronary perfusion after coronary air embolism

Coronary air embolism (CAE) can occur after heart surgery whenever air is present in the left heart or proximal aorta. When CAE occurs, its sequelae can range from electrocardiographie changes of ischmeia to severe myocardial dysfunction and cardiac arrest. Since N₂O has been shown to have detrimental effects in the presence of coronary obstructions, as well as the tendency to enlarge air emboli, the authros test the hypotheses that N₂O would enhance the deleterious effects fo CAE, and that discontinuing N₂O at the time of CAE would minimize those effects. The effects of ventilation with and without N₂O on the cardiac insult due to left anterior descending CAE (0.02 ml · kg^-1) were studied in 27 swine. Global cardiovascular changes that occurred after CAE included decreases in cardiac output, systemic arterial and coronary perfusion pressure, and LV dP/dt, as well as increases in LVEDP. These parameters returned towards baseline over time when N₂O was discontinued at the time of CAE. Maintenance of N₂O in the inspired gas after CAE occurred was uniformly fatal within 2-4 min in this model. Regional myocardial ischemia was significantly greater in animals receiving N₂O, as documented by : 1) a greater incidence of elevations of epicardial ST-segments exceeding 3 mm from baseline in embolized and non-embolized coronary artery distributions, 2) a greater incidence of dysrhythmias (greater than 6 PVCs · min^-1), 3) longer duration of depression of coronary blood flow, 4) longer duration of post-ischemic coronary hyperemia, and 5) larger decreases with less recovery over time of regional myocardial lactate extraction. The authors conclude that N₂O should not be used following cardiopulmonary bypass, since even discontinuing it at the time of CAE is not sufficient to eliminate its deleterious effects.