The effectiveness of 02 inhalation on the acutely-induced ischemic myocardium in dogs was investigated. In 22 open-chest mongrel dogs, the left anterior descending coronary artery was partially occluded to reduce coronary flow. The regional coronary vein accompanying the artery was cannulated to obtain coronary venous blood. Switching of inspiratory gas from room air to 40% 02 produced an elevation of coronary venous 02 saturation from 35.8 + 12.7 (s.D.) to 41.1 + 11.9% and shifting of

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... l lactate production to utilization (from -0.9+36.9 to 5.0+36.7%), indicating that 40% 02 inhalation ameliorated ischemia. Application of 100% 02 inhalation caused even more beneficial effects ; coronary venous 02 saturation was elevated to 50.6±12.6% and myocardial lactate extraction was improved to 7.8+ 40.5%. The present study indicated that 40% 0 inhalation was effective and 100% 02 inhalation even more effective in ameliorating acutely-induced myocardial ischemia. Decreases in myocardial contractile force and left ventricular size and suppresion of sympathoadrenal activity might be possible mechanisms for these beneficial effects. 02 inhalation therapy ; myocardial infarction ; angina pectoris ; coronary circulation