Effects of carbon monoxide or low oxygen gas mixture inhalation on regional oxygenation, blood flow, and small vessel blood content of the rabbit heart

The effects of lowering arterial O₂ content, approximately 30%, by inspiration of low O₂ or CO gas mixtures on regional myocardial relative tissue\(P_{O_2 } \), perfusion and small vessel blood content were studied in anesthetized, thoracotomized New Zealand white rabbits. Relative tissue\(P_{O_2 } \) and perfusion were determined polarographically.⁵⁹FeCl₃ was used to determine small vessel blood content. In control, relative tissue\(P_{O_2 } \), perfusion and small vessel blood content averaged 33.1 mm Hg, 64.9 ml/min/100 g and 4.3 ml/100 g respectively in the subepicardium (EPI) and 22.7, 53.6 and 4.2 in the subendocardium (ENDO) of the left ventricle. Both hypoxic conditions increased regional blood flow, but to a lesser extent in the ENDO. Relative ENDO tissue\(P_{O_2 } \) fell more markedly than EPI in both conditions. Small vessel blood content increased more with CO than low O₂. Regional O₂ consumption, calculated by Krogh analysis, increased under both conditions. The response to lowered O₂ content is thus an increase in flow, metabolic rate and the number of open capillaries with a lowered driving pressure for O₂. The effects of these types of hypoxia appear more severe in the ENDO.