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 PO2, perfusion and small vessel blood content were studied in anesthetized, thoracotomized New Zealand white rabbits. Relative tissue PO2 and perfusion were determined polarographically. 59FeCl3 was used to determine small vessel blood content. In control, relative tissue PO2, 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 PO2 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.