Peripheral circulatory responses to 96 hours of eucapnic hypoxia in conscious sheep

Abstract

Conscious sheep acclimatizing to hypoxia (Pa O 2 40 mm Hg, Pa CO 2 24 mm Hg) respond with increases in cardiac output (Q̇co) and cerebral blood flow lasting for 24 and 48 h, respectively. Coronary flow increases ini a sustained fashion, while there are progressive decrease in renal, splenic and pancreatic flows. In the present study, 5 adult ewes were exposed to similar levels of normobaric hypoxia (Pa O 2 40 mm Hg) but the Pa CO 2 was maintained at eucapnic levels (32 mm Hg). V̇E increased (+210%) while V̇ O 2 decreased by 35%. Ventilatory sensitivity to CO 2 was unchanged. Ȯco (thermodiluation) was elevated for 96 h (+20%) as stroke volume was maintained at normoxic levels and heart rate increased (+36%). Pulmonary artery pressure increased (+35%) along with plasma catecholamine levels (+116–196%). There were sustained elevations of cerebral flow (radiolabelled microspheres) from 79.1 (±9.2 SEM) to 121.6 ml·min −1·100 g −1 (±10.8), coronary flow from 183 (±22.1) to 373 ml·min −1·100 g −1 (±46.3), diaphragm flow (+400%) and intercostal muscle flow (+186%) with no apparent redistribution of Ċco. Therefore, the cardiac and peripheral circulatory response pattern are altered significantly in eucapnic hypoxia. The rate of O 2 delivery to brain and several abdominal viscera is higher.