Cardiovascular adjustments to acute hypoxemia superimposed on chronic hypoxemia in lambs.

Cardiovascular responses to acute hypoxemia are in part mediated through adrenergic and chemoreceptor stimulation. In chronic hypoxemia the response to these stimuli may be blunted. Therefore, we determined whether the cardiovascular responses to acute hypoxemia superimposed on 3-4 wk of chronic hypoxemia were blunted in lambs with an experimental cardiac right-to-left shunt (combination of atrial septal defect and variable pulmonary stenosis). Cardiovascular variables and regional blood flows were determined during chronic hypoxemia and after acutely reducing the arterial oxygen saturation by increasing the cardiac right-to-left shunt. Arterial oxygen saturation decreased (65 +/- 7 to 40 +/- 7%, P < 0.001) and systemic blood flow increased (164 +/- 63 to 233 +/- 100 ml.min-1.kg-1, P < 0.01), maintaining systemic oxygen supply and oxygen uptake. Blood flow to the myocardium (P < 0.01), the adrenals (P < 0.05), and the brain (0.05 < P < 0.10) increased, and oxygen supply to these organs was maintained. Conversely, blood flow to the kidneys and the gastrointestinal tract was unaltered, so that oxygen supply to these organs was decreased. The responses to acute hypoxemia in chronically hypoxemic lambs were similar to those previously reported in normoxemic lambs. We conclude that the cardiovascular responses to acute hypoxemia in chronically hypoxemic lambs are not blunted.