Reductions in cardiac output in hypoxic young pigs: systemic and regional perfusion and oxygen metabolism.

We tested the hypotheses that, in hypoxic young pigs, reductions in cardiac output restrict systemic oxygen transport to a greater extent than does hypoxia alone and that compensatory responses to this restriction are more effective in higher than in lower priority vasculatures. To study this, 10- to 14-day-old instrumented awake hypoxic (arterial oxygen tension = 39 Torr) pigs were exposed to reduced venous return by inflation of a right atrial balloon-tipped catheter. Blood flow was measured with radionuclide-labeled microspheres, and oxygen metabolism was determined with arterial and venous oxygen contents from appropriate vessels. Hypoxia resulted in a reduction in oxygen tension; increases in cardiac output and perfusion to brain (72% over baseline), heart, adrenal glands, and liver without reductions to other organs except for the spleen; reductions in systemic and intestinal oxygen delivery; and increases in systemic and intestinal oxygen extraction without changes in systemic, cerebral, or intestinal oxygen uptake. During hypoxia, decreasing venous return was associated with increases in arterial lactic acid concentration and central venous pressure; attenuation of the hypoxia-related increase in cardiac output; sustained increases in brain (72% over baseline) and heart perfusion; reductions in lung (bronchial artery), pancreatic, renal, splenic, and intestinal (-50% below baseline) perfusion; decreases in systemic and gastrointestinal oxygen delivery; sustained increases in systemic and intestinal oxygen extraction; and decreases in intestinal oxygen uptake, without changes in cerebral oxygen metabolism. We conclude that when venous return to the heart is reduced in hypoxic young pigs, the hypoxia-related increase in cardiac output was attenuated and the relative reduction in cardiac output was associated with preserved cerebral oxygen uptake and compromised intestinal oxygen uptake. Regional responses to hypoxia combined with relative reductions in cardiac output differ from that of hypoxia alone, with the greatest effects on lower priority organs such as the gastrointestinal tract.