Systemic and intestinal limits of O2 extraction in the dog

When systemic delivery of O2 (QO2 = QT X CaO2, where QT is cardiac output and CaO2 is arterial O2 content) is reduced by bleeding, the systemic O2 extraction ratio [ER = (CaO2 - CVO2)/CaO2, where CVO2 is venous O2 content] increases until a critical limit is reached below which O2 uptake (VO2) becomes limited by O2 delivery. During hypovolemia, reflex increases in mesenteric arterial tone may preferentially reduce gut blood flow so that the onset of O2 supply dependence occurs in the gut before other regions. We compared the critical O2 delivery (QO2c) and critical extraction ratio (ERc) of whole body and an isolated segment (30-50 g) of small bowel in seven anesthetized paralyzed dogs ventilated with room air. Systemic QO2 was reduced in stages by controlled hemorrhage as arterial O2 content was maintained, and systemic and gut VO2 and QO2 were measured at each stage. Body QO2c was 7.9 +/- 1.9 ml X kg-1 X min-1 (ERc = 0.69 +/- 0.12), whereas gut O2 supply dependency occurred when gut QO2 was 34.3 +/- 11.3 ml X min-1 X kg gut wt-1 (ERc = 0.63 +/- 0.09). O2 supply dependency in the gut occurred at a higher systemic QO2 (9.7 +/- 2.7) than whole-body QO2c (P less than 0.05). The extraction ratio at the final stage (maximal ER) was less in the gut (0.80 +/- 0.05) than whole body (0.87 +/- 0.06). Thus during reductions in systemic QO2, gut VO2 was maintained by increases in gut extraction of O2.(ABSTRACT TRUNCATED AT 250 WORDS)