Systemic hemorrhage augments local O2 extraction in canine intestine

When O2 delivery (QO2) to a tissue is reduced, microvascular adjustments facilitate increases in O2 extraction, thereby delaying the onset of O2 supply-limited metabolism until a critically low QO2 is reached. The present study investigated the contribution of the autonomic nervous system to these adjustments by measuring O2 extraction in isolated intestine. In anesthetized dogs, a 30- to 50-g segment of intestine was vascularly isolated and its QO2 was decreased in stages by reducing the speed of an occlusive pump. In a normovolemic group (n = 11), blood volume was maintained to minimize sympathetic tone while flow to the intestine was reduced. In a hypovolemic group (n = 7), blood volume was removed in stages to augment sympathetic tone as flow to the intestinal segment was simultaneously reduced. A hypovolemic alpha-adrenergic-blocked (alpha-blocked) group (n = 6) was identical to the corresponding alpha-adrenergic intact (alpha-intact) group except that alpha-adrenergic effects were inhibited with phenoxybenzamine (3 mg/kg). The systemic critical O2 extraction ratio in the alpha-blocked group (69 +/- 6%) was less than that in the alpha-intact group (77 +/- 7%; P = 0.05). In the intestine, the critical O2 extraction ratio was significantly poorer in the normovolemic (45 +/- 11%) group than in either hypovolemic group (alpha-intact: 69 +/- 3%, P < 0.00005; alpha-blocked: 62 +/- 9%, P < 0.005). These findings demonstrate that systemic hemorrhage significantly augments critical O2 extraction in intestine during progressive local stagnant hypoxia and suggest that nonadrenergic vasoconstrictor mechanisms may play an important role.