As systemic delivery of O2 (Q̇O2 = Q̇T x Ca(O2)) is reduced during progressive hemorrhage, the O2 extraction ratio [(Ca(O2 - CV(O2))/Ca(O2)] increases until a critical delivery is reached below which O2 uptake (VO2) becomes limited by delivery (O2 supply dependence). When tissue metabolic activity and O2 demand are increased or reduced, the critical Q̇O2 required to maintain V̇O2 should rise or fall accordingly, unless other changes in the distribution of a limited Q̇O2 precipitate the onset of O2 supply dependence at a different critical extraction ratio. We compared the critical Q̇O2 and critical extraction ratio in 23 normothermic (38°C), hyperthermic (41°C), or hypothermic (34°C) dogs during stepwise reduction in delivery produced by bleeding, as arterial O2 content was maintained. Dogs were anesthetized, paralyzed, and mechanically ventilated. Hypothermia reduced whole-body V̇O2 by 31%, whereas hyperthermia increased V̇O2 by 20%. The critical Q̇O2 was significantly reduced during hypothermia (5.6 ± 0.95 ml·min-1·kg-1) (P < 0.05) and increased during hyperthermia (8.9 ± 1.1) (P ≃ 0.06) compared with normothermic controls (7.4 ± 1.2). The extraction ratio at the onset of supply dependency was significantly increased in hyperthermia (0.76 ± 0.05) compared with hypothermia (0.65 ± 0.10) (P < 0.05), and the normothermic critical extraction was 0.71 ± 0.1. These results suggest that higher body temperatures are associated with an improved ability to maintain a V̇O2 independent of Q̇O2, since a higher fraction of the delivered O2 can be extracted before the onset of O2 supply dependence, relative to lower body temperatures.
ASJC Scopus subject areas
- Physiology (medical)