Abstract
Background: During acellular replacement of an acute blood loss, hyperoxic ventilation (HV) increases the amount of O<sub>2</sub> physically dissolved in the plasma and thereby improves O<sub>2</sub> supply to the tissues. While this effect could be demonstrated for HV with inspiratory O<sub>2</sub> fraction (FiO<sub>2</sub>) 0.6, it was unclear whether HV with pure oxygen (FiO<sub>2</sub> 1.0) would have an additional effect on the physiological limit of acute normovolemic anemia. Methods: Seven anesthetized domestic pigs were ventilated with FiO<sub>2</sub> 1.0 and subjected to an isovolemic hemodilution protocol. Blood was drawn and replaced by a 6% hydroxyethyl starch (HES) solution (130/0.4) until a sudden decrease of total body O<sub>2</sub> consumption (VO<sub>2</sub>) indicated the onset of O<sub>2</sub> supply dependency (primary endpoint). The corresponding hemoglobin (Hb) concentration was defined as ‘critical Hb' (Hb<sub>crit</sub>). Secondary endpoints were parameters of myocardial function, central hemodynamics, O<sub>2</sub> transport and tissue oxygenation. Results: HV with FiO<sub>2</sub> 1.0 enabled a large blood-for-HES exchange (156 ± 28% of the circulating blood volume) until Hb<sub>crit</sub> was met at 1.3 ± 0.3 g/dl. After termination of the hemodilution protocol, the contribution of O<sub>2</sub> physically dissolved in the plasma to O<sub>2</sub> delivery and VO<sub>2</sub> had significantly increased from 11.7 ± 2 to 44.2 ± 9.7% and from 29.1 ± 4.2 to 66.2 ± 11.7%, respectively. However, at Hb<sub>crit</sub>, cardiovascular performance was found to have severely deteriorated. Conclusion: HV with FiO<sub>2</sub> 1.0 maintains O<sub>2</sub> supply to tissues during extensive blood-for-HES exchange. In acute situations, where profound anemia must be tolerated (e.g. bridging an acute blood loss until red blood cells become available for transfusion), O<sub>2</sub> physically dissolved in the plasma becomes an essential source of oxygen. However, compromised cardiovascular performance might require additional treatment.
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