Why hyperoxia matters during acute anemia.

Background: Given the low physical solubility of oxygen (O2) in plasma, little value is attached to hyperoxic ventilation (FiO2 1.0) as a modality for improving O2 transport and tissue oxygen supply when hypoxemia (i.e., O2 partial pressure (paO2) <60 mmHg) is absent. Because recent experimental and clinical data conflict with this notion, we used mathematical modeling to reevaluate efficacy of hyperoxic ventilation in improving tissue oxygenation in the absence of hypoxemia by specifying its theoretical efficacy in terms of hemoglobin (Hb) equivalents.

Methods: A mathematical approach was used based on the assumption that efficacy of hyperoxic ventilation depends on the additional amount of O2 dissolved in plasma and is influenced by the high biological availability of the additional O2 provided at high paO2. This approach was used to calculate the amount of additional Hb necessary to increase the amount of utilizable O2 to the same extent as hyperoxic ventilation (the so-called "Hb equivalent").

Results: Although former estimations strongly underestimated efficacy of hyperoxic ventilation (Hb equivalent, 1-2 g/dL), our more exact mathematical approach revealed a theoretical Hb equivalent of hyperoxic ventilation in the range of 3-7 g/dL, which depended on basic physiological conditions like pulmonary function, Hb concentration, and peripheral shunt perfusion.

Conclusion: Hyperoxic ventilation establishes a highly available source of O2 that can be utilized effectively for tissue oxygenation. Although further experimental studies are required to quantify this theoretically calculated amount of utilizable O2, these results suggest that the tissue oxygenation efficacy of hyperoxic ventilation, even in absence of hypoxemia, is grossly underestimated in daily clinical practice.