Dependence of the gradient between arterial and end-tidal PCO2 on the fraction of inspired oxygen

Abstract

Background End-tidal PCO2 (Pe′CO2) is routinely used in the clinical assessment of the adequacy of ventilation because it provides an estimate of PaCO2. How well Pe′CO2 reflects PaCO2 depends on the gradient between them, expressed as ΔPa-e′CO2. The major determinant of ΔPa- e′CO2 is alveolar dead space (Vdalv). The fraction of inspired O2 (FiO2) is not thought to substantially affect ΔPa-e′CO2 in anaesthetized patients. We hypothesized that a high FiO2 may indeed increase ΔPa-e′CO2 by preferentially vasodilating well-perfused alveoli, resulting in the redistribution of blood flow to these alveoli from poorly perfused alveoli and an increase in Vdalv. We therefore investigated the effects of changes in FiO2 on ΔPa-e′CO2 and Vdalv. Methods With Institutional Review Board approval and informed consent, we studied 20 ASA III supine patients undergoing elective lower abdominal surgery under combined general and epidural anaesthesia. At constant levels of ventilation, FiO2 levels of 0.21, 0.33, 0.5, 0.75, and 0.97 were applied in a random order and ΔPa-e′CO2 and Vdalv were calculated. Results The ΔPa-e′CO2 values were, in order of ascending FiO 2, mean [standard error of the mean (sem)] 0.13 (0.04), 0.28 (0.08), 0.29 (0.09), 0.44 (0.11), and 0.53 (0.09) kPa. The corresponding values of Vdalv were 25.5, 33.8, 35.8, 48.9, and 47.4 ml. Each successive hyperoxic level showed a significant increase in ΔPa-e′CO2 except between the 0.330.5 and 0.750.97 FiO2 levels. Conclusions These data demonstrate that ΔPa-e′CO2, in anaesthetized patients depends on FiO2. © 2011 The Author.