Variability of blood gases, pulse oximeter saturation, and end-tidal carbon dioxide pressure in stable, mechanically ventilated trauma patients.

We evaluated the short-term variability of PaO2, PaCO2, pulse oximeter saturation (SpO2), and end-tidal PCO2 (PETCO2) in mechanically ventilated trauma patients. All patients were stable and undisturbed during the evaluation periods. Blood gases were obtained from an arterial catheter 4 times at 20-minute intervals. SpO2 and PETCO2 were recorded when the blood gases were obtained. Fifty evaluations were made in 26 patients; 24 patients were evaluated twice, with greater than or equal to 24 hours between evaluation periods. Variability was expressed as coefficient of variation (%CV) for each evaluation period. The median %CVs were 3.6% for PaO2 (95th percentile = 9.8%), 0.5% for SpO2 (95th percentile = 1.4%), 2.8% for PaCO2 (95th percentile = 7.4%), and 2.4% for PETCO2 (95th percentile = 7.1%). The overall correlation between PaCO2 and PETCO2 was r = 0.80, and the mean difference between PaCO2 and PETCO2 was 0.9 +/- 3.6 mm Hg. The variability of PETCO2 was similar to the variability of PaCO2. However, the variability of PaO2 was considerably greater than that of SpO2, which was probably related to the shape of the oxyhemoglobin dissociation curve and the relatively high saturations of the patients in this study. Variability of blood gases, SpO2, and PETCO2 should be considered when these values are clinically interpreted.