The accuracy of transcutaneous PCO2 in subjects with severe brain injury: a comparison with end-tidal PCO2.

BACKGROUND In patients suffering from brain injury, end-tidal PCO2 (PETCO2 ) monitoring is controversial, but transcutaneous PCO2 (PtcCO2 ), which is noninvasive and utilizes immediate display, may be an alternative method. We hypothesized that PtcCO2 would be more accurate than PETCO2 for monitoring PaCO2 in patients with severe brain injury. METHODS A prospective observational study included consecutive mechanically ventilated adult subjects who had acute brain injury and an arterial catheter in place. When an arterial blood gas analysis was required, the PETCO2 and PtcCO2 values were simultaneously recorded. The agreement between the PETCO2 , PtcCO2 , and PaCO2 measurements (reference) was determined using the Bland-Altman method. The number of outliers defined by the formula ([PETCO2 or PtcCO2 ] - PaCO2 ) > ± 4 mm Hg indicated the proportion of measurements that were considered clinically unacceptable. RESULTS A total of 25 subjects were included in the study, and 85 simultaneous measurements of PaCO2 , PtcCO2 , and PETCO2 were obtained. The bias and precision between PaCO2 and PtcCO2 were -0.75 and 6.23 mm Hg, respectively. The limits of agreement ranged from -12.97 to 11.47 mm Hg. The bias and precision between PaCO2 and PETCO2 were 0.68 and 5.82 mm Hg, respectively. The limits of agreement ranged from -10.72 to 12.08 mm Hg. There were 34 (40%) outliers for the PtcCO2 sensor and 34 (40%) outliers for the PETCO2 sensor (P > .99). CONCLUSIONS The accuracy of PtcCO2 was not superior to that of PETCO2 for assessing PCO2 levels and should not be used to monitor these levels in subjects with severe brain injury.