Changes in arterial partial pressure of carbon dioxide (Paco2) may alter cerebral perfusion in critically ill patients with acute brain injury. Consequently, international guidelines recommend normocapnia in mechanically ventilated patients with acute brain injury. The measurement of end-tidal capnography (Etco2) allows its approximation. Our objective was to report the agreement between trends in Etco2 and Paco2 during mechanical ventilation in patients with acute brain injury.
Retrospective monocenter study was conducted for 2 years. Critically ill patients with acute brain injury who required mechanical ventilation with continuous Etco2 monitoring and with 2 or more arterial gas were included. The agreement was evaluated according to the Bland and Altman analysis for repeated measurements with calculation of bias, and upper and lower limits of agreement. The directional concordance rate of changes between Etco2 and Paco2 was evaluated with a 4-quadrant plot. A polar plot analysis was performed using the Critchley methods.
We analyzed the data of 255 patients with a total of 3923 paired ΔEtco2 and ΔPaco2 (9 values per patient in median). Mean bias by Bland and Altman analysis was −8.1 (95 CI, −7.9 to −8.3) mm Hg. The directional concordance rate between Etco2 and Paco2 was 55.8%. The mean radial bias by polar plot analysis was −4.4° (95% CI, −5.5 to −3.3) with radial limit of agreement (LOA) of ±62.8° with radial LOA 95% CI of ±1.9°.
Our results question the performance of trending ability of Etco2 to track changes in Paco2 in a population of critically ill patients with acute brain injury. Changes in Etco2 largely failed to follow changes in Paco2 in both direction (ie, low concordance rate) and magnitude (ie, large radial LOA). These results need to be confirmed in prospective studies to minimize the risk of bias.