Arterial to end‐tidal carbon dioxide difference in children undergoing mechanical ventilation of the lungs during general anaesthesia

Summary Capnography (ET CO 2 ) is routinely used as a non‐invasive estimate of arterial carbon dioxide (Pa CO 2 ) levels in order to modify ventilatory settings, whereby it is assumed that there is a positive gap between Pa CO 2 and ET CO 2 of approximately 0.5 kP a. However, negative values (ET CO 2 > Pa CO 2 ) can be observed. We retrospectively analysed arterial to end‐tidal carbon dioxide differences in 799 children undergoing general anaesthesia with mechanical ventilation of the lungs in order to elucidate predictors for a negative gap. A total of 2452 blood gas analysis readings with complete vital sign monitoring, anaesthesia gas analysis and spirometry data were analysed. Mean arterial to end‐tidal carbon dioxide difference was −0.18 kP a (limits of 95% agreement −1.10 to 0.74) and 71.2% of samples demonstrated negative values. The intercept model revealed Pa CO 2 to be the strongest predictor for a negative Pa CO 2 ‐ET CO 2 difference. A decrease in Pa CO 2 by 1 kP a resulted in a decrease in the Pa CO 2 ‐ET CO 2 difference by 0.23 kP a. This study demonstrates that ET CO 2 monitoring in children whose lungs are mechanically ventilated may paradoxically lead to overestimation of ET CO 2 (ET CO 2 > Pa CO 2 ) with a subsequent risk of unrecognised hypocarbia.