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

Capnography (ETCO₂) is routinely used as a non‐invasive estimate of arterial carbon dioxide (PaCO₂) levels in order to modify ventilatory settings, whereby it is assumed that there is a positive gap between PaCO₂ and ETCO₂ of approximately 0.5 kPa. However, negative values (ETCO₂ > PaCO₂) 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 kPa (limits of 95% agreement ?1.10 to 0.74) and 71.2% of samples demonstrated negative values. The intercept model revealed PaCO₂ to be the strongest predictor for a negative PaCO₂‐ETCO₂ difference. A decrease in PaCO₂ by 1 kPa resulted in a decrease in the PaCO₂‐ETCO₂ difference by 0.23 kPa. This study demonstrates that ETCO₂ monitoring in children whose lungs are mechanically ventilated may paradoxically lead to overestimation of ETCO₂ (ETCO₂ > PaCO₂) with a subsequent risk of unrecognised hypocarbia.