Onodi C, Bühler PK, Thomas J, Schmitz A, Weiss M.
Anaesthesia. 2017 Jul 11. doi: 10.1111/anae.13969. [Epub ahead of print]
The abstract is as follows:
The authors state that 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. Furthermore, they also state that negative values (ETCO2 > PaCO2 ) can be observed in children. Therefore, the authors 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. The authors concluded 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.
Comments by Bhavani Shankar Kodali MD
Negative arterial to end tidal PCO2 values have been reported earlier in children and pregnant patients. I have found negative arterial to end-tidal PCO2 differences, or gradients during general anesthesia for cesarean delivery and anesthesia for tubal ligation. The readers are encouraged to refer to the section of under physiology of this website, www.capnography.com. The link is below
The physiological explanation why negative gradients can occur has been explained in the above section.