doi:10.1093/bja/ael311 Nitrous oxide for monitoring fluid absorption in volunteers†

Background. We studied whether nitrous oxide (N2O) added to a fluid allows the infused volume to be quantified by measuring N2O in the expired air during normal breathing. If so, N2O might serve as a tracer of fluid absorption during endoscopic surgery. Methods. Twelve male volunteers received continuous and intermittent i.v. infusions (5–45 min) of fluid containing 40 ml litre1 of N2O. Breath N2O and CO2 concentrations were measured every second via a flared nasal cannula, a standard nasal cannula, or a Hudson mask. Results. An expression for the amount of infused fluid was obtained by calculating the area under the N2O concentration–time curve for samples representative for exhalation (CO2>median) and then dividing this area by the median CO2 for the remaining samples. The N2O method then estimated fluid volumes of between 50 and 1400 ml within a 95 % prediction interval of ±200 ml. There were differences of up to 14 % in results between the airway devices tested, but the volunteers preferred the flared nasal cannula. N2O showed a distinctly higher 3 min variability during intermittent infusion, which could indicate whether fluid absorption is directly intravascular or extravascular. No adverse effects were seen. Conclusions. N2O method does not require forced end-expiratory breath sampling but still predicts an administered fluid volume with high precision. N2O variability can probably be used to distinguish immediately between intravascular and perivesical fluid absorption during surgery