Monitoring of fluid absorption with nitrous oxide during transurethral resection of the prostate

Background: The fluid absorption that occurs during transurethral resection of the prostate (TURP) can be indicated and quantified by the ethanol method. Recently, nitrous oxide (N₂O) was tested in animals and volunteers and seemed to be more accurate and safe. The present study compared these two methods in surgical patients.
Methods: Eighty-six TURPs were performed at two hospitals using an irrigating fluid that contained 3% mannitol, 1% ethanol and 0.004% N₂O (40 ml/l). The ethanol concentration was measured by end-expiratory tests every 10 min. The N₂O concentration was measured by a flared nasal cannula every second. Fluid absorption was calculated based on a regression equation (ethanol method) from the area under the curve based on the samples where CO₂ >median (N₂O method).
Results: Thirteen patients (15%) absorbed >300 ml of fluid as indicated by the ethanol method. The median volume was 707 ml (range 367–1422). Ethanol yielded higher figures for fluid absorption up to 700–800 ml, whereafter the N₂O method indicated that the absorption was larger. Over the entire range, the mean difference between the two methods at the end of any 10-min period of TURP was only +45 ml, although the 95% limits of agreement were quite separated (−479 to +569 ml).
Conclusions: The N₂O method does not require forced breath sampling and was successfully apply clinically. However, there was a dose-dependent difference in result between the ethanol and N₂O methods, which markedly separated the limits of agreement for a wider range of fluid absorption events.