S210 Closed-circuit anesthesia (CCA) involves the administration of that amount of gases to the circuit needed to replace patient uptake and other losses from the circuit. Because the amount of gases sampled by multigas analyzers makes up a significant portion (200 ml/min) of the fresh gas flow during CCA, sampled gases have been redirected to the circuit. However, the reference chamber of the paramagnetic O2 analyzer in the Datex multigas analyzer entrains air (25-42 ml/min) which is used as the reference gas, and this air is mixed with the sampled gases at the exhaust of the analyzer. This contributes to nitrogen (N2) accumulation during CCA. [1] Datex has developed a new module whose paramagnetic oxygen analyzer can use either air or 100% O2 as the reference gas (Datex-Engstrom Compact Airway Modules M-CAiOV, Datex-Engstrom. Helsinki, Finland). We investigated N2 accumulation during CCA using this new analyzer with 100% O2 as the reference gas. METHODS: After obtaining informed consent, 26 ASA I-III patients undergoing elective surgery received general endotracheal anesthesia with isoflurane [iso] in O2. Sampled gases were redirected from the exhaust of the gas analyzer into the circle system. After denitrogenation (end-expiratory N2 (EtN2) < 5%) and intravenous induction of anesthesia, CCA with iso in O2 was started. The EtN2 concentrations (%), calculated by the monitor as 'balance gas' (100 - Et (O2) - Etiso - EtCO2), were recorded every minute. Using linear regression analysis, the EtN2 after 55 min (EtN2 55 min) was correlated with the Et (N2) concentration when the circuit was closed (EtN2 0 min) and with the Et (N2) 5 min after closure (EtN2 5 min). After 1 hour, the system was opened. RESULTS: EtN2 increased exponentially: EtN2 = 1.1 + 10.2 * e-t/41.4 (r2 = 0.46) (Figure 1). EtN2 0 min, EtN2 5 min, and EtN2 55 min were 1.1 +/- 0.9, 2.4 +/- 1.5, and 8.7 +/- 3.6, respectively (mean +/- SD). Correlations (r2) between both EtN2 0 min and EtN2 5 min and the EtN2 55 min were 0.0 and 0.49, respectively.Figure 1CONCLUSIONS: Datex-Engstrom multigas analyzers use infrared technology to measure nitrous oxide, carbon dioxide and potent anesthetic concentrations. Oxygen is measured using paramagnetic oxygen analysis, and N2 concentrations are calculated as 'balance gas'. Until now, air has been used as the reference gas in the oxygen analyzer. While endogenously released N2 accounts for an increase of N2 of 5 to 10% over the first two hours. [2] air entrainment by the monitor quickly becomes the main source of N2 increase. [1] With the new module that allows 100% O2 to be used as the reference gas, significantly less N2 accumulates. This may simplify pharmacokinetics studies of potent inhaled anesthetics using a CCA technique. The new module is not yet commercially available, because the accuracy of the oxygen analyzer depends on a continuous source of oxygen. Until then, another means of oxygen monitoring has to be used simultaneously.