MEASUREMENT OF EXPIRED ALCOHOL CONCENTRAT IONS WITH A NEW ELECTROCHEMICAL SENSOR - A MODEL INVESTIGATION TO DETERMINE INTERFERENCE WITH VOLATILE ANESTHETICS AND CLINICAL-APPLICATION

Absorption transurethral (TURF) and percutaneous nephrolitholapaxy (PN L) into veins or delayed absorption due to fluid extravasation may res ult in a TURF syndrome. The measurement of end-tidal breath alcohol co ncentration let AC) as a monitor of absorption of irrigating fluid lab elled with 2% ethanol is limited by the disturbance of infrared sensor s by volatile anaesthetics and nitrous oxide (N2O) (Fig. 2). An electr ochemical sensor is acceptable for this method. The aim of the present study was the evaluation of breath alcohol measurements using an elec trochemical sensor device (Alcomed 3010, Envitec). The stability of th e sensor in the presence of volatile anaesthetics was examined using a lung model. In a clinical investigation, the device was then applied to spontaneously breathing or mechanically ventilated patients inhalin g volatile anaesthetics during endoscopic urological surgery. Method. A two-chamber lung model filled with water for performing noninvasive measurements at the mouth of a patient has already been introduced by Brunner et al. (Fig. 1). With the addition of different amounts of eth anol to the temperature-controlled water, a constant ethanol concentra tion is achievable in the air above the water that is dependent on adj ustments of the ventilator. Increasing concentrations of volatile anae sthetics (isoflurane, enflurane, halothane, and sevoflurane) were adde d to the fresh gas flow (21 O-2/31 N2O) and etACs were measured using the manually triggered self-absorbent electrochemical sensor. First, r egression equations were established between breath alcohol concentrat ions and increased volatile anaesthetic concentrations. Regression equ ations were then established between end-tidal anaesthetic gas concent rations and vaporizer adjustments in order to rule out an influence of ethanol on the anaesthetic nas monitor Ultima V (Datex). In the clini cal investigation, 53 intubated and ventilated patients (33 undergoing PNL. 20 undergoing TURF) and 48 patients breathing spontaneously (32 with inhalation anaesthesia, 16 with spinal anaesthesia) were investig ated. The etAC was measured with the Alcomed 3010 and compared with ga s-chromatographically registered blood alcohol concentrations (BAG). T he study had previously been approved by the Ethical Committee of the Medical University of Luebeck. Patients with liver disease and a histo ry of toxic abuse were excluded. Only one value per patient (maximum B AG) was included in the statistics in order to avoid a cluster effect. Results. The lung model experiments demonstrated that the measurement of etAC with an electrochemical sensor is free of interference by vol atile anaesthetics (Table 1). The slope of the regression between the measured alcohol concentration and increased concentrations of anaesth etics did not differ significantly from baseline values. The measureme nt of end-tidal anaesthetic concentrations was not significantly diffe rent from vaporizer adjustments in the presence of increased alcohol c oncentrations (Table 2). During the clinical investigation, a regressi on between etAC and BAC was determined for both groups. For the group of patients breathing spontaneously, the correlation coefficient was 0 .961 and the regression equation revealed etAC = 0.5677BAC-0.1303 (Fi g. 5). However, in the group of ventilated patients a biphasic course was shown that was dependent on BAC (Fig. 6). At BAC < 0.4 parts per t housand, a similar correlation (r = 0.856) to the spontaneously breath ing group could be seen (regression equation: etAC = 0.617BAC-0.020). Above 0.4 parts per thousand BAC there was no acceptable correlation (r = 0.444, regression equation: etAC = 0.202BAC + 0.104). Conclusion s. The tested electrochemical sensor does not interfere with volatile anaesthetics and N2O as demonstrated by a lung model. There is a good correlation between etAC and BAC measurements in patients breathing sp ontaneously with special regard to the slope of the regression (s = 0. 57). The application to ventilated patients during the administration of inhalation anaeshtesia should be restricted, with attention paid to the non-linear course o the regression.