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Correspondence

A plastic plate facilitating the monitoring of fluid absorption during general anaesthesia

OLSSON, J.; HAHN, R. G.

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European Journal of Anaesthesiology: June 1999 - Volume 16 - Issue 6 - p 418-423

Sir:

Early detection and treatment of irrigating fluid absorption is of importance for the safety of patients undergoing certain endoscopic procedures, notably transurethral resection of the prostate and transcervical resection of the endometrium. Ethanol monitoring is a non-invasive method for indicating and quantifying such absorption. If a tracer amount of ethanol is added to the irrigating fluid, the volume of fluid absorbed can be estimated from the amount of ethanol measured in the patient's exhaled breath. The monitoring requires only a device for measuring the breath ethanol concentration, an irrigating fluid containing ethanol and a nomogram. In clinical studies, the pocket-sized Alcolmeter S-D2 (Lions Ltd, S. Glamorgan, Wales), with an electrochemical sensor, has generally been used to measure the ethanol concentration in the expired breath. An irrigating fluid containing glycine 1.5% plus ethanol 1% is licensed in most EU countries. Several valid nomograms have been published and are available free of charge [1].

Ethanol monitoring is best evaluated during regional anaesthesia, but the monitoring is also reliable during general anaesthesia [2-4]. However, the Alcolmeter which is easy to operate in awake patients, might be more troublesome to handle when the patient is ventilated. Although apparatus is available for continuous monitoring of the breath ethanol level [5], most anaesthetists prefer to use the Alcolmeter also when the operation is performed under general anaesthesia. The recommended approach is to drill a hole in a Capnomac adapter large enough for the inlet on the Alcolmeter and to place it between the tracheal tube and the ventilation system [3]. The device is connected to the adapter and placed on the patient's forehead where measurements can be effected every 10 min at the end of expiration, just as in awake patients. The cumbersome part is keeping the Alcolmeter in place as it must be maintained at a 90° angle to the adapter. To achieve and maintain the straight angle, it is necessary to place a piece of cloth or drape between the patient's forehead and the Alcolmeter and to fix this arrangement with plastic tape (Fig. 1). Sampling can be performed with ease only if this arrangement is made. However, the setup might be uncomfortable and the Alcolmeter sometimes loses its position.

Fig. 1
Fig. 1:
The old recommendation of how to arrange a Lions Alcolmeter S-D2 in a breathing circuit.

We have tried several arrangements other than the set-up recommended today. Our attempts have included placing an adapter with a side-hole in the breathing circuit to which the Alcolmeter can be connected every time a breath sample is to be obtained. We have found that these solutions usually result in erroneous figures for the breath alcohol level. Further work showed that the Alcolmeter is extremely sensitive to an increase in dead space. The device needs some 2.0 mL of air for a reliable analysis, with compensation for the dead space volume of the device (1.2 mL). Any arrangement that increases this dead space volume will result in falsely low values. Thus, the test portion of the air in the expiratory cycle must be taken using a true 'side-stream' technique. We also confirmed that accurate data can only be obtained close to the tracheal tube. The heat and moisture exchanger and the soda lime together absorb about 30% of the ethanol in the breathing system. In contrast, extremely low fresh gas flows or volatile gases do not seem to induce any bias in the breath analysis [4].

A solution to the problem of positioning the Alcolmeter close to the breathing circuit during general anaesthesia is to use a stabilizing plate. The white plastic plate shown in Fig. 2 has been designed to replace the Capnomac adapter and provides perfect stability at the 90° angle of the Alcolmeter between the tracheal tube and the ventilation system. The device is made of POM (polyoxymethylene), which is also called polyacetal plastic. Polyoxymethylene is very resistant to heat (up to 150°C) and various detergents and cleaning agents. The device measures 185 × 77 × 10 mm and has a central indentation into which the Alcolmeter is lowered and fixed from behind by an excentric plastic screw. In the front, a tube of the same material and with connections for standard anaesthetic use is placed perpendicularly through the plate. This tube is penetrated by the inlet pipe of the Alcolmeter to establish a stable 'side-stream' position (Fig. 3). This simple design clearly facilitates the use of ethanol monitoring during general anaesthesia. The plate can be built easily by a hospital technician.

Fig. 2
Fig. 2:
The arrangement of the new plastic plate, which is designed for achieving the best possible stability when the alcohol concentration is measured.
Fig. 3
Fig. 3:
The inlet pipe of the Alcolmeter is placed in a sidestream position in the breathing circuit without increasing dead space volume when an analysis is made.

J. OLSSON

R. G. HAHN

Departments of Anaesthesia, Central Hospital, Sundsvall and Söder Hospital, Stockholm, Sweden

References

1 Hahn RG. Ethanol monitoring of irrigating fluid absorption. Eur J Anaesth 1996; 13: 102-115.
2 Hultén J, Sarma VJ, Hjertberg H, Palmquist B. Monitoring of irrigating fluid absorption during transurethral prostatectomy. Anaesthesia 1991; 46: 349-353.
3 Stalberg HP, Hahn RG, Jones AW. Ethanol monitoring of transurethral prostatic resection during inhalation anesthesia. Anesth Analg 1992; 75: 983-988.
4 Olsson J, Hahn RG. Analysis of ethanol in expired air during low-flow isoflurane anaesthesia. Br J Anaesth 1996; 76: 85-89.
5 Hahn RG, Larsson H, Ribbe T. Continuous monitoring of fluid absorption in transurethral surgery. Anaesthesia 1995; 50: 327-331.
© 1999 European Society of Anaesthesiology