Limitations of Present Dosing Systems for Gases and Volatile Anaesthetics

Abstract

The spreading application of anaesthesia techniques based on the administration of reduced fresh gas flows [4] places a greater emphasis on the demand for “precise” dosing of both volatile agents and anaesthetic gases. Occasionally, a more precise administration of anaesthetic gases is demanded for future anaesthesia delivery systems [1]. Due to their construction principles, vaporizers of conventional design are of only limited suitability for use with comparatively low gas flows, being optimized for flow-rates between 4 and 101/min [16]. Nevertheless, recent studies on units used clinically indicated a high incidence (36%) of significant deviations (> ±20%) between nominal and actual concentrations in this flow range as well [21]. As easy-to-use instruments enabling both a simple and a continuous measurement of anaesthetic concentrations have become available only recently, the differences between the concentrations set and actually delivered remained largely unrecognized during routine clinical work. In the following, the extent of these discrepancies will be described on the basis of our own investigations. Some of the underlying causes will be illuminated, and the accuracy limits of gas-dowing systems will be discussed.