Comparison of use of an infrared anesthetic gas monitor and refractometry for measurement of anesthetic agent concentrations

Comparison of an infrared anaesthetic agent analyser (Datex-Ohmeda) with refractometry for measurement of isoflurane, sevoflurane and desflurane concentrations

OBJECTIVE

To assess agreement between infrared (IR) analysers and a refractometer for measurements of isoflurane, sevoflurane and desflurane concentrations and to demonstrate the effect of customized calibration of IR analysers.

STUDY DESIGN

In vitro experiment.

SUBJECTS

Six IR anaesthetic monitors (Datex-Ohmeda) and a single portable refractometer (Riken).

METHODS

Both devices were calibrated following the manufacturer's recommendations. Gas samples were collected at common gas outlets of anaesthesia machines. A range of agent concentrations was produced by stepwise changes in dial settings: isoflurane (0-5% in 0.5% increments), sevoflurane (0-8% in 1% increments), or desflurane (0-18% in 2% increments). Oxygen flow was 2 L minute(-1) . The orders of testing IR analysers, agents and dial settings were randomized. Duplicate measurements were performed at each setting. The entire procedure was repeated 24 hours later. Bland-Altman analysis was performed. Measurements on day-1 were used to yield calibration equations (IR measurements as dependent and refractometry measurements as independent variables), which were used to modify the IR measurements on day-2.

RESULTS

Bias ± limits of agreement for isoflurane, sevoflurane and desflurane were 0.2 ± 0.3, 0.1 ± 0.4 and 0.7 ± 0.9 volume%, respectively. There were significant linear relationships between differences and means for all agents. The IR analysers became less accurate at higher gas concentrations. After customized calibration, the bias became almost zero and the limits of agreement became narrower.

CONCLUSIONS AND CLINICAL RELEVANCE

If similar IR analysers are used in research studies, they need to be calibrated against a reference method using the agent in question at multiple calibration points overlapping the range of interest.