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Evaluation of a new method of assessing depth of sedation using two-choice visual reaction time testing on a mobile phone

A. Thomson*, A.F. Nimmo*, B. Tiplady*, J.B. Glen

*Department of Anaesthesia, Critical Care and Pain Medicine Royal Infirmary of Edinburgh

Introduction

An objective measure of the degree of sedation produced by propofol would be useful both in research studies and in clinical practice. A previous study suggested that visual reaction time (VRT) might be suitable for monitoring level of sedation¹ but the subjects in that study had to wear special goggles connected to a personal computer. We assessed whether two-choice VRT measured using a specially programmed mobile telephone was correlated with level of propofol sedation.

Methods

Unpremedicated ASA I and II patients were studied immediately before elective surgery. After baseline recordings of VRT, propofol was given by effect-site target-controlled infusion with a customised system (Marsh model with a k_e0 = 0.8/min). Sedation was commenced with an effect-site target concentration (C_eT) of 0.3 mcg/ml and the target was increased in 0.2 mcg/ml increments until the patient became too drowsy to carry out the test. At each level of sedation, once the calculated effect-site concentration (C_eCALC) reached the target, two-choice VRT was measured twice using the ArrowRT programme (www.penscreen.com) on a mobile telephone. Each test lasted one minute. In addition, the patient was asked to rate their level of sedation using visual analogue scales (VAS) and an observer’s assessment was made using the validated Observer’s Assessment of Alertness/ Sedation (OAA/S) scale².

Results

20 patients (13 M; aged 46 ± 10 (mean ± SD), range  28 – 64) were studied. VRT increased with increasing propofol effect-site concentration in all patients (figure). There was little change in VRT at a C_eCALC up to 0.7, but a marked increase in VRT at higher propofol concentrations. The increase in VRT from baseline before patients became unable to carry out the test was 187 ± 96 % (mean + SD) with a range of 85 – 472 %. Patient VAS scores also increased with increasing propofol C_eCALC but the last score measured before the patient became too drowsy to complete further VAS assessments was very variable (range 4 – 99 mm). OAA/S score fell with increasing propofol C_eCALC. In all 19 subjects who completed the study to the point where they were unable to carry out the VRT and VAS tests, OAA/S score had fallen to 3. OAA/S score was better correlated with VRT (Spearman’s r = -0.85) than with VAS (r = - 0.64).

Discussion

Increasing levels of propofol sedation cause an increase in two-choice VRT that is particularly marked just before the patient becomes too drowsy to carry out the test. VRT appears superior to patient-rated VAS scores in the assessment of depth of propofol sedation. We have shown that VRT can be measured using a specially programmed mobile telephone which is portable and easy to use.

References

1. Kim KM, Jeon WJ, Lee DH, et al. Acta Anaesthesiol Scand 2004; 48: 1033–1037

2. Chernik DA, Gillings D, Laine H et al. J Clin Psychopharmacol 1990; 10: 244–251