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Reaction time monitored patient maintained propofol sedation: a volunteer safety study

Allam S¹, O’Brien C², Anderson KJ¹, Macpherson A², Gambhir S¹, Leitch JA² and Kenny GNC¹

Departments of Anaesthesia, University of Glasgow/ Glasgow Royal Infirmary¹, Glasgow Dental Hospital²

Introduction: Effect-site controlled patient maintained propofol sedation (ePMS) was found by our group to be safe and effective in patients having dental surgery, but when healthy volunteers deliberately attempted to over sedate themselves, potentially unsafe levels of sedation were reached in some (1). As a patient’s effect-site propofol concentration (Ce) and level of sedation increases, their reaction time increases also (2). We have incorporated a reaction time monitor into our current ePMS system’s handset in the hope of reducing this risk of over sedation. The aim of this study was to reassess its safety in a further 20 healthy volunteers.

Methods: 20 healthy volunteers (ASA 1-2) were recruited and an average baseline reaction time (RT) was recorded (handset demand button pressed in response to its vibration). Propofol sedation was commenced at Ce of 1 mcg/ml and the handset continued to vibrate at one minutely intervals during sedation to monitor RT. As before the volunteer could increase target Ce in increments of 0.2 mcg/ml (up to max 3 mcg/ml) by pressing the demand button twice within one second, provided calculated Cp and Ce had equilibrated within 10%. In addition, the patient’s RT compared to baseline was used in an algorithm as follows: RT £150% of baseline would allow an increment, RT 150%-200% would maintain current level for 2 minutes, RT 200%-300% would decrease target Ce by 0.2 mcg/ml and RT > 300% would decrease by 0.4 mcg/ml. Volunteers were encouraged to use the button “to make themselves unconscious”. End points were defined as loss of verbal contact (OAAS scale less than 3), oxygen saturation (SaO₂ < 90%, airway intervention, apnoea, 3 consecutive reductions in target Ce due to RT slowing or after 30 minutes if none of the above.

Results: All 20 volunteers successfully completed the study maintaining verbal contact throughout in accordance with the definition of conscious sedation. The average maximum propofol Ce (range) was 1.7 (1.2-2.4) mcg/ml. The mean lowest SaO₂ (range) was 97% (93-99%). No supplementary oxygen or airway intervention was required in any volunteer. 10 volunteers ended the study due to 3 consecutive target Ce reductions because of prolonged RT, and 10 after maintaining sedation for 30 minutes. 19/20 volunteers indicated they would use this technique for future sedation.

The chart shows the number of volunteers reaching each observer’s assessment of alertness sedation score category.

Discussion: This study indicates that the addition of RT monitoring has improved the safety of propofol ePMS. It may be possible to use this system in the absence of an anaesthetist. Further work is needed to assess its safety and efficacy in patient populations.

References:

1) Anderson KJ, Leitch JA, Green JS, Kenny GN. Effect-site controlled patient maintained sedation: a volunteer safety study. Anaesthesia 2005; 60(3): 235-8

2) Allam S, Chapman R, Anderson KJ, Kenny GN. Effect of propofol on patient reaction time (to be submitted).