Effect-site controlled patient maintained propofol sedation (ePMS): a volunteer safety study

Green J, Anderson K, Leitch J, Kenny GNC

Glasgow Hospitals; Western/Gartnavel, Dental and GRI

Introduction

We have developed an ePMS machine that delivers propofol to achieve a target effect-site concentration (Ce) - regulated by a patient controlled handset. The object of this study was to assess the safety of the system in volunteers deliberately attempting to over-sedate themselves.

Methods

20 healthy volunteers (ASA 1&2) were recruited and sedation was commenced with a set target Ce of 1.0 μg.ml^-1. The system infuses propofol to a plasma concentration (Cp) of upto 200% of the set Ce target. Cp is then reduced to meet the rising Ce at the set target level. The patient handset became active when Cp had fallen to within 10% of the target Ce. Once active, a successful double press of the button (within one second) increased the target Ce by 0.2mcg/ml.. After 6 minutes without a successful press of the handset the system automatically reduced Ce by 0.2 μg.ml^-1 (“machine step-down”). End points were defined as loss of verbal contact (abbreviated objective observers’ assessment of alertness sedation scale sedation scale 2), machine step-down, SpO2<90%, requirement for airway support measures or apnoea. At this point the subject was then asked to obey commands (open their eyes or stick their tongue). It was noted whether they could still successfully press the button.   

Results (mean (SD) unless otherwise stated)

All 20 volunteers successfully used the system to sedate themselves. All completed the study by 25 (5) minutes with an average maximum Ce of 2.7 (0.6) mg.ml^-1. Reasons for ending the study are shown in the table.

Of the volunteers who desaturated: two took a deep inspiration when instructed to, and the saturation immediately recovered; and the remaining two required supplementary oxygen.

Conclusion

The ePMS system is not safe in its current form when volunteers deliberately attempt to over sedate.

Discussion

Half the volunteers who ended with machine step-down could still press the button successfully, suggesting they lost interest or forgot to press the button rather than could not manage the required coordination. One who desaturated could still press the button.