Oxford Meeting - November 2002

The LMA

“Lets Monitor Awareness?”

 Duncan Young, Steve Wilson, Janet Burton, Helen Higham, John Sear

Nuffield Department of Anaesthetics, Oxford.

If the pressure in the cuff of an LMA is monitored during an anaesthetic fluctuations can be seen. These fluctuations were noted to alter in amplitude with changes in anaesthetic depth, especially at the end of the procedure. These fluctuations might be of value as a monitor of depth of anaesthesia. A device was produced which kept the mean cuff pressure constant but recorded and presented short-term fluctuations in the pressure. LMA International, who commissioned the device, called it a Patient Response Monitor (PRM).

We tested the hypothesis that the PRM might be used as a monitor of depth of anaesthesia in a series of volunteer experiments. Fit (ASA 1-2) adults were anaesthetised with a single agent, either sevoflurane or propofol via a TCI device, and the depth of anaesthesia varied as widely as was practical. Sevoflurane was used in end-tidal concentrations of 0.5, 0.8, 1.1, and 1.5 age corrected MAC and propofol in target concentrations of 2, 3.5, 4.8 and 8 ug ml^-1. After induction and LMA placement the anaesthetic depth was reduced, then increased, and finally reduced with a painful stimulus added at each level. Spontaneous ventilation was used throughout the study.

There is no “gold standard” depth of anaesthesia monitor, so the PRM was compared with drug levels (end-tidal sevoflurane and arterial propofol levels), auditory evoked potentials, and the bispectral index (BIS).

The PRM device accurately regulated LMA cuff pressure. The PRM device provided four variables as potential measures of depth of anaesthesia. Of these the positive peak cuff pressure and the logarithmically transformed activity cuff pressure (the signal power) tracked the alternative measures of depth of anaesthesia best. The PRM tracked the bispectral index most closely, was related to measured changes in auditory evoked potential amplitude and had little correlation with end-tidal or arterial drug levels.

The PRM device appeared to be recording respiratory synchronous changes in cuff pressure across the range of depth of anaesthesia studied. No respiratory asynchronous movements suggestive of spontaneous pharyngeal movement were seen. These experiments could not determine if there was respiratory synchronous contraction of the pharyngeal muscles, or if the changes were pressure fluctuations in the airway transmitted to the cuff.

The device may have some utility in measuring depth of anaesthesia in unparalysed patients and so act as a guide to anaesthetic administration. It is unlikely to detect awareness in paralysed patients.