Increasing the blood target concentration of remifentanil does not improve the accuracy of closed loop propofol anaesthesia

S.E. Milne, G.N.C. Kenny. University Department of Anaesthesia, Royal Infirmary, Glasgow, UK

 

Background and goal of study

Closed Loop Anaesthesia (CLAN) has been previously described 1. It uses the Auditory Evoked Potential Index (AEPI) 2 derived from the auditory evoked response as an indicator of anaesthetic depth to control the administration of TCI propofol.

We hypothesised that increasing the target concentration of remifentanil would reduce the variability in the AEPI due to surgical stimulation, and that this would improve the accuracy of CLAN.

 

Material and methods

After hospital ethics committee approval and obtaining informed consent, 30 ASA 1 or 2 patients undergoing day stay surgery were recruited. Patients were randomised to receive TCI remifentanil at 2, 4 or 8 ng/ml and anaesthesia was maintained with TCI propofol controlled by CLAN. The CLAN system attempted to maintain the AEPI at 35 and the mean absolute difference between this preset value of AEPI and the actual AEPI was measured.

 

Results and discussion

The mean absolute difference between the preset and actual AEPI at each concentration of remifentanil is shown in the table.

 

Remifentanil concentration (ng/ml)

2

4

8

Number of patients

10

10

10

Mean absolute AEPI difference (SD)

3.50 (0.82)

3.74 (1.10)

3.31 (1.20)

 

Although a retrospective power analysis showed that statistical significance at the 5% level with 80 % power could be achieved by increasing each group size to 130, we believe that the small difference between groups is of no clinical relevance.

 

Conclusions

Increasing the target concentration of remifentanil from 2 to 8 ng/ml does not improve the ability of the CLAN system to maintain a preset AEPI.

 

References

1.       Anesthesiology 1992;77:A328.

2.       Anaesthesia 1997;52:1030-1036.