Effect Site Concentration of Remifentanil and Pupillary Response to Noxious Stimulation

Eba JM, Engelman E, Schmartz D, Ewalenko P, Barvais L

 

Background and Goal of Study. During anesthesia, µ-agonist opioid effect is estimated on base of imprecise clinical parameters. Pupil diameter increases after noxious stimulation but this reactivity has been shown to decrease with increasing doses of opioids¹. This study aims at evaluating the relation between pupil reactivity and increasing concentrations of remifentanil (RF) with a repetitive standard noxious stimulus, i.e. a 5 seconds 100 Hz tetanic stimulation (TET) applied on the ulnar nerve.

Materials and Methods After ethics committee approval and written informed consent, 12 patients were sudied (ASA 1 or 2, no eye disease minor peripheral surgery). After premedication with 0.5 mg oral alprazolam,  iv anesthesia was started with propofol, at increased concentrations till loss of consciousness (LOC). RF was then started and titrated by increments of 1ng/ml up to 5ng/ml of effect site concentration². At the awake state, LOC and each RF plateau (RF1 to RF5), blood pressure (BP), heart rate (HR), BIS (A2000), and pupil size were recorded. TET was applied at each step from LOC, pupil dilation measured and the difference between pupil size after and before TET (Dpup) calculated. Statistical analysis included ANOVA and Tukey post hoc test with p£0.05 considered as significant.

Results and Discussions. Mean (range) age was 41 yr (22-65), weight 65 kg  (50-78), height 168 cm (154-186) and propofol concentration at LOC 3.5 mg/ml (3-4). BP and HR decreased progressively, without significance between each RF plateau.  BIS awake dropped from 96 to 46 at LOC (p<0.05) then remained unchanged. TET induced no change. Dpup decreased progressively from 1.54 mm to 0.13 mm. A linear correlation between Dpup and RF concentration was found (R= - 0.82) ( see figure).

Conclusion. Decreased pupil response after TET at RF increasing concentrations up to 5 ng/ml is a more sensitive index of m-agonist effect than clinical parameters in young healthy patients.

References

1. Larson et al.  Anesthesiology 1997; 87: 849-55.

2. Minto et al.  Anesthesiology 1997; 86 : 10-23.