Implicit Memory And Noxious Stimulation Under Propofol / Remifentanil Anaesthesia

Dr Lequeux, Pr Sosnowski, Pr Cantraine, Pr Barvais

Erasmus Hospital, Université Libre de Bruxelles, Belgium

St-Pierre Hospital, Université Libre de Bruxelles, Belgium

Background and Goal of Study:

A noxious stimulation seems to be an important factor for implicit memory (unconscious recall) to be processed during general anaesthesia. However it is unclear whether this stimulation enhances implicit memory because it awakens the patients or because it causes a sympathetic reaction that stimulates the amygdale complex involved in implicit memory processing (1).

Materials and Methods:

20 young healthy volunteers were randomised in 2 groups in a crossover design and anaesthetised with a TCI of propofol (using Infusion Toolbox (ITB) (2) with the set of Marsh) with a stepwise increase of the target blood concentration (Cp) until loss of consciousness (LOC). A noxious stimulus was then applied (hand in cold water: 2-4°C) and a first set of 10 French words repeated 3 times was played via headphones. Afterwards, while Cp propofol was sill maintained, a TCI of remifentanil (ITB set of Minto) was started with a progressive stepwise increase of Cp until no withdrawal reaction was observed after a similar stimulus while a second set of words was played. Implicit memory was tested after recovery with the word stem completion test (1). Mann-Whitney, paired and non paired-T tests were used as appropriate for statistical analysis.

Results and Discussions:

17 over the 20 volunteers were awakened by the noxious stimulation during propofol infusion and none during propofol/remifentanil infusion. Cp propofol at LOC and Cp remifentanil at no withdrawal reaction were respectively 2.9 μg/ml ± 0.6 and 2.0 ng/ml ± 0.6. Systolic blood pressure (BP) and heart rate (HR) significantly increased (p<0.05) compared to LOC values (BP: 109 mmHg ± 11, HR: 66 bpm ± 11) when the noxious stimulation was applied during propofol infusion (BP: 123 mmHg ± 10, HR: 77 bpm ± 11), evidencing a sympathetic stimulation, but not when the same stimulation was applied during propofol/remifentanil infusion (BP: 106 mmHg ± 9, HR: 59 bpm ± 10). No patient showed explicit memorisation. Implicit memory was evidenced during propofol infusion but not during propofol/remifentanil infusion. The 3 volunteers who remained unconscious during the noxious stimulation under propofol infusion remembered significantly (p<0.05) less words than the 17 others and not more (p=0.26) than controls (0.3 ± 0.6 vs 2.1 ± 1.0 vs 0.9 ± 0.7). Implicit memory was thus present only if the patients were awakened by the noxious stimulation and not because of the sympathetic reaction due to the stimulation.

Conclusion(s):

Under propofol TCI anaesthesia in young healthy volunteers, implicit memory is evidenced only when a noxious stimulation is associated with a short period of consciousness.

References:

(1) Andrade J.Br J Psychol 1995; (86): 479-506

(2) Coussaert E. J Clin Monit Comput. 2000; (16): 3-10.