Effects of depth of isoflurane anaesthesia on a cognition task in mice
Porto and Vila Real, Portugal
* E-mail: amaria{at}ibmc.up.pt
Editor—Several clinical studies have suggested the existence of postoperative cognitive dysfunction (POCD), especially in elderly patients.1 2 However, human studies carry many variables.3 As the sole effects of anaesthesia are difficult to study, we used mice without performing surgery to study the role of anaesthesia in cognitive dysfunction.
Thirty 10–12-week-old inbred male mice (approved by DGV, Portugal) were randomly assigned into three groups: control group (animals not anaesthetized); Group I (anaesthetized with 1% isoflurane); and Group II (anaesthetized with 2% isoflurane). The animals were placed individually in an induction chamber, and anaesthesia was induced with 3% isoflurane (Isoflo, Esteve Farma, Carnaxide, Portugal) in 100% oxygen with a delivery rate of 5 litre min–1 until loss of righting reflex. After induction, the animals were moved into a plastic zip bag and placed in dorsal recumbence. Anaesthesia was then maintained with isoflurane in 100% oxygen with a flow of 1.5 litre min–1. Heart rate and ventilatory frequency were recorded over intervals of 10 min. Body temperature was maintained at 37±2°C by a homeothermic blanket (N-HB101-S-402, Panlab, Barcelona, Spain) placed under the zip bag. Animals remained anaesthetized for 1 h. Isoflurane concentration was monitored in the exhausted air with an agent gas monitor (Datex Capnomac Ultima, Helsinki, Finland), and no stimuli were applied. At recovery, all animals received 100% oxygen until the gain of righting reflex. Animals from the control group were not anaesthetized; however, they were manipulated and placed inside the induction chamber for 1 min (average time until loss of the righting reflex in anaesthetized animals) and were thereafter returned to their home cage to avoid isolation stress.
One week before the procedure, a food restriction schedule was established to ensure feeding motivation in the spatial learning task.4 At 28 and 56 h after anaesthesia, mice were tested in an eight-arm radial maze (RAM) where animals had to find a piece of cereal. During the test, only one of the choice arms was baited, and this arm remained the same throughout the experiment. Each trial was over when the mouse entered the correct arm and ate the reward. Each animal made 15 test trials per session. An error was recorded when a mouse entered an unrewarded arm. Number of errors before correct choice in each trial, and time to complete the trial was also recorded.
Student's t-testing for independent samples showed that Group II had a significantly higher heart rate (P<0.0004) and a lower ventilatory frequency (P<0.0002) 20 min after induction of anaesthesia compared with Group I. No differences were detected between the groups concerning the number of errors and time to complete the task using repeated measures ANOVA. Twenty-eight hours post-anaesthesia, the control group had a higher number of trials with zero errors and a lower number of trials with one error compared with Group II, using Kruskal–Wallis and Mann–Whitney test (Fig. 1). At 56 h post-anaesthesia, animals did not show any difference at this level.
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), outlier]. XXP<0.01 and XP<0.05 compared with the control group regarding number of trials with 0 and 1 error, respectively, using Mann–Whitney test.Some studies showing cognitive impairment after surgery have indicated anaesthesia as one of the possible causes.1 However, there are very few studies relating depth of anaesthesia and cognitive dysfunction.5 In rodents, the majority of studies compare cognitive performance of anaesthetized and non-anaesthetized animals.6 Our previous study4 showed that the lowest isoflurane concentration impaired learning of mice in a T-maze test. In the present study, we used RAM in order to avoid possible ceiling effects of the T-maze task. Both studies had the main concern of comparing cognition between animals anaesthetized with two very different levels of isoflurane and non-anaesthetized animals without the confounding effects induced by surgical procedures.
The number of errors showed no difference between the groups. This is the most important measure to evaluate the cognitive performance of mice because it indicates the animals' decisions. Moreover, the only difference detected (number of trials with 0 and 1 error) was not presented 56 h post-anaesthesia, indicating the lack of long-lasting effects on this memory task.
In general, the highest depth of isoflurane did not cause cognitive impairment compared with a lower anaesthetic level and with non-anaesthetized adult mice. Hence, in this case, higher concentrations of anaesthetics may be used without compromising cognition. Further studies involving humans should minimize the variables inherent to clinical studies in order to evaluate the true effect of the anaesthesia in POCD.
Fundação para a Ciência e Tecnologia (POCTI/CVT/59056/2004).
References
1 Dodds C, Allison J. Postoperative cognitive deficit in the elderly surgical patient. Br J Anaesth (1998) 81:449–62.
2 Monk TG, Weldon BC, Garvan CW, et al. Predictors of cognitive dysfunction after major noncardiac surgery. Anesthesiology (2008) 108:18–30.[Web of Science][Medline]
3 Moller JT, Cluitmans P, Rasmussen LS, et al. Long-term postoperative cognitive dysfunction in the elderly ISPOCD 1 study. ISPOCD investigators. International Study of Post-Operative Cognitive Dysfunction. Lancet (1998) 351:857–61.[CrossRef][Web of Science][Medline]
4 Valentim AM, Alves HC, Olsson IA, Antunes LM. The effects of depth of isoflurane anesthesia on the performance of mice in a simple spatial learning task. J Am Ass Lab Animal Science (2008) 47:16.
5 Farag E, Chelune GJ, Schubert A, Mascha EJ. Is depth of anesthesia, as assessed by the bispectral index, related to postoperative cognitive dysfunction and recovery? Anesth Analg (2006) 103:633–40.
6 Culley DJ, Baxter MG, Yukhananov R, Crosby G. Long-term impairment of acquisition of a spatial memory task following isoflurane–nitrous oxide anesthesia in rats. Anesthesiology (2004) 100:309–14.[CrossRef][Web of Science][Medline]
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