Sevoflurane is a rapid-acting volatile anaesthetic with low partition coefficients [1–3]. The low blood-gascoefficient (0.63) should shorten the time to induction and recovery from anaesthesia with sevoflurane. This should be useful when rapid induction and recovery are desirable, as in out-patient anaesthesia. Therefore, there are increasing demands to study sevoflurane.
It is essential to ensure equipotent concentrations of volatile anaesthetics for comparative purposes, for the successful use of the rat as an experimental model. Therefore, the determination of minimum alveolar concentration (MAC) for rats is necessary. However, there is only limited information about the MAC values of sevoflurane in this species. Cook et al. reported MAC to be 2.5% using unpublished observations. Crawford et al. compared haemodynamic and organ blood flow responses during halothane and sevoflurane anaesthesia, and determined the MAC value for sevoflurane to be 2.40±0.05% in spontaneously breathing rats. The present study was designed to determine the effect of age on the MAC of sevoflurane in rats.
Methods and materials
The study was approved by the animal care committee of Yamanashi Medical University. Nine-week-old, male Wistar ST (SLC Co. Ltd, Shizuoka, Japan) rats weighing 310–340 g (younger, n=40) and rats older than 13 months, weighing 500–580 g (older, n=38) were studied. They were maintained in a temperature-controlled environment with food and water ad libitum and a 12-h light/dark cycle. To minimize variations in anaesthetic requirements induced by circadian rhythm, all MAC determinations were performed and completed between 10.00 and 15.00 hours. All animals were placed in a box chamber and only the tail protruded from a tiny hole in the chamber. The rats all breathed spontaneously during exposure to sevoflurane, which was given with oxygen using a calibrated vaporizer (PPVΣ, Penlon, Oxon, UK). The anaesthetic concentration was monitored with an infrared anaesthetic agent analyser (model AM-1, Acoma, Tokyo, Japan). Core body temperature was measured using a rectal probe and maintained at 37°C with a heating lamp when necessary. Under sevoflurane anaesthesia, eight of the 40 younger rats and eight of the 38 older rats were instrumented using a silastic catheter placed in the abdominal aorta via the femoral artery to allow for arterial blood gas sampling. These animals were used for MAC determination 1 h after recovering from the instrumentation.
MAC was determined using a method described previously [6–8]. A long haemostat was applied to the rat's tail for 1 min to ratchet lock. The tail was always stimulated proximal to an earlier test site. Gross purposeful movements of the head, extremities, or body were taken as a positive test. When a positive response occurred, sevoflurane concentration was increased by 10% of the preset value and the stimulus was repeated after a 15 min equilibration. When no response was noted initially, the concentration was decreased by 10% and the stimulus reapplied after a 15 min equilibration. MAC was calculated for each animal as the mean of the highest inspired anaesthetic concentration at which there was a positive response and the lowest concentration at which no response was observed. Arterial blood gas samples were taken when MAC was determined.
Between (younger vs. older) and within (non-instrumented vs. instrumented) group, comparisons were analysed using unpaired Student's t-tests. Data are presented as mean ±SD.
Arterial blood gases in younger and older rats are presented in Table 1. There were no significant differences between younger and older rats for any measurements. MAC for sevoflurane in younger rats was significantly higher than that in the older rats. In addition, instrumentation did not affect MAC values in either younger or older rats (Table 2).
These data clearly demonstrate that MAC for sevoflurane in younger rats was significantly higher than that in the older ones. This agrees with the previous results of MAC for halothane and isoflurane . Cook et al. reported the unpublished observation of Mazze who found the MAC of sevoflurane to be 2.5% in Fischer 344 rats. However, they were unpublished observations. This MAC value compared with those in our younger and older rats. The difference may result from the difference in age because the strain of rat does not influence the MAC value . Mazze may have used 12-month-old rats. These data suggest that any comparisons of MAC values should account for age differences.
Crawford et al., in a study comparing the haemodynamic and organ blood flow responses to halothane and sevoflurane anaesthesia, determined the MAC value for sevoflurane to be 2.40±0.05% in spontaneous breathing Sprague-Dawley rats weighing 260–320 g. The age of their rats seems to be similar to that of our younger rats. However, the MAC values were not the same. This may be because of the limited number (n=8) of animals in their study.
Because only the inspired anaesthetic concentration was measured, a correction may be necessary to estimate alveolar concentration. White et al. measured both the inspired and the end-tidal anaesthetic concentrations of halothane and isoflurane and reported that the inspired-to-alveolar differences for halothane were still ≈9–13%, while the differences for isoflurane were only 3–6%, 2–3 h after induction of anaesthesia. As the blood/gas partition coefficient of sevoflurane is lower than that of isoflurane , inspired-to-alveolar sevoflurane concentration ratios were smaller than the ratios for isoflurane. The low solubility of sevoflurane in blood could allow the substitution of inspired for alveolar anaesthetic concentrations in the rat, if this is measured more than 2 h after induction of anaesthesia. However, the most accurate data are obtained when alveolar concentrations are measured.
Slight respiratory depression occurred in both older and younger rats reported here. However, no animals developed hypoxaemia or metabolic acidosis in either group. Slight respiratory acidosis does not influence MAC values . MAC was also not affected by limited instrumentation in this study. This result is consistent with that of Wouters et al. who reported that chronic instrumentation did not affect MAC. However, they used chronically instrumented animals at least 7 days after surgery, whereas in the present report rats were studied only 1 h after awakening. Acute surgical stress might have influenced MAC, although there were no significant differences in MAC values between non-instrumented and instrumented animals.
In conclusion, the data suggest that the age of the animal must be considered for successful use of the rat as an experimental model when studying volatile anaesthetics and that limited instrumentation does not influence anaesthetic requirements for sevoflurane.
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