Serum glucocorticoid concentrations after halothane and isoflurane anaesthesia in New Zealand white rabbits

STRESS is associated with an increase in the activity of the hypothalamic-pituitary-adrenal (HPA) axis. It has been shown that some anaesthetics may stimulate a stress response (Gonzalez Gil and others 2001). Halothane and isoflurane are widely used inhalant anaesthetics. This short communication reports the response of the HPA axis to these anaesthetics, assessed in 20 New Zealand white rabbits from data collected on serum corticosteroid and cortisol levels. The rabbits were maintained under conventional conditions, a cycle of 12 hours’ light followed by 12 hours’ darkness; temperature of 20 to 22°C; 50 to 55 per cent relative humidity; 10 to 15 air changes/hour) and were allocated into two groups: group 1 comprised 10 rabbits anaesthetised with halothane (Fluothane; Zeneca Farma) and group 2 comprised 10 rabbits anaesthetised with isoflurane (Isoflurano; Laboratorios Inibsa). The animals were handled daily for one week before the experiments to minimise any stress associated with blood sampling. All experimental manipulations were performed between 09.15 and 12.45. Anaesthesia was induced by using a face mask attached to an Ayre’s T-piece (with Jackson-Rees modification) breathing system. Either halothane or isoflurane was delivered at a rising concentration of 0·5 per cent increments in vaporiser setting every 30 seconds, to a measured, delivered concentration of 3·5 per cent halothane (minimum alveolar concentration [MAC] 1·39 [standard deviation 0·23] per cent) or 4·5 per cent isoflurane (MAC 2·05 [0·18] per cent) (Drummond 1985). Both anaesthetics were supplied in a fresh gas flow rate of four litres of oxygen/minute. The trachea of each rabbit was intubated and anaesthetic was also supplied at a concentration of 1·5 per cent halothane or 2·5 per cent isoflurane in a flow of two litres of oxygen/minute. Anaesthesia was maintained for 30 minutes. Blood samples (1·5 ml) were obtained from the central ear artery, using 21 G needles, before induction, and one, 10 and 30 minutes, and one, two, 24, 48 and 72 hours after intubation. Serum corticosterone and cortisol levels were measured in each blood sample by a competitive enzyme immunoassay previously validated for this species by Illera and others (1993). The depth of anaesthesia was monitored by the pedal withdrawal, ear pinch, palpebral, corneal and righting reflexes. Heart rate was calculated from the tracings of an electrocardiogram; respiratory rate was recorded by visually counting the number of breaths. Data collected on glucocorticoid levels, and heart and respiratory rates were subjected to a two-way analysis of variance with repeated measures in each group to assess changes with time, followed by the Duncan t test when appropriate. Differences were considered significant at P<0·05. Both anaesthetics increased the serum corticosterone levels during the maintenance of anaesthesia: after 30 minutes of halothane anaesthesia corticosterone levels had risen to 27·23 (2·36) ng/ml compared with a baseline level of 2·0 (0·69) ng/ml, and with isoflurane anaesthesia corticosterone levels had risen to 14·06 (1·61) ng/ml compared with a baseline level of 2·22 (0·71) ng/ml (Fig 1a). The corticosterone levels were significantly higher in the halothane group from one to 30 minutes (22·84 [2·80] ng/ml at 10 minutes) compared with the isoflurane group (8·39 [1·60] ng/ml at 10 minutes). From one minute to two hours both halothaneand isoflurane-induced anaesthesia increased serum cortisol levels: after one hour of halothane anaesthesia cortisol levels had risen to 3·17 [0·25] ng/ml compared with a baseline level of 0·73 (0·15) ng/ml, and with isoflurane anaesthesia cortisol levels had risen to 3·14 (0·46) ng/ml compared with a baseline level of 0·56 (0·08) ng/ml (Fig 1b). Although the induction of and recovery from anaesthesia was faster in the isoflurane group than in the halothane group there were no significant changes (P<0·05) in the times required for the loss and return of reflexes for both anaesthetics. The mean (se) respiratory rate decreased between one minute and one hour of anaesthesia in group 1, from 106·2 (5·27) breaths/minute at baseline level to 52·4 (3·07) breaths/minute at 10 minutes to 84·8 (6·5) breaths/minute at one hour and between one and 30 minutes in group 2 from 102·1 (5·72) breaths/minute at baseline to 53·2 (4·42) breaths/minute at 10 minutes to 56·8 (5·1) at 30 minutes. Heart rate significantly decreased between one and 30 minutes after intubation in group 1 (228·9 [16·98] beats/minute at 10 minutes) compared with the baseline rate (308·7 [10·43] beats/minute) and the rate in group 2 at 10 minutes (313·9 [9·79] beats/minute). The effects of inhalation anaesthetics on adrenal function have not been well elucidated in New Zealand white rabbits. The results from this study show a consistent response of the FIG 1: Effects of halothane and isoflurane on serum (a) corticosterone levels and (b) cortisol concentrations. * P<0·05, ** P<0·01, *** P<0·001 significant difference between halothane and isoflurane groups, C Significant difference (P<0·05) from baseline values (at 0 minutes). m Minutes, h Hours ■ Halothane ■ Isoflurane