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The role of ascorbic acid and xylitol in etomidate-induced adrenocortical suppression in humans

Schraag, S.*; Pawlik, M.*; Mohl, U.*; Böhm, B. O.; Georgieff, M.*

European Journal of Anaesthesiology: July 1996 - Volume 13 - Issue 4 - p 346-351
Original Papers
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Etomidate-induced suppression of cortisol biosynthesis is a result of a blockade of 11-beta-hydroxylation in the adrenal gland, mediated by the imidazol radical of etomidate. Since the generation of steroids requires reductive and energy rich equivalents, the present study examined whether supplementation with ascorbic acid or xylitol, a major source of NADPH, could attenuate adrenal suppression by etomidate in human subjects by promoting the turnover rate of 11-beta-hydroxylase. During continuous etomidate/alfentanil anaesthesia for pelviscopic surgery 30 female patients received either Ringer's lactate, xylitol (0.25 g kg−1h−1) or ascorbic acid (0.5 g h−1) intravenously (i.v.). The plasma concentrations of cortisol, aldosterone and dehydroepiandrosterone (DHEA) were recorded for 5 h after end of surgery and a stimulation with synthetic ACTH was performed. The results showed no evidence of a clinically relevant attenuating effect of ascorbic acid or xylitol on etomidate-induced adrenocortical suppression. However, the observed suppression of cortisol levels was not enough to allow an attenuating affect to be measured.

*Clinic of Anaesthesiology and †Medical Clinic Endocrinology Section, University of Ulm Medical School, Steinhövelstraße 9, D-89075 Ulm, Germany

Accepted June 1995

Correspondence: Dr S. Schraag.

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Introduction

Etomidate-induced adrenocortical suppression has been discussed extensively, since Ledingham and Watt [1] in 1983 observed a higher mortality in critically ill patients who received long-term sedation with etomidate. This finding was attributed to a direct suppression of steroidogenesis in the adrenal gland [2,3]. More recent investigations revealed the molecular explanation for the suppressive effect. It could be demonstrated, that etomidate's free imidazol radical affects the adrenal mitochondrial steroidogenesis by inhibition of cytochrome P450-linked microoxygenase systems [4,5].

Early in 1974 Grabner et al.[6] found an attenuating effect of xylitol on dexamethasone-induced adrenal blockade in rabbits. Xylitol is entirely metabolized in the pentose phosphate pathway, and therefore provides additional NADPH for reductive biosynthesis. As the supply and the action of NADPH as a reductive substrate in the microoxygenase system is rate limiting, the observed attenuation could be explained by this mechanism.

On the other hand Boidin and colleagues [7] considered a possible beneficial effect of ascorbic acid on etomidate-induced adrenal suppression, since it is suggested, that etomidate also blocks ascorbic acid metabolism. Resynthesis of ascorbic acid cannot occur because of the blockade of cytochrome P450. Depletion of the ascorbic acid pool would then cause inhibition of the steroidogenesis by reduced hydroxylation and electron transfer in the cytochrome P450 microoxygenase system [8].

To verify the clinical relevance of these hypotheses, the present study investigated the influence of ascorbic acid and xylitol on etomidate-induced adrenocortical suppression under clinical conditions in humans.

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Methods

Patients and study groups

After approval by the University of Ulm Ethics committee 30 consecutive female ASA physical status I and II patients aged 18-61 years (33.9±9.6, median ±SD) undergoing lower abdominal pelvicendoscopic surgery, gave written informed consent to be randomly assigned to one of the following three groups: Group one received an infusion of Ringer's lactate, group two an infusion of xylitol (0.25 g kg−1 h−1, Tutofusin Malat®, Kabi Pharmacia, Erlangen, Germany) and group three an infusion of ascorbic acid (0.5 g h−1, Cebion®, E. Merck, Darmstadt, Germany). The i.v. infusion was started 15 min prior to induction and lasted until 90 min after the end of anaesthesia. No woman was included who had a known history of endocrine disease or was taking hormone containing drugs, including oral contraceptives.

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Anaesthetic technique

All patients underwent the same anaesthetic procedure and were premedicated with clorazepate 20 mg orally 2 h before surgery. Anaesthesia was induced and maintained with etomidate (Etomidat-Lipuro®, B. Braun, Melsuugen, Germany) and alfentanil (Rapifen®, Janssen Beerse, Belgium) given as a bolus injection followed by continuous infusion (Table 1) using syringe pumps (Medfusion 2100®, Medex, Duluth, USA). Oral intubation was facilitated by vecuronium (Norcuron®, Organon Boxtel, The Netherlands) 0.1 mg kg−1. Ventilation was controlled (Cicero®, Dräger, Lübeck, Germany) with nitrous oxide and oxygen, keeping the FiO2 at 0.35. Arterial blood pressure and heart rate were monitored by automatic non-invasive monitor and ECG during anaesthesia. End-tidal CO2 was kept in a normocapnic range.

Table 1

Table 1

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Hormone assays

The plasma cortisol, aldosterone and dehydroepiandrosterone (DHEA) levels were measured using blood samples drawn from an indwelling venous catheter at the following times: 15 min before induction of anaesthesia as a baseline value (T1), 15 min after induction (T2), 60 min after induction (T3), 60 min after end of surgery (T4), 30 min after administration of synthetic ACTH (T5) and 5 h after surgery (T6). The samples were collected in heparinized vials, immediately centrifuged at 0°C and stored at −70°C for further analysis. The plasma concentrations were measured with commercially available radio-immunoassays (cortisol, Labordiagnostika Gödecke, Freiburg, aldosterone and DHEA, Serono Diagnostics, Munich). Normal values for cortisol at 8.00 h are 5-25 μg dL−1, for aldosterone in supine position 10-160 pg mL−1 and for DHEA 1.1-7.2 ng mL−1 in women. The amount of cross-reacting antibodies with other steroid hormones was negligible.

An ACTH stimulation test was performed 60 min after end of surgery by injecting synthetic ACTH (Synacthen®, Ciba-Geigy, Basel, Switzerland) 250 μg i.v.

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Statistical analysis

Means and variances (75 and 95% confidence intervals) for the steroid hormone levels were determined for each of the three study groups with respect to times of analysis (T1-T6). In addition these data were tested with the non-parametric Kruskal-Wallis statistic using an overall α-error of <0.05. In all cases, the Holm procedure [9] was used to adjust the α-risk for multiple comparisons. Temporal changes in cortisol, aldosterone and DHEA plasma concentrations were evaluated using repeated measures analysis of variance (Anova), with P<0.05 considered significant. Data processing was performed with an Apple Macintosh personal computer using the Statview 4 statistical software package (Abacus Concepts Inc., Berkeley Ca. USA).

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Results

There were no significant differences among the three treatment groups with respect to age, weight, cumulative anaesthetic doses and duration of anaesthesia (Table 2). The largest etomidate dose in the Ringer's lactate group was because of surgery in one case with an overall duration exceeding 3 h. This is reflected in the standard deviation.

Table 2

Table 2

During the study no serious side effect occurred and the anaesthetic was well tolerated. However, 42% of the patients experienced post-operative nausea and vomiting.

The time course of the plasma concentrations for cortisol, aldosterone and dehydroepiandrosterone (DHEA) are shown in Figs. 1-3 as means and 95% confidence intervals. It must be noted, that all groups have a large variability in baseline values for cortisol and aldosterone, though every procedure started at 8.00 h. The temporal changes in hormone levels were similar for all three study groups. Whereas the levels of aldosterone dropped below the lower normal limit after induction of anaesthesia and remained there during the whole observation period, the decline in cortisol levels was modest, did not fall short of normal values and showed a rising trend towards samples T5 and T6. The ascorbic acid group showed the smallest variation during the observed intervals. The plasma concentrations of DHEA, as a precursor of cortisol biosynthesis, began to rise and remained high for 60 min after the end of surgery (T4), which was statistically significant for Ringer's lactate, not for xylitol or ascorbic acid groups. The rise in the median values for DHEA in the ascorbic acid group were low and did not exceed the upper normal values. Other than this, no significant differences could be detected between the groups.

Fig. 1

Fig. 1

Fig. 2

Fig. 2

Fig. 3

Fig. 3

In no groups did adrenal gland stimulation with synthetic ACTH result in an increase in cortisol levels. These remained between 5 and 10 μg 100 dL−1.

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Discussion

The use of etomidate has been shown to be a safe and smooth induction agent, especially in the elderly and in patients with cardiac diseases. Etomidate when compared with other agents, provides cardiovascular stability with little or no effect on myocardial metabolism [10]. Beyond that, etomidate offers a favourable pharmacokinetic profile to maintain anaesthesia using a continuous infusion technique [11].

However, etomidate administration in human subjects is accompanied by a dose dependent suppression of adrenal steroid synthesis. The etomidate imidazol radical blocks both side-chain cleavage of cholesterol, which is rate limiting in cortisol synthesis, and mitochondrial 11-beta-hydroxylase. The latter mechanism requires energy-rich phosphates, such as NADPH, molecular oxygen, reductive equivalents and the presence of the cytochrome P450-Fe++ complex [5].

This study was carried out to assess whether the adrenal inhibition caused by etomidate could be attenuated in human subjects by supplementation with ascorbic acid, as an essential reductive equivalent and NADPH, which is provided by metabolized xylitol in the pentose phosphate pathway.

Our results failed to support this theory. It could not be demonstrated, that the supplemented substances are able to increase the turnover rates of the 11-beta-hydroxylation significantly. We must explain the reasons for this and therefore how the various results of other investigators can be interpreted.

Boidin et al.[7] studied the effect of ascorbic acid on etomidate toxicity in 10 patients undergoing vascular surgery during etomidate/fentanyl anaesthesia. They received either ACTH or ascorbic acid (1000 mg) i.v. and a twofold rise in the cortisol levels in the ascorbic acid group was observed. It was suggested that ascorbic acid acts as an electron donor [12], Kitabachi [12] postulated, that two electrons are donated to atomic oxygen, which, together with a hydrogen ion received from NADPH, forms a hydroxyl group. On the other hand, Nathan et al.[13] found no difference in cortisol plasma concentration after 4 h in 16 patients receiving an ascorbic acid bolus (1000 mg) or saline infusion prior to induction of etomidate anaesthesia.

The only indicator for a supportive effect of ascorbic acid during etomidate administration, in our results, was the slight increase in DHEA levels in the ascorbic acid group compared with the xylitol or Ringer's lactate groups, which showed a more pronounced rise. However, this difference was only statistically significant for Ringer's lactate 60 min after the end of surgery and was nearly equaled later by the xylitol group. Weber et al.'s data [14] indicate a predominant inhibitory effect of etomidate on corticosteroid synthesis by relative selective inhibition of 11-beta-hydroxylase compared with other imidazols, such as ketoconazole. This blockade results in an accumulation of steroid precursors, which are metabolized via alternative pathways, such as androgen synthesis. So the DHEA synthesis may represent the extent to which 11-beta-hydroxylase is suppressed by etomidate.

In contrast, two other studies do not support the effectiveness of ascorbic acid in promoting 11-beta-hydroxylation. Yanagibashi et al.[15] found in bovine glomerulosa cells, that ascorbate and NADPH provides reductive equivalents that only support the last step in aldosterone synthesis. Laney et al.[16] examined the influence of ascorbic acid intake on plasma cortisol and tissue ascorbate levels after ACTH treatment in guinea pigs. Their results suggest that the plasma cortisol response and the decrease in adrenal ascorbate levels after ACTH are unrelated to ascorbic acid status and conclude that the absolute ascorbic acid level in the adrenal glands is not critical for steroidogenesis.

In summary, the role of ascorbic acid and energy providing equivalents in a supportive treatment for etomidate induced adrenocortical suppression remains unclear. On the other hand, our data indicate, that the overall variation and distribution of cortisol levels during etomidate anaesthesia is high and that the absolute suppression is not striking enough to quantify an attenuating effect. Since aldosterone levels were totally suppressed in the presence of etomidate, we assume that the residual capacity for steroid synthesis depends on the generation of cortisol. Recently, Crozier et al.[17] investigated the effect of continuous etomidate infusion during cardiac surgery. Their results also indicate that the cortisol response is preserved.

On the basis of these data, the suggestion that there is an appreciable and clinically important perioperative risk from etomidate induced adrenocortical suppression has to be discussed with caution, as cortisol depletion is incomplete.

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References

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Keywords:

ANAESTHETICS, intravenous, etomidate; ASCORBIC ACID; XYLITOL; ADRENAL

© 1996 European Academy of Anaesthesiology