In the current study, we used the electroencephalographic BSR as the feedback in a closed-loop system to continuously deliver intravenous infusions of etomidate, methoxycarbonyl etomidate, and carboetomidate, with the goal of maintaining rats at approximately equivalent hypnotic depths. In the first part of our study, we compared the sedative–hypnotic doses required to maintain a constant level of hypnosis (i.e., a BSR of 40% in the presence of isoflurane, 1%) and then measured the rate with which the BSR recovered after the infusion was discontinued. Our studies revealed that the sedative–hypnotic doses required to maintain a constant hypnotic depth and the rate of BSR recovery varied significantly among the three agents, with methoxycarbonyl etomidate > carboetomidate > etomidate. In the second part of our study, we determined the extent to which adrenocortical function was suppressed during closed-loop sedative–hypnotic infusion and assessed the rate of adrenocortical recovery after such infusions were discontinued. We found that adrenocortical function was suppressed during continuous infusions of etomidate and methoxycarbonyl-etomidate. However, on terminating the infusion, adrenocortical function recovered within 30 min with methoxycarbonyl etomidate but remained suppressed beyond an hour with etomidate. Carboetomidate had no effect on adrenocortical function either during or after continuous infusion.
We used an electroencephalogram-based closed-loop system to administer the sedatives–hypnotics because it establishes an unbiased dosing regimen to achieve approximately equivalent hypnotic depths.21,23,24
Because all three of our study drugs are thought to produce hypnosis via
the same mechanism (i.e.
, enhancement of γ
-aminobutyric acid type A receptor function in the brain), we believe that an electroencephalographic parameter, such as the BSR that varies with sedative–hypnotic dose, provides a reasonable quantitative measure of relative hypnotic depth and controls for differences in hypnotic potency and duration of action among agents when dosing.19,20,25,26
We chose a target BSR of 40% primarily because it is near the midpoint of the BSR dynamic range (0–100%) and could be maintained in our studies using reasonable quantities of methoxycarbonyl etomidate in rats. Although this represents a deep level of hypnosis, all of our experiments were performed in a background of isoflurane, 1%, which reduces the intravenous sedative–hypnotic dose required to reach this BSR. Inspection of , , and shows that, in the presence of isoflurane, 1%, a BSR of 40% can be achieved with bolus doses of our intravenous agents, which are only approximately 1.5 to 3 times their respective ED50
values for loss of righting reflexes (LORRs) in the absence of isoflurane; the ED50
values for LORR in the absence of isoflurane are 1 mg/kg etomidate, 5 mg/kg methoxycarbonyl etomidate, and 7 mg/kg carboetomidate in Sprague–Dawley rats.19,20
Because all three agents likely produce hypnosis via
the same receptor mechanism (and probably by binding to the same molecular site on the γ
-aminobutyric acid type A receptor), the interactions that etomidate, methoxycarbonyl etomidate, and carboetomidate make with isoflurane are strongly expected to be equivalent with respect to hypnosis.19,20
Therefore, we believe that relative dosing among the three intravenous hypnotic agents may be compared using this electroencephalographic approach in the presence of isoflurane.
With the administration of all three sedatives–hypnotics, the mean BSR measured in our rats increased and remained near our target BSR of 40% during closed-loop continuous infusion. However, the total dose delivered varied greatly among the three agents. For example, the total doses of etomidate, carboetomidate, and methoxycarbonyl etomidate required to keep the BSR at 40% for 15 min (in the presence of isoflurane, 1%) were 4.7 ± 1.6, 70 ± 16, and 174 ± 27 mg/kg, respectively. These doses correspond to ED50
multiples for LORR of 4.7, 10, and 35, respectively.19,20
The higher relative dose of methoxycarbonyl etomidate needed to maintain a BSR of 40% likely results from its faster elimination; methoxycarbonyl etomidate is rapidly hydrolyzed by esterases and has an ultrashort duration of hypnotic action when given as a single bolus.19
Consistent with that conclusion, the time constant with which the BSR decreased on discontinuing the infusion was significantly shorter with methoxycarbonyl etomidate than with carboetomidate and etomidate (37 s vs.
3.9 min and 11.7 min, respectively). However, in our experiments, the BSR did not completely return to the preinfusion baseline even 15–20 min after terminating the methoxycarbonyl etomidate infusion. This probably does not reflect the presence of accumulated metabolites because infusion of methoxycarbonyl etomidate's hydrolysis products (i.e.
, methoxycarbonyl etomidate carboxylic acid and methanol) did not increase the BSR higher than the low baseline value. In addition, with carboetomidate, the peak BSR tended to occur more slowly. This is broadly consistent with previous behavioral studies19
showing that LORR occurred more slowly with carboetomidate compared with etomidate (33 ± 22 vs.
4.5 ± 0.6 s).
To assess the adrenocortical effects of sedative–hypnotic infusions, we used a second group of rats and added a protocol in which ACTH1–24
was administered and blood was drawn to measure ACTH1–24
–stimulated serum corticosterone every 30 min. The first ACTH1–24
dose was given at the start of the closed-loop infusion, and the first blood was drawn immediately after the infusion was complete. Thus, the corticosterone concentration in the first blood sample (INF blood sample) reflects adrenocortical responsiveness to ACTH1–24
during a continuous sedative–hypnotic infusion. Our results showed that, during infusion, both methoxycarbonyl etomidate and etomidate suppressed ACTH1–24
–stimulated adrenocortical steroid synthesis. This suppression was unlikely the result of metabolite accumulation because infusion of methoxycarbonyl etomidate's hydrolysis products had no effect on serum corticosterone concentrations. Similarly, etomidate's carboxylic acid metabolite is considered to have no significant effect on steroid synthesis.27
After the infusions of etomidate and methoxycarbonyl etomidate were completed, serum corticosterone concentrations increased with additional doses of ACTH1–24
, reflecting, at least in part, recovery of adrenocortical function. Because methoxycarbonyl etomidate is more rapidly metabolized to inactive metabolites than etomidate, it seems reasonable to conclude that the significantly higher serum corticosterone concentrations in the methoxycarbonyl etomidate postinfusion samples, compared with the etomidate samples, reflect methoxycarbonyl etomidate's faster rate of in vivo
However, we cannot exclude other possible explanations or contributions, such as the lower affinity of methoxycarbonyl etomidate for 11β
-hydroxylase (which could explain why serum corticosterone concentrations tended to be higher during infusion of methoxycarbonyl etomidate vs.
etomidate) or faster dissociation of methoxycarbonyl etomidate from 11β
-hydroxylase (if drug dissociation is the rate-limiting step leading to recovery of adrenocortical function).28–30
In contrast to etomidate and methoxycarbonyl etomidate, carboetomidate produced no adrenocortical suppression at any point because the serum corticosterone concentrations in all blood samples drawn from rats in the carboetomidate group were not significantly different from those in the control group. Presumably, this reflects carboetomidate's low affinity for 11β
-hydroxylase, the cytochrome P450 enzyme in the adrenal gland that is most sensitive to inhibition by etomidate and is necessary for corticosterone, cortisol, and aldosterone synthesis.5,12,20
Although rats are a valuable model for studying the actions of intravenous sedative–hypnotic agents, they differ from humans in important ways. First, rats are typically less sensitive than humans to the hypnotic actions of these agents. Although the anesthetic induction doses of etomidate and propofol in humans are approximately 0.2–0.3 and 2–2.5 mg/kg, respectively, these doses are insufficient to produce even LORR in rats.19
Second, in vitro
and in vivo
metabolism of ester-containing drugs (including etomidate) occurs much more quickly in rats than humans.31
For example, remifentanil and esmolol have in vitro
metabolic half-lives of only 0.5 and 2.3 min, respectively, in rat blood compared with 37 and 27.2 min, respectively, in human blood.32,33
Remifentanil's in vivo
elimination half-life is approximately 1 min in rats and longer than 10 min in humans.34–36
Because of these differences between rats and humans, we expect maintenance doses of methoxycarbonyl etomidate and carboetomidate to be 1–2 orders of magnitude lower in humans than rats on a weight-adjusted basis.
In conclusion, the sedative–hypnotic doses required to maintain a constant level of hypnosis and the rate of hypnotic recovery on infusion termination varied, with methoxycarbonyl etomidate > carboetomidate > etomidate. Serum corticosterone concentrations were reduced during continuous infusions of etomidate and methoxycarbonyl etomidate; however, on infusion termination, serum corticosterone levels recovered more quickly with methoxycarbonyl etomidate than with etomidate. Carboetomidate had no effect on serum corticosterone concentrations during or after continuous infusion. This suggests that methoxycarbonyl etomidate and carboetomidate may have clinical utility as continuously infused sedative–hypnotic maintenance agents when hemodynamic stability is desired.