The influence of hemorrhagic shock on etomidate: a pharmacokinetic and pharmacodynamic analysis

How is depth of anaesthesia assessed in experimental pigs? A scoping review

BACKGROUND

Despite the large number of pigs involved in translational studies, no gold standard depth of anaesthesia indicators are available. We undertook a scoping review to investigate and summarize the evidence that sustains or contradicts the use of depth of anaesthesia indicators in this species.

METHODS

Medline, Embase and CAB abstract were searched up to September 22nd 2022. No limits were set for time, language and study type. Only original articles of in vivo studies using pigs or minipigs undergoing general anaesthesia were included. The depth of anaesthesia indicators reported in the selected papers were divided in two categories: A, indicators purposely investigated as method to assess depth of anaesthesia; B, indicators reported but not investigated as method to assess depth of anaesthesia.

RESULTS

Out of 13792 papers found, 105 were included after the screening process. Category A: 17 depth of anaesthesia indicators were found in 19 papers. Studies were conducted using inhalant anaesthetics as the main anaesthetic agent in the majority of the cases (13/19 = 68.4%), while 3/19 (15.8%) used propofol. The most investigated depth of anaesthesia indicators were bispectral index (8/19 = 42.1%) and spectral edge frequency 95% (5/19 = 26.3%). Contrasting results about the specific usefulness of each depth of anaesthesia indicators were reported. Category B: 23 depth of anaesthesia indicators were found in 92 papers. The most reported depth of anaesthesia indicators were: motor response following a stimulus (37/92 = 40.2%), depth of anaesthesia scores (21/92 = 23.3%), bispectral index (16/92 = 17.8%) and spectral edge frequency 95% (9/92 = 9.8%).

CONCLUSION

Results highlight the lack of scientifically valid and reliable indicators to ensure adequate depth of anaesthesia in pigs.

Etomidate and its Analogs: A Review of Pharmacokinetics and Pharmacodynamics

Etomidate is a hypnotic agent that is used for the induction of anesthesia. It produces its effect by acting as a positive allosteric modulator on the γ-aminobutyric acid type A receptor and thus enhancing the effect of the inhibitory neurotransmitter γ-aminobutyric acid. Etomidate stands out among other anesthetic agents by having a remarkably stable cardiorespiratory profile, producing no cardiovascular or respiratory depression. However, etomidate suppresses the adrenocortical axis by the inhibition of the enzyme 11β-hydroxylase. This makes the drug unsuitable for administration by a prolonged infusion. It also makes the drug unsuitable for administration to critically ill patients. Etomidate has relatively large volumes of distributions and is rapidly metabolized by hepatic esterases into an inactive carboxylic acid through hydrolyzation. Because of the decrease in popularity of etomidate, few modern extensive pharmacokinetic or pharmacodynamic studies exist. Over the last decade, several analogs of etomidate have been developed, with the aim of retaining its stable cardiorespiratory profile, whilst eliminating its suppressive effect on the adrenocortical axis. One of these molecules, ABP-700, was studied in extensive phase I clinical trials. These found that ABP-700 is characterized by small volumes of distribution and rapid clearance. ABP-700 is metabolized similarly to etomidate, by hydrolyzation into an inactive carboxylic acid. Furthermore, ABP-700 showed a rapid onset and offset of clinical effect. One side effect observed with both etomidate and ABP-700 is the occurrence of involuntary muscle movements. The origin of these movements is unclear and warrants further research.

Physiologically Difficult Airway in the Patient with Severe Hypotension and Metabolic Acidosis

The expertise to recognize and manage the difficult airway is essential in anesthesiology. Conventionally, this refers to anatomical concerns causing difficulties with facemask ventilation and/or with tracheal intubation. Severe derangements in patients' physiology can make induction and intubation likewise difficult, and approximately 30% of critically ill patients had cardiovascular collapse subsequently to intubation. We present the case of a 58-year-old male with a past medical history of type II diabetes and hypertension who presented with altered mental status due to severe metabolic acidosis with a pH of 6.8 on admission to the intensive care unit. The anesthesia team was called to urgently intubate the patient. Upon arrival, the patient was localizing to pain and was hypocapnic, tachycardic, and hypotensive despite ongoing therapy with norepinephrine, vasopressin, and bicarbonate drips. Bedside point-of-care ultrasound showed hyperdynamic left ventricle with no other abnormalities. The patient was induced with IV ketamine, and dissociation occurred with maintenance of spontaneous respirations, which was followed by laryngoscopy and intubation causing only minimal hemodynamic changes. The patient was subsequently dialyzed and treated supportively. He was discharged from the hospital two weeks later—neurologically intact and at his baseline. Combination of hypotension and severe metabolic acidosis is particularly a challenging setting for airway management and a major risk factor for adverse events, including cardiopulmonary arrest. Hemodynamically stable induction agents should be preferred. In addition, sustaining spontaneous ventilation and avoiding periods of apnea in the peri-intubation period is paramount—any buildup of CO₂ could push a critically low pH even lower and cause cardiovascular collapse. Sympathomimetic properties of ketamine make this induction agent a particularly appealing choice in this setting. This case report further supports the concept that severe physiologic perturbations—in which conventional induction techniques are not feasible—should be included in the current definition of a difficult airway.

The Influence of Hemorrhagic Shock on the Disposition and Effects of Intravenous Anesthetics: A Narrative Review

The need to reduce the dose of intravenous anesthetic in the setting of hemorrhagic shock is a well-established clinical dogma. Considered collectively,; the body of information concerning the behavior of intravenous anesthetics during hemorrhagic shock, drawn from animal and human data, confirms that clinical dogma and informs the rational selection and administration of intravenous anesthetics in the setting of hemorrhagic shock. The physiologic changes during hemorrhagic shock can alter pharmacokinetics and pharmacodynamics of intravenous anesthetics. Decreased size of the central compartment and central clearance caused by shock physiology lead to an altered dose-concentration relationship. For most agents and adjuncts, shock leads to substantially higher concentrations and increased effect. The notable exception is etomidate, which has relatively unchanged pharmacokinetics during shock. Increased concentrations lead to increased primary effect as well as increased side effects, notably cardiovascular effects. Pharmacokinetic changes are essentially reversed for all agents by fluid resuscitation. Propofol is unique among agents in that, in addition to the pharmacokinetic changes, it exhibits increased potency during shock. The pharmacodynamic changes of propofol persist despite fluid resuscitation. The persistence of these pharmacodynamic changes during shock is unlikely to be due to increased endogenous opiates, but is most likely due to increased fraction of unbound propofol. The stage of shock also appears to influence the pharmacologic changes. The changes are more rapid and pronounced as shock physiology progresses to the uncompensated stage. Although scant, human data corroborate the findings of animal studies. Both the animal and human data inform the rational selection and administration of intravenous anesthetics in the setting of hemorrhagic shock. The well-entrenched clinical dogma that etomidate is a preferred induction agent in patients experiencing hemorrhagic shock is firmly supported by the evidence. Propofol is a poor choice for induction or maintenance of anesthesia in severely bleeding patients, even with resuscitation; this can include emergent trauma cases or scheduled cases that routinely have mild or moderate blood loss.

Trauma Airway Management: Induction Agents, Rapid Versus Slower Sequence Intubations, and Special Considerations

Trauma patients who require intubation are at higher risk for aspiration, agitation/combativeness, distorted anatomy, hemodynamic instability, an unstable cervical spine, and complicated injuries. Although rapid-sequence intubation is the most common technique in trauma, slow-sequence intubation may reduce the risk for failed intubation and cardiovascular collapse. Providers often choose plans with which they are most comfortable. However, developing a flexible team-based approach, through recognition of complicating factors in trauma patients, improves airway management success.

Emergency induction of anaesthesia in the prehospital setting: a review of the anaesthetic induction agents

The standard of prehospital care is improving in many trauma systems around the world. For patients surviving the primary injury, the optimal prehospital interven tions remain debatable. Current evidence suggests that patients with severe head injury may benefit from advanced airway management, most commonly per formed by rapid sequence induction of anaesthesia and orotracheal intubation. The ‘best choice’ induction agent remains unclear, and choice seems to depend on local preferences and the skill mix of the prehospital care team. In this review we look at the recent evidence for selected hypnotic agents.

Effects of continuous infusion of etomidate at various dose rates on adrenal function in dogs

BACKGROUND

Etomidate is a commonly used sedative in intravenous anesthesia. The aim of this study was to compare the effects of various etomidate doses administered by continuous infusion on adrenal function in dogs under general anesthesia.

METHODS

Thirty-six healthy adult male dogs were randomly divided into six groups. Sodium pentobarbital alone was administered to the control group (group C); five experimental groups (E1, E2, E3, E4, and E5) were also given etomidate at doses of 10, 15, 20, 25, and 30 μg · kg(-1) · min(-1), respectively, to maintain anesthesia. Heart rate (HR), mean arterial pressure (MAP), and bispectral index (BIS) were monitored. Serum cortisol, aldosterone, adrenaline, and noradrenaline levels were measured, and HR, MAP, and BIS values recorded, before intubation (T0), and at 1 h, 2 h, and 3 h after intubation (T1-3).

RESULTS

Cortisol and aldosterone levels in groups E1-5 decreased as the doses and times of continuous infusion of etomidate increased. The cortisol level was significantly decreased compared with baseline at T3 in group E1 and at T1-3 in groups E2-5 (P < 0.05). Compared with the corresponding levels in group C, cortisol levels were significantly lower than T0 values at T3 in group E1 and at T1-3 in groups E2-5 (P < 0.05). The aldosterone level was significantly lower at T3 in group E2 and at T1-3 in groups E3-5 (P < 0.05). Significant reductions in cortisol levels at T2-3 in group E2 and at T1-3 in groups E3-5 compared with group C were also observed (P < 0.05). The plasma adrenaline and noradrenaline levels, HR, MAP, and BIS in groups E1-5 were within the normal range at the different times and with the different doses (P > 0.05).

CONCLUSIONS

Cortisol and aldosterone levels decreased with time and continuous infusion of etomidate; there were no significant changes in adrenaline and noradrenaline levels, HR, MAP, and BIS in any group.

Pro/con debate: Is etomidate safe in hemodynamically unstable critically ill patients?

Etomidate is an induction agent known for its smooth intubating conditions and cardiovascular stability. Studies, however, have shown that a single dose of etomidate can result in a prolonged adrenal insufficiency. The impact of this in patients with sepsis has been a matter for debate. This review presents a pro/con case for using etomidate in hemodynamically unstable critically ill patients and provides guidance for alternative induction techniques and when the use of etomidate might be justified despite these concerns.

Clinical and molecular pharmacology of etomidate

This review focuses on the unique clinical and molecular pharmacologic features of etomidate. Among general anesthesia induction drugs, etomidate is the only imidazole, and it has the most favorable therapeutic index for single-bolus administration. It also produces a unique toxicity among anesthetic drugs: inhibition of adrenal steroid synthesis that far outlasts its hypnotic action and that may reduce survival of critically ill patients. The major molecular targets mediating anesthetic effects of etomidate in the central nervous system are specific γ-aminobutyric acid type A receptor subtypes. Amino acids forming etomidate binding sites have been identified in transmembrane domains of these proteins. Etomidate binding site structure models for the main enzyme mediating etomidate adrenotoxicity have also been developed. Based on this deepening understanding of molecular targets and actions, new etomidate derivatives are being investigated as potentially improved sedative-hypnotics or for use as highly selective inhibitors of adrenal steroid synthesis.

Population pharmacokinetics and pharmacodynamics of brief etomidate infusion in healthy volunteers

This study established the pharmacokinetic and pharmacodynamic relationships of the bispectral index (BIS) and Observer's Assessment of Alertness/Sedation (OAA/S) scale with effect site drug concentrations during and after brief etomidate infusion. Eighteen American Society of Anesthesiologists status I or II volunteers received etomidate (0.2%) infusion at 5 mg/min until the loss of eyelash reflexes, and spontaneous recovery was allowed. Data for plasma etomidate concentrations, BIS, and OAA/S were collected every minute and analyzed by NONMEM. A 2-compartment pharmacokinetic model and a pharmacodynamic sigmoid E(max) model fit the data best, with volumes of distribution at central and peripheral compartments of 4.45 and 74.90 L, respectively, and systemic and intercompartmental clearances of 0.63 and 3.16 L/min, respectively. t(1/2)k(e0) was 1.550 min. EC(50) values were 0.526 and 0.554 µg/mL, and gamma values were 2.25 and 6.24 for BIS and OAA/S, respectively. The prediction probability between OAA/S and BIS was 0.8. The slopes of the curves suggest that BIS is a better monitor of depth of sedation and hypnosis, whereas OAA/S may be more useful for monitoring sleep versus wakefulness. These results should be interpreted within the context of short-term etomidate infusion of less than 10 minutes.

Anaesthesia in haemodynamically compromised emergency patients: does ketamine represent the best choice of induction agent?

In rapid sequence induction of anaesthesia in the emergency setting in shocked or hypotensive patients (e.g. ruptured abdominal aortic aneurysm, polytrauma or septic shock), prior resuscitation is often suboptimal and comorbidities (particularly cardiovascular) may be extensive. The induction agents with the most favourable pharmacological properties conferring haemodynamic stability appear to be ketamine and etomidate. However, etomidate has been withdrawn from use in some countries and impairs steroidogenesis. Ketamine has been traditionally contra-indicated in the presence of brain injury, but we argue in this review that any adverse effects of the drug on intracranial pressure or cerebral blood flow are in fact attenuated or reversed by controlled ventilation, subsequent anaesthesia and the greater general haemodynamic stability conferred by the drug. Ketamine represents a very rational choice for rapid sequence induction in haemodynamically compromised patients.

A physiological model to evaluate drug kinetics in patients with hemorrhagic shock followed by fluid resuscitation. Application to amoxicillin-clavulanate

PURPOSE

To build a physiologically based pharmacokinetic model describing drug kinetics in interstitial fluid in case of hemorrhagic shock, and to propose a simple method to determine the subset of influential parameters that may be estimated with the data at hand.

METHODS

The model, which accounts for alterations of regional blood flows and body water distribution, was fitted to amoxicillin and clavulanate kinetic data, assessed in 12 trauma patients with hemorrhagic shock by comparison with 12 healthy volunteers. The predictions were the free concentrations of amoxicillin and clavulanate in 14 organs.

RESULTS

In all tissues of trauma patients, the rate of distribution was lower, but the steady-state level was higher than those in healthy participants. Blood volume was reduced by 25% and blood flow in organs other than lung, brain, and heart were reduced by 18%. Compared with healthy subjects, the time that free amoxicillin concentration remained above 8 mg/L in the interstitial fluid of trauma patients was higher in blood and muscles, and lower in the tendon compartment.

CONCLUSIONS

The results and predictions were consistent with the knowledge in this field. The model may be useful to optimize clinical trial designs and drug dosing regimens.

Influence of Endotoxin-Induced Sepsis on the Requirements of Propofol-Fentanyl Infusion Rate in Pigs

Endotoxin-induced sepsis in pigs is a recognized experimental model for the study of human septic shock. Generally, pigs are brought into general anaesthesia before sepsis is induced. It is our experience that drug dosages of propofol and fentanyl need to be reduced during endotoxin-induced sepsis, in order to prevent respiratory and cardiovascular depression, but the scientific evidence for this observation is lacking. Therefore, we measured the consumption of propofol and fentanyl at equal level of anaesthesia in pigs with (n = 5) and without (n = 5) endotoxin-induced sepsis, using the cerebral state index (CSI) as measure of anaesthetic depth. Infusion rates of propofol (P < 0.01) and fentanyl (P < 0.05) were significantly lower in septic pigs. Pigs with endotoxin-induced sepsis had an infusion rate of 2.2 mg/kg/hr (S.D. 0.5) for propofol and 12 microg/kg/hr (S.D. 2) for fentanyl, whereas healthy pigs had infusion rates of 3.5 mg/kg/hr (S.D. 0.6) and 17 microg/kg/hr (S.D. 4), respectively. CSI was equal in both groups throughout the experiment, and had a lowest average value of 47 (S.D. 10) at t = 30 in healthy pigs and reached a highest average value of 67 (S.D. 19) at t = 240 in pigs with endotoxin-induced sepsis. Anaesthetic depth was sufficient, assessed clinically, throughout the experiment in both groups. We concluded that the consumption of propofol and fentanyl was significantly reduced in pigs with endotoxin-induced sepsis. In the present study, we adjusted the level of anaesthesia according to clinical signs, and found good agreement with CSI.

Disposition and pharmacodynamics of propofol during isovolaemic haemorrhage followed by crystalloid resuscitation in humans

AIMS

The purpose of this study was to estimate the changes in unbound propofol concentration and pharmacodynamics of propofol during isovolaemic haemorrhage followed by crystalloid resuscitation.

METHODS

Ten patients undergoing measure elective surgery were enrolled in this study. Anaesthesia was maintained by 60% nitrous oxide in oxygen, fentanyl 10-20 microg kg-1 and an infusion of propofol at 8 mg kg-1 h-1 until the end of the operation. Radial arterial samples were collected for measurement of propofol concentration just before the start of the operation, and at the point when blood loss was >10 ml kg-1, 20 ml kg-1 and 30 ml kg-1. Cardiac output (CO), haemoglobin values and plasma concentrations of albumin were also determined. Patients were resuscitated with lactated Ringer's solution to maintain a mean arterial blood pressure (+/-20% of prehaemorrhage). Bispectral index (BIS) was measured continuously.

RESULTS

Mean blood pressure, heart rate and CO were well maintained during the operation in all patients. Haemoglobin values and plasma albumin concentrations decreased significantly during surgery. There were no significant differences in total propofol concentrations across the time points. The unbound propofol concentration was increased from 0.10+/-0.040 microg ml-1 to 0.17+/-0.041 microg ml-1 after the haemorrhage of 30 ml kg-1 (P<0.05). BIS was significantly decreased from 47+/-5.9 to 39+/-3.7 (P<0.05) after the haemorrhage of 30 ml kg-1.

CONCLUSIONS

The hypnotic potency of propofol is increased during isovolaemic haemorrhage in crystalloid resuscitated patients even if CO is maintained.

Etomidate depresses lumbar dorsal horn neuronal responses to noxious thermal stimulation in rats

Etomidate is a widely used IV anesthetic, but little is known about its analgesic properties, in particular, its effects on spinal cord neuronal responses to noxious stimuli. We hypothesized that etomidate would depress lumbar neuronal responses to noxious heat. Rats (n = 15) were anesthetized with isoflurane (1.2%) and laminectomy was performed to record single unit activity. Lumbar neuronal responses to noxious thermal (52 degrees C, 12 s) stimulation of the hindpaw were recorded before and every 2 min (up to 13 min postinjection) after administration of etomidate. The responses at peak effect of etomidate (as a percentage of the control response) were 63% +/- 16%, 63% +/- 16%, 38% +/- 25%, 36% +/- 30%, and 41% +/- 26% for the 0.125, 0.25, 0.5, 1 and 2 mg/kg doses, respectively. The responses quickly recovered, usually by the 10-min period postinjection. Similar responses were obtained in decerebrate, isoflurane-free rats administered etomidate and in isoflurane-anesthetized rats administered propofol. These data demonstrate that etomidate depresses spinal cord neuronal responses to noxious stimulation and is a possible mechanism by which this drug might produce analgesia.

Wirkortäquilibration, Anschlagzeit, „time to peak effect“

Contrary to the situation in "classical" clinical pharmacology, non-steady state phenomena play a fundamental role for clinical pharmacology in anesthesia. Their understanding is of tantamount importance for the safe and efficient application of drugs relevant to anesthesia. Concepts like optimised target-controlled infusion (TCI), effect compartment targeting and the small margin of error tolerable during maintained spontaneous ventilation, force the anesthesiologist to acquire a firm understanding of the difference between the concentration time course at the effect side vs. time course of the plasma concentration. The underlying concepts, their application for the rational use of muscle relaxants, propofol with TCI systems, volatile anaesthetics and opioids will be discussed.

[Clinical applications of intravenous anaesthetics pharmacology: the example of hypnotics and opioids]

OBJECTIVE

To review the general principles of pharmacokinetics and pharmacodynamics models by focusing on intravenous anaesthetics (hypnotics and opioids).

DATA SOURCES

Medline references, lectures from the French congress of anaesthesiology and intensive care medicine, abstracts.

DATA SYNTHESIS

Pharmacokinetic and pharmacodynamic modelling allows simple estimation of becoming of anaesthetic drugs in the body, instead of classical pharmacologic approach. However, pharmacokinetic as well as pharmacodynamic parameters are often considered as resulting from complex mathematic approaches and remain then poorly used in practice by physicians. The aim of this article is to simply expose concepts underlying PK-PD models building and to explain significance of the main PK-PD parameters (first-order rate constants, k(e0), T(1/2)k(e0), T(peak), context-sensitive half-time, context-sensitive decrement times). Clinical consequences for using intravenous anaesthetic drugs (hypnotics and opioids) are exposed either during bolus injection or continuous infusion, when injected alone or co-administered.