CBF and CMRO2 during continuous etomidate infusion supplemented with N2O and fentanyl in patients with supratentorial cerebral tumour. A dose-response study

The effect of general anaesthetics on brain lactate release

The effects of anaesthetic agents on brain energy metabolism may explain their shared neurophysiological actions but remain poorly understood. The brain lactate shuttle hypothesis proposes that lactate, provided by astrocytes, is an important neuronal energy substrate. Here we tested the hypothesis that anaesthetic agents impair the brain lactate shuttle by interfering with astrocytic glycolysis. Lactate biosensors were used to record changes in lactate release by adult rat brainstem and cortical slices in response to thiopental, propofol and etomidate. Changes in cytosolic nicotinamide adenine dinucleotide reduced (NADH) and oxidized (NAD⁺) ratio as a measure of glycolytic rate were recorded in cultured astrocytes. It was found that in brainstem slices thiopental, propofol and etomidate reduced lactate release by 7.4 ± 3.6% (P < 0.001), 9.7 ± 6.6% (P < 0.001) and 8.0 ± 7.8% (P = 0.04), respectively. In cortical slices, thiopental reduced lactate release by 8.2 ± 5.6% (P = 0.002) and propofol by 6.0 ± 4.5% (P = 0.009). Lactate release in cortical slices measured during the light phase (period of sleep/low activity) was ~25% lower than that measured during the dark phase (period of wakefulness) (326 ± 83 μM vs 430 ± 118 μM, n = 10; P = 0.04). Thiopental and etomidate induced proportionally similar decreases in cytosolic [NADH]:[NAD⁺] ratio in astrocytes, indicative of a reduction in glycolytic rate. These data suggest that anaesthetic agents inhibit astrocytic glycolysis and reduce the level of extracellular lactate in the brain. Similar reductions in brain lactate release occur during natural state of sleep, suggesting that general anaesthesia may recapitulate some of the effects of sleep on brain energy metabolism.

Effects of anesthesia on cerebral blood flow, metabolism, and neuroprotection

Administration of anesthetic agents fundamentally shifts the responsibility for maintenance of homeostasis from the patient and their intrinsic physiological regulatory mechanisms to the anesthesiologist. Continuous delivery of oxygen and nutrients to the brain is necessary to prevent irreversible injury and arises from a complex series of regulatory mechanisms that ensure uninterrupted cerebral blood flow. Our understanding of these regulatory mechanisms and the effects of anesthetics on them has been driven by the tireless work of pioneers in the field. It is of paramount importance that the anesthesiologist shares this understanding. Herein, we will review the physiological determinants of cerebral blood flow and how delivery of anesthesia impacts these processes.

Etomidate--a review of robust evidence for its use in various clinical scenarios

Etomidate is an intravenous hypnotic with a favourable clinical profile in haemodynamic high-risk scenarios. Currently, there is an active debate about the clinical significance of the drug's side effects and its overall risk-benefit ratio. Etomidate-induced transient adrenocortical suppression is well documented and has been associated with increased mortality in sepsis. In surgical patients at risk of hypotensive complications, however, a review of current literature provides no robust evidence to contraindicate a single-bolus etomidate induction. Large randomised controlled trials as well as additional observational data are required to compare safety of etomidate and its alternatives.

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.

Newer agents for rapid sequence intubation: etomidate and rocuronium

The emergency airway management of children and adolescents with critical illnesses may necessitate rapid sequence intubation with a sedating and a neuromuscular blocking agent. Etomidate and rocuronium have become increasingly popular for the sedation and paralysis, respectively, of pediatric patients in rapid sequence intubation, and there are many advantages to the use of both agents. Both etomidate and rocuronium have a rapid onset of action, and both agents are relatively free of hemodynamic adverse effects. Etomidate does, however, suppress adrenal function, and consequently, its use in patients with septic shock is controversial. Rocuronium can produce optimal intubating conditions without the serious complications that can accompany succinylcholine. The available evidence supports the safety of etomidate and rocuronium in rapid sequence intubation but also suggests that more prospective studies are needed in pediatric patients.

Anesthetic considerations for intraoperative management of cerebrovascular disease in neurovascular surgical procedures

Despite new surgical methods and interventions a considerable number of patients who undergo neurovascular procedures emergently or electively have substantial mortality, morbidity, and disability. Sound knowledge of pathophysiology of cerebral hypoperfusion, reliable and timely information from monitoring devices, and appropriate choice of therapeutic intervention is essential for successful anesthetic management of these patients. The management of perioperative vasospasm and temporary ischemia during aneurysm clipping require an understanding of cerebral vascular pathophysiology and neuroprotective measures.

Deep Hypothermic Circulatory Arrest for Complex Cerebral Aneurysms: Lessons Learned

OBJECTIVE

Deep hypothermic circulatory arrest is a useful adjunct for treating complex aneurysms. Decreased cerebral metabolism and resultant ischemic tolerance create an environment suitable for devascularizing high-risk lesions. However, the advent of modern imaging modalities, innovative cerebral revascularization strategies, and the emergence of endovascular stenting and coiling limit the number of aneurysms requiring this surgical intervention. We present 66 patients with intracranial aneurysms who underwent surgical clipping under deep hypothermic arrest and attempt to identify patients well-suited for this procedure.

METHODS

This study was conducted during a 15-year period and examined patients with aneurysms of the anterior and posterior cerebral circulation. Demographics, aneurysm characteristics, and surgical factors were evaluated as predictors of functional outcome.

RESULTS

Patient age and the duration of cardiac arrest were independent predictors of early clinical outcome (P &lt; 0.05). Our experience suggests that the ideal patient is younger than 60 years old and harbors few medical comorbidities. Individuals with large aneurysms of the anterior communicating artery, internal carotid artery bifurcation, posterior inferior cerebellar artery, midbasilar, or vertebral arteries and with an absence of thrombosis and calcium may be most likely to experience favorable outcomes. Circulatory arrest should not exceed 30 minutes. Postoperative computed tomographic scanning and timely anesthetic emergence allow for early detection of hemorrhage. Complete dissection of the aneurysm before bypass and avoiding extreme hypothermia yield a low incidence of life-threatening postoperative hematomas.

CONCLUSION

Hypothermic circulatory arrest is a useful technique for neuroprotection during the clipping of complex cerebral aneurysms. This procedure, however, has several associated risks. Patient factors, pathoanatomic characteristics, and surgical parameters may be used to guide patient selection.

Moyamoya disease and anesthesia

Moyamoya disease is a condition that results from bilateral stenosis or obstruction of the intracranial arteries at the base of the brain. Patients exhibit ischemic symptoms, and vascular reconstruction is the therapy of choice. Surgical treatment for Moyamoya disease is often complicated by cerebral ischemia, so the goal in perioperative management is to maintain the balance between oxygen supply and demand in the brain. This report presents three cases of Moyamoya disease in patients under 3 years of age, and discusses anesthesia management issues for pediatric patients with this condition.

The safety of etomidate for emergency rapid sequence intubation of pediatric patients

OBJECTIVE

To determine whether pediatric patients given etomidate for rapid sequence intubation (RSI) in the ED develop clinically important hypotension or adrenal insufficiency.

METHODS

Retrospective review of 100 consecutive patients younger than age 10 years given etomidate for RSI in the ED at two academic medical centers. Data were abstracted from ED and in-patient medical records. Clinically important hypotension was defined as a decrease in systolic blood pressure (BP) measurement to below one standard deviation (SD) of mean normal for age. Clinically important adrenal insufficiency was defined as the need for exogenous corticosteroid replacement for suspected adrenal insufficiency at any time during hospitalization.

RESULTS

BP measurements before and within 20 minutes after etomidate administration for RSI were recorded on 84 intubations (84%). The mean change in BP between pre-intubation and post-intubation measurements was a decrease of 1 mmHg (95% CI: -6 mm Hg to +7 mm Hg, P = 0.83). When expressed as a percentage of normal BP for age, the mean change in BP was a decrease of 1% (95% CI: -7% to +6%, P = 0.82). Four patients (4.8%; 95% CI: 1.3-11.7%) had a systolic BP decrease to below one SD of mean normal for age. Fourteen patients received corticosteroids during hospitalization, but none (0/99, 95% CI: 0-3.7%) for suspected adrenal insufficiency.

CONCLUSIONS

We found no evidence of clinically important adrenocorticoid suppression and a low incidence of clinically important hypotension when using etomidate for emergent pediatric RSI. Because other induction agents may also result in hypotension, prospective comparison studies are needed to further evaluate the safety of etomidate in this patient population.

A review of etomidate for rapid sequence intubation in the emergency department

Currently, there is no one drug that is the agent of choice for induction in rapid sequence intubation in the emergency department (ED). All agents currently used as induction agents in the ED offer distinct advantages for various clinical conditions, but each has a significant side effect profile and specific contraindications that limit its use in many common clinical settings. A review of the data available from the anesthesia literature suggests that etomidate possesses many properties that may make it the agent of choice for rapid sequence intubations in the ED. These advantages include excellent pharmacodynamics, protection from myocardial and cerebral ischemia, minimal histamine release, and a hemodynamic profile that is uniquely stable. Disadvantages include a lack of blunting of sympathetic response to intubation, a high incidence of myoclonus, prominent nausea and vomiting, potential activation of seizures in patients with epileptogenic foci, and impaired glucocorticoid response to stress. Further studies are needed to evaluate the advantages and disadvantages of the use of etomidate for rapid sequence intubation in the ED.

Focal cerebral ischemia during anesthesia with etomidate, isoflurane, or thiopental: a comparison of the extent of cerebral injury

An investigation was performed to compare the cerebral protective properties of etomidate, isoflurane, and thiopental. In separate groups of spontaneously hypertensive rats, etomidate, isoflurane, or thiopental was administered to achieve and maintain burst-suppression of the electroencephalogram (3-5 bursts/min) for the duration of the experiment. A fourth group received 1.2 minimal alveolar concentration halothane. All groups underwent 3 hours of middle cerebral artery occlusion and then 2 hours of reperfusion. Thereafter, the animals were killed and the volume of injured brain was determined by staining with 2,3,5-triphenyltetrazolium. Physiological parameters did not differ among the four groups during the investigation, with the exception that hemolysis occurred in the etomidate group (free hemoglobin levels, approximately 0.4 g.dl-1). The volume of injured brain in the thiopental group (56 +/- 10 mm3) was significantly smaller than that in the halothane control group (99 +/- 13 mm3). The volumes of injured brain in the etomidate and isoflurane groups (145 +/- 11 mm3 and 139 +/- 14 mm3, respectively) were significantly larger than those in the control and thiopental groups. We speculate that the apparently detrimental effect of etomidate may be the result of the binding of nitric oxide of cerebral endothelial origin by the iron component of free hemoglobin. Intracranial pressure was not recorded, and in the isoflurane group, there may have been adverse effects on cerebral perfusion pressure associated with vasodilation caused by high concentrations of isoflurane. The results are consistent with a protective effect by barbiturates.

Rapid-sequence intubation in head trauma

Elevated intracranial pressure commonly is associated with severe head injury. Emergency airway management technique in the patient who has sustained severe head injury must optimize conditions for intubation, minimize the adverse effects of intubation, and permit rapid and effective management of the elevated intracranial pressure. Disturbances in autoregulation make the injured brain particularly susceptible to the adverse effects of alterations of systemic blood pressure. Airway manipulation without adequate pharmacologic support can cause precipitous changes in systemic hemodynamic parameters. This article reviews the physiologic and pathologic relationship among airway management, systemic hemodynamic parameters, and intracranial pressure. Specific recommendations for the use of neuromuscular blocking agents, anesthetic induction agents, and adjunctive medications are provided.

The effects of midazolam on cerebral blood flow and oxygen consumption. Interaction with nitrous oxide in patients undergoing craniotomy for supratentorial cerebral tumours

Cerebral blood flow and the cerebral metabolic rate of oxygen were measured in 30 patients during craniotomy for supratentorial cerebral tumours by a modification of the Kety-Schmidt technique using Xenon 133 intravenously. Anaesthesia was induced with midazolam 0.3 mg/kg, fentanyl and pancuronium, and maintained with midazolam as a continuous infusion, fentanyl, pancuronium and nitrous oxide in oxygen or oxygen in air. The concentration of midazolam in the blood of 10 patients was about 300 ng/litre during two measurements; the patients' lungs were ventilated with N2O in oxygen. The concentration of midazolam in the blood of another 10 patients was doubled to about 600 ng/litre during the second flow measurement; the patients' lungs were ventilated with N2O/O2. The concentration of midazolam in the blood of the third group of 10 patients was doubled to 600 ng/litre during the second flow measurement; the patients' lungs were ventilated with oxygen in air. No relationship was found between the dose of midazolam and cerebral blood flow or oxygen consumption. Nitrous oxide in combination with midazolam also had no effect on these variables.

Midazolam and flumazenil in neuroanaesthesia

Of the numerous benzodiazepines currently available, only a few are used in anaesthetic practice. Midazolam is utilised as a premedicant, sedative, and an induction agent and produces minimal depression of ventilation or of the cardiovascular system. The anticonvulsant activity is similar to that of diazepam. In neuroanaesthesia midazolam may be an acceptable alternative to other intravenous induction agents like barbiturates or etomidate in patients with compromised intracranial compliance. Midazolam produces dose-related reductions in cerebral blood flow and cerebral oxygen consumption. However, midazolam does not necessarily prevent increases in intracranial pressure during induction of anaesthesia. In the electroencephalogram (EEG) midazolam produces a reduction in alpha-activity, an increase in theta-delta-activity and low-voltage beta-activity. Midazolam like other benzodiazepines does not affect early components of auditory and somatosensory evoked responses, whereas late cortical responses may be suppressed. Flumazenil, a specific benzodiazepine antagonist, has been demonstrated to be effective in reversing benzodiazepine-induced sedation. In the EEG midazolam-induced changes vanish almost instantaneously. Flumazenil is also effective in restoring the amplitudes of cortical auditory and somatosensory evoked responses. However, as has been demonstrated in head-injured patients and animal experiments after incomplete cerebral ischaemia, increases in cerebral blood flow and intracranial pressure cannot be excluded after flumazenil administration. When utilising flumazenil for wake-up procedures in midazolam-sedated patients with pathological intracranial compliance, flumazenil has to be titrated very carefully in low doses over prolonged periods.

Cerebral blood flow, cerebral metabolic rate of oxygen and relative CO2 reactivity during neurolept anaesthesia in patients subjected to craniotomy for supratentorial cerebral tumours

In 10 patients subjected to craniotomy for supratentorial cerebral tumours in neurolept anaesthesia, cerebral blood flow (CBF) and cerebral metabolic rate of oxygen (CMRO2) were measured twice peroperatively by a modification of the Kety & Schmidt technique, using 133Xe. The relative CO2 reactivity was assessed indirectly as the % change of the arteriovenous oxygen difference (AVDO2) per mm change in PaCO2. The patients were premedicated with diazepam 10-15 mg perorally. For induction, thiopentone 4-6 mg/kg, droperidol 0.2 mg/kg and fentanyl 5 micrograms/kg were used, and for maintenance N2O 67% and fentanyl 4 micrograms/kg/h. During the first flow measurement the median and range of CBF was 30 ml/100 g/min (range 17-45), of AVDO2 8.0 vol % (range 4.1-9.5), and of CMRO2 2.28 ml O2/100 g/min (range 1.57-2.84). During the second CBF study, AVDO2 increased to 9.3 vol % (range 3.4-11) (P less than 0.05), and CMRO2 increased to 2.51 ml O2/100 g/min (range 1.88-3.00) P less than 0.05, while CBF was unchanged. The CO2 reactivity was present in all studies, median 1.8%/mmHg (range 0.5-15.1). The correlation coefficients between jugular venous oxygen tension/saturation, respectively, and CBF were high at tensions/saturations exceeding 4.0 kPa and 55%, indicating that hyperperfusion is easily unveiled by venous samples from the jugular vein during this anaesthesia.

Cerebral blood flow, cerebral metabolic rate of oxygen and relative CO2-reactivity during craniotomy for supratentorial cerebral tumours in halothane anaesthesia. A dose-response study

Fourteen patients were studied during craniotomy for small supratentorial cerebral tumours. Cerebral blood flow (CBF) and cerebral metabolic rate of oxygen (CMRO2) were measured twice by a modification of the Kety-Schmidt technique using 133Xe intravenously. Anaesthesia was induced with thiopental 4-6 mg kg-1, fentanyl and pancuronium, and maintained with an inspiratory halothane concentration of 0.45% in nitrous oxide 67% at a moderate hypocapnic level. In one group of patients (n = 7) the inspiratory halothane concentration was maintained at 0.45% throughout anaesthesia. About 1 h after induction of anaesthesia CBF and CMRO2 averaged 35 +/- 2 ml 100 g-1 min-1 and 2.7 +/- 0.3 ml O2 100 g-1 min-1 (mean +/- s.c. mean), respectively. During repeat studies 1 h later CBF and CMRO2 did not change. In another group of patients (n = 7) an increase in halothane concentration from 0.45% to 0.90% was associated with a significant decrease in CMRO2 from 2.3 +/- 0.1 to 2.0 +/- 0.1 ml O2 100 g-1 min-1. The CO2-reactivity measured after the second flow measurement was preserved. It is concluded that halothane in this study induces a dose-dependent decrease in cerebral metabolism, an increase in CBF while CO2-reactivity is maintained.

Clinical pharmacokinetic considerations in the treatment of increased intracranial pressure

Life-threatening increased intracranial pressure can be reversed by a variety of drugs. These compounds all have some disadvantages, producing rebound effects, severe coma or cardiovascular depression and electrolyte imbalance. However, reduction of intracranial pressure is a prerequisite for recovery and the benefits of treatment outweigh the risks. Dexamethasone is rapidly eliminated, the short half-life (about 3 hours) indicating that dosage intervals should be kept small. As yet, however, its therapeutic efficacy has not been clearly demonstrated. Therefore, an association between pharmacokinetics and pharmacodynamics cannot be established. Osmotic diuretics are the most widely used agents for reduction of intracranial pressure. Pharmacokinetics show a very close relationship to changes in serum osmolality, but there are large variations in the clearance. For the use of osmotics, the blood-brain barrier must be intact. Osmotic diuretics may lead to intracerebral oedema or to acute renal failure as serum osmolality increases. Considering the pharmacokinetics of each drug, and the dynamics of intracerebral pressure and osmolality, an intermittent, individually titrated dosage should be administered, with simultaneous monitoring of intracranial pressure. Frusemide (furosemide) can be used as an adjunct, to enhance the effect of osmotic diuretics. Its pharmacokinetics are limited by renal function, depending on age as well as on the extent of renal impairment. Altered renal elimination of concomitantly administered drugs, and electrolyte imbalances should be anticipated when diuretics are used. Barbiturates are certain to decrease intracranial pressure in humans by an as yet unknown mechanism. Their administration is recommended for patients that do not respond to conventional therapy. As barbiturates can result in deep coma, knowledge of their pharmacokinetics is of great importance for recovery. Following single doses, pentobarbitone has a relatively long elimination half-life (about 22 hours). However, after repeated administration for several days, its elimination may be enhanced due to autoinduction. Thiopentone kinetics are characterised by distribution and redistribution into deep peripheral compartments. Administration of high and frequent doses leads to considerably delayed recovery. This is not true for methohexitone, which shows comparable pharmacokinetics after single and multiple dose administration. Elimination depends on liver blood flow. Thus, recovery from methohexitone-coma is rapid. Rapid elimination is also an important characteristic of etomidate and alphaxalone/alphadolone, two non-barbiturate hypnotics.(ABSTRACT TRUNCATED AT 400 WORDS)

Changes in CMRO2, EEG and concentration of etomidate in serum and brain tissue during craniotomy with continuous etomidate supplemented with N2O and fentanyl

Fourteen patients with supratentorial cerebral tumours were anaesthetized with continuous etomidate infusion (30 or 60 micrograms kg-1 min-1) supplemented with N2O 67% and fentanyl. Peroperatively cerebral blood flow (CBF) and cerebral metabolic rate of oxygen (CMRO2) were measured twice by the Kety and Schmidt method. Simultaneously with the CBF measurements, blood for serum etomidate was sampled and EEG was recorded in 2-min periods in 12 patients. In 10 patients a brain biopsy for etomidate was taken peroperatively and correlated with the other data. The results indicate a dose-dependant increase in serum etomidate and brain tissue etomidate, a decrease in CMRO2 and suppression of EEG activity. In individual studies an increase in serum etomidate or a decrease in CMRO2 correlated to a suppression of the EEG activity, and vice versa. However, the wide variations in these relationships within and between patients make any conclusion regarding CMRO2 impossible from the EEG recording, infusion rate of etomidate or serum concentration of etomidate.