The effects of etomidate on cerebral metabolism and blood flow in a canine model for hypoperfusion

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.

The Use of the Excimer Laser-assisted Anastomosis Technique Alleviates Neuroanesthesia During Cerebral High-flow Revascularization

In patients with complex intracranial aneurysms or skull base tumors, parent vessel occlusion and flow replacement by high-flow bypass surgery is a demanding therapy, both for the neurosurgeon and the neuroanesthesiologist. One reason for this is the need for prolonged temporary occlusion of a major cerebral artery, which carries a high risk of perioperative ischemia and necessitates versatile neuroprotective measures during anesthesia. Recently, a novel excimer laser-assisted nonocclusive anastomosis (ELANA) technique has been introduced, circumventing the need for temporary occlusion of cerebral vessels. We hypothesized that the use of this ELANA technique would facilitate also the neuroanesthesiologic management of these patients. To test this, we reviewed the details of the neuroanesthesiologic management of patients undergoing ELANA high-flow bypass surgery at our institution. Twenty-nine patients with giant aneurysms (n=27) or skull base tumor (n=2) who were undergoing parent vessel occlusion and permanent flow replacement by high-flow bypass surgery using the ELANA technique were investigated retrospectively. The records of the patients were analyzed for induction and maintenance of anesthesia, fluid therapy, transfusion requirements, hemodynamic parameters, and brain protective strategies. Although we are not able to provide a sufficient body of cohort data to compare the neuroanesthesiologic management of patients undergoing the conventional anastomosis technique with management using the ELANA technique, in each of our reported cases the conventional anastomosis technique would have entailed a high probability of prolonged temporary occlusion that would, in turn, have warranted intensive brain-protective strategies. The observation that use of the ELANA technique precluded the necessity of brain-protective strategies without entailing perioperative cerebral infarction suggests that the ELANA technique supports the neurosurgeon in creating difficult permanent intracranial anastomoses and also facilitates neuroanesthesiologic management of patients undergoing cerebral high-flow revascularization procedures.

Neuroprotective effect of etomidate on functional recovery in experimental spinal cord injury

OBJECTIVE

Primary impact to the spinal cord causes rapid oxidative stress after injury. To protect neural tissue, it is important to prevent secondary pathophysiological mechanisms. Etomidate, a strong antiexcitotoxic agent, stimulates the gamma aminobutyric acid (GABA) receptors. The purpose of this study was to investigate neurobehavioral and histological recovery and to evaluate the biochemical responses to treatment of experimental spinal cord injury (SCI) in rats with etomidate or methylprednisolone (MP) or both etomidate and MP.

MATERIAL AND METHODS

Seventy-two rats were randomly allocated into six groups: a control group (laminectomy alone), a trauma group (laminectomy+trauma), a methylprednisolone group (30 mg/kg MP), an etomidate group (2 mg/kg), a methylprednisolone and etomidate combined treatment group (30 mg/kg MP and 2 mg/kg etomidate) and a vehicle group. Six rats from each group were killed at the 24th hour after the injury. Malondialdehyde, glutathione, nitric oxide and xanthine oxidase levels were measured. Neurological functions of the remaining rats were recorded weekly. Six weeks after injury, all of those rats were killed for histopathological assessment.

RESULTS

Etomidate treatment revealed similar neurobehavioral and histopathological recovery to MP treatment 6 weeks after injury. Combined treatment did not provide additional neuroprotection.

CONCLUSION

Etomidate treatment immediately after spinal cord injury has similar neuroprotection to MP. In spite of different neuroprotection mechanisms, combined treatment with MP and etomidate does not provide extra protection.

Outcome of acutely ischemic brain tissue in prolonged middle cerebral artery occlusion: a serial positron emission tomography investigation in the baboon

Thrombolysis within 3 to 6 hours of symptom onset is recommended therapy for acute middle cerebral artery (MCA) stroke, but recent imaging studies in humans suggest that the penumbra may last much longer in some patients. It is therefore important to study the events that take place with occlusions that last longer than 6 hours. Based upon positron emission tomography (PET), the tissue with high oxygen extraction fraction (OEF) is at risk of infarction. In a previous sequential PET study in anesthetized baboons, we documented that when reperfusion was initiated at 6 hours after MCA occlusion, the region with the acutely highest OEF was not incorporated within the final magnetic resonance imaging (MRI)-defined infarct, suggesting reperfusion prevented such demise. In agreement with this hypothesis, we report here using the same sequential PET paradigm with final chronic-stage volume MRI that a 20-hour MCA occlusion resulted in, on average, 36% of the highest OEF area being recruited into the final infarct. We also found that the portion of the highest OEF area that went on to infarct had at the earliest time-point significantly lower cerebral blood flow and cerebral oxygen metabolism (mean reductions relative to unoccluded side, 56% and 32%, respectively) than the portion that did not (41% and 11%, respectively) and that some reperfusion occurred in the latter at second time-point, that is, before recanalization. Thus, apart from duration of occlusion, the fate of the at-risk tissue is predicated by the initial severity of the ischemia as well as by early secondary events such as partial spontaneous reperfusion.

Brain protection during neurosurgery

The initial concept of brain protection during neurosurgery is based on research done in the 1970s-1980 which established the concept that by decreasing cerebral metabolic rate the brain could survive longer periods of ischemia. The first section of this chapter reviews some of this initial research that promoted the use of barbiturates for cerebral protection. The second section reviews current anesthetic drugs and their potential for cerebral protection in addition to the benefits of blood pressure, temperature and glucose control. The final section discusses the "new mechanisms of cerebral protection" and the role old and new drugs may play in the future for brain protection during neurosurgery.

Effects of etomidate administration on cerebral collateral flow

OBJECTIVE

Augmentation of blood flow to collateral-dependent tissue (CDT) as a result of selective vasodilation of collateral vessels has been shown to occur with various stimuli after middle cerebral artery occlusion. Etomidate, a carboxylated imidazole derivative, is a nonbarbiturate anesthetic that is used clinically both as an anesthetic and as a neuroprotective agent. The effect etomidate has on collateral cerebral vessels is unknown. The purpose of our studies was to test whether etomidate selectively augmented cerebral blood flow (CBF) to CDT during ischemia as an additional mechanism of neuroprotection.

METHODS

A left craniotomy was performed in each of 14 dogs, with the animals under halothane anesthesia. A branch of the middle cerebral artery was occluded and cannulated distally for determination of CDT using a "shadow flow" technique. CBF and vascular pressures were measured and used to calculate vascular resistance. An etomidate infusion (0.1 mg/kg of body weight/min administered intravenously) was started, and CBF and vascular pressures were measured at 10 and 40 minutes. Hypotension was then induced, and CBF and pressures were again measured.

RESULTS

CBF was significantly reduced in all regions of the brain, including CDT, when etomidate was infused. CDT showed a 53.7% reduction in flow, whereas normal CBF was reduced by at least 63.4%. During hypotension, blood flow to CDT was reduced by an additional 42.7%, whereas normal cerebrum was reduced by at least 22.7%. Vascular resistance was increased in all vessels during etomidate infusion.

CONCLUSION

The neuroprotective effects of etomidate do not seem to be through the augmentation of collateral or global CBF.

Temporary occlusion of the middle cerebral artery in intracranial aneurysm surgery: time limitation and advantage of brain protection

The risk of focal infarction secondary to the induced reversible arrest of local arterial flow during microsurgical dissection of middle cerebral artery (MCA) aneurysms was evaluated further to define the optimal approach to temporary arterial occlusion. To compare the effectiveness of potential brain-protection anesthetics, a group of patients treated with the intravenous agents propofol, etomidate, and pentobarbital, administered individually or in combination, was compared to a group treated with the inhalational agent isoflurane. Forty-nine consecutive MCA aneurysm surgeries involving the temporary clipping of the parent vessel were retrospectively reviewed. Thirty-eight patients received intravenous brain-protection (IVBP) anesthesia. Groups of patients with and without infarctions, and receiving and not receiving IVBP anesthesia, were compared based on the duration and nature of temporary arterial occlusion. Postoperative radiographic evidence of new infarction was used as the threshold for failure of occlusion tolerance. The overall infarction rate was 22.4% (11 of 49 patients), including 15.8% (six of 38 patients) in the IVBP group versus 45.5% (five of 11 patients) in the group that did not receive brain protection (NBP). In the NBP group, the mean duration of temporary occlusion was 3.9 +/- 2.2 minutes for patients without infarction versus 12.2 +/- 4.3 minutes for patients with focal infarction (p < 0.01). In contrast, the mean duration was 13.6 +/- 10.6 minutes for patients without infarction and 18.5 +/- 9.9 minutes for patients with infarction in the IVBP group. All patients (four of four) in the NBP group who underwent occlusion lasting 10 minutes or longer suffered an infarction versus five of 23 patients in the IVBP group (p < 0.0001). Patients with multiple aneurysms were found to be at increased risk of developing focal infarction, whereas those treated with intermittent temporary clip application were at decreased risk. It is concluded that patients in whom focal iatrogenic ischemia is induced during MCA aneurysm clip ligation have a significant advantage compared with those receiving isoflurane when they are given pentobarbital as the primary neuroprotective agent or when they receive propofol or etomidate titrated to achieve electroencephalographic burst suppression, particularly if more than 10 minutes of occlusion time is required. It is also concluded that 10 minutes is a general guideline for safe, temporary occlusion of the MCA. The use of intermittent temporary arterial occlusion and its use in patients with multiple aneurysms need further evaluation before specific recommendations can be made.

Temporary occlusion of the middle cerebral artery in intracranial aneurysm surgery: time limitation and advantage of brain protection

The risk of focal infarction secondary to the induced reversible arrest of local arterial flow during microsurgical dissection of middle cerebral artery (MCA) aneurysms was evaluated further to define the optimal approach to temporary arterial occlusion. To compare the effectiveness of brain-protection anesthetics, a group of patients treated with the intravenous agents, propofol, etomidate, and pentobarbital, administered individually or in combination, was compared to a group treated with the inhalational agent isoflurane.

Forty-nine consecutive MCA aneurysm surgeries involving the temporary clipping of the parent vessel were retrospectively reviewed. Thirty-eight patients received intravenous brain-protection (IVBP) anesthesia. Groups of patients with and without infarctions, and receiving and not receiving IVBP, were compared based on the duration and nature of temporary arterial occlusion. Postoperative radiographic evidence of new infarction was used as the threshold for failure of occlusion tolerance. The overall infarction rate was 22.4% (11 of 49 patients), including 15.8% (six of 38 patients) in the IVBP group versus 45.5% (five of 11 patients) in the isoflurane (ISO) group. In the ISO group, the mean duration of temporary occlusion was 3.9 ± 2.2 minutes for patients without infarction versus 12.2 ± 4.3 minutes for patients with focal infarction (p < 0.01). In contrast, the mean duration was 13.6 ± 10.6 minutes for patients without infarction and 18.5 ± 9.9 minutes for patients with infarction in the IVBP group. All patients in the ISO group who underwent occlusion lasting 10 minutes or longer suffered an infarction versus five of 23 patients in the IVBP group. Patients with multiple aneurysms were found to be at increased risk of developing focal infarction, whereas those treated with intermittent temporary clip application were at a decreased risk.

It is concluded that patients in whom focal iatrogenic ischemia is induced during MCA aneurysm clip ligation have a significant advantage compared with those receiving ISO when they are given pentobarbital as the primary neuroprotective agent or when they receive propofol or etomidate titrated to achieve electroencephalographic burst suppression, particularly if more than 10 minutes of occlusion time is required. It is also concluded that 10 minutes is a general guideline for safe, temporary occlusion of the MCA. The use of intermittent temporary arterial occlusion and patients with multiple aneurysms need further evaluation before specific recommendations can be made.

Sequential studies of severely hypometabolic tissue volumes after permanent middle cerebral artery occlusion. A positron emission tomographic investigation in anesthetized baboons

BACKGROUND AND PURPOSE

In the positron emission tomography literature, markedly hypometabolic brain tissue (oxygen metabolism < 1.3 to 1.7 mL.100 g-1.min-1) has often been equated with irreversible damage in the human brain. By serial positron emission tomography measurements, we investigated the temporal evolution of the volume of severely hypometabolic brain tissue after permanent middle cerebral artery occlusion in anesthetized baboons with, as a perspective, the development of rational therapeutic strategies.

METHODS

Seven anesthetized and ventilated baboons underwent sequential positron emission tomography examinations with the 15O steady-state technique before and 1, 4, 7, and 24 hours and 14 to 29 days after occlusion. In each baboon the infarct volume was calculated by quantitative histological procedures after 19 to 41 days of occlusion.

RESULTS

The sequential measurement of regional oxygen metabolism demonstrated an extension (for > or = 24 hours) of the volume of severely hypometabolic tissue as defined by both absolute and relative metabolic thresholds, and this profile of evolutivity is observed no matter the threshold used. Mean (+/- SEM) infarction volume of 2.4 +/- 0.6 cm3 was comparable to a tissue volume with oxygen consumption < 40% of contralateral metabolism. The volume of hypometabolic tissue was essentially stable at the 1-, 4-, and 7-hour postocclusion studies, increased markedly at the 24-hour study point, and increased even further in the chronic-stage study (on average, 17 days after occlusion). The tissue that eventually displayed a severely hypometabolic state at the final measurement showed a significant decrease of oxygen metabolism and cerebral blood flow at each time analyzed. In that tissue, the oxygen extraction fraction increased significantly at 1 hour (although not thereafter).

CONCLUSIONS

The extension of severely hypometabolic volume after middle cerebral artery occlusion reinforces the concept of a dynamic penumbra and suggests the existence of a relatively large window of therapeutic opportunity in which it may be possible to develop neuroprotective strategies. Our study suggests that maximum infarct volume is determined at some time between 24 hours and 17 days after permanent middle cerebral artery occlusion in anesthetized baboons.

Propylene glycol toxicity following continuous etomidate infusion for the control of refractory cerebral edema

Continued elevations in Intracranial Pressure (ICP) following traumatic or ischemic compromise are known to cause markedly increased morbidity and mortality. Because of the side effects of barbiturates including hypotension and prolonged recovery time, the use of shorter-acting anesthetic agents to control ICP has been considered. Etomidate, when administered by continuous infusion, has been shown to decrease cerebral metabolism resulting in a secondary decrease in cerebral blood flow with minimal changes in cerebral perfusion pressure. We initially intended to randomize 20 patients prospectively into a study protocol that would assess the effects of either pentobarbital or the cardioprotective agent etomidate on ICP and cardiac performance. Given the sequelae of the therapy, we were only able to randomize seven patients with cerebral edema refractory to medical management to receive either etomidate or pentobarbital in a blinded fashion. Three patients who received etomidate developed renal compromise (mean low creatinine clearance 41 ml/min, range 37-44 ml/min) which was initially noted at 24 hours. We believed that this represented an adverse effect that was probably related to the study drug and the study was stopped. Each patient received a 0.30 mg/kg IV induction of etomidate and then 0.02 mg/kg/min continuous infusion for 24-72 hours titrated burst suppression. All patients also received dexamethasone 2 mg IV every six hours to prevent the adrenocortical insufficiency that might occur as a consequence of etomidate-induced suppression of cortisol synthesis. Intracranial pressure decreased (mean = 12mmHg) following the initiation of etomidate. Cardiac parameters remained unchanged (cardiac output 4.8 +/- .6 liters/min).(ABSTRACT TRUNCATED AT 250 WORDS)

Limited protective effects of etomidate during brainstem ischemia in dogs

To evaluate etomidate as a neuroprotective agent in the brain stem, 33 dogs were divided into seven groups and were exposed to isolated, reversible brainstem ischemia in the presence or absence of etomidate using a newly developed canine model of brainstem ischemia. Brainstem auditory evoked potentials (BAEP) and regional cerebral blood flow were measured during ischemia and for 5 hours after reperfusion. This model provides a potential physiological environment in which to test the efficacy of putative brainstem ischemic protective strategies. During ischemia, BAEP were abolished in all animals. Without etomidate 10 minutes of ischemia was of short enough duration to allow complete recovery of BAEP. Ischemia of 20 or 30 minutes' duration resulted in minimal recovery. The dose of etomidate administered did not suppress BAEP or brainstem cardiovascular response to ischemia. In animals receiving etomidate and rendered ischemic for 20 minutes, a significant but only temporary recovery in BAEP was seen. Etomidate failed to have a significant effect in animals rendered ischemic for 30 minutes. The minimal effect of etomidate on the current measures of brainstem function is in contrast to etomidate's known suppressive effect on cortical electroencephalogram and predicts that etomidate does little to alter brainstem metabolism. Etomidate's failure to provide for permanent recovery of BAEP suggests that the drug does not give sufficient protection from ischemia to the brainstem neurons in the auditory pathway. If these auditory neurons reflect brainstem function as a whole, etomidate may not be the protective agent of choice during temporary arterial occlusion of posterior circulation.

Acute stroke intervention with intraarterial urokinase infusion

PURPOSE

A preliminary evaluation of the efficacy and safety of treating patients with acute stroke with intraarterial urokinase infusions was performed.

PATIENTS AND METHODS

Twelve patients with acute stroke were treated within 8 hours of symptom onset (average, 5 hours). Thrombolysis was performed within the middle cerebral (n = 10), internal carotid (n = 1), and basilar (n = 1) arteries. Urokinase (160,000-500,000 IU) was infused through microcatheters placed into or adjacent to the thrombi.

RESULTS

Thrombolysis was angiographically successful in nine patients (75%), all of whom had long-term neurologic improvement. No or minimal neurologic deficits were present in six patients (50%). Thrombolysis failed in three patients (25%); one patient died and two developed severe permanent neurologic deficits. No hemorrhagic complications occurred.

CONCLUSION

Preliminary results suggest that intraarterial urokinase infusion may be effective and safe for treating patients with acute stroke. Potentially devastating neurologic damage was averted or lessened in nine patients (75%). No additional neurologic damage was caused by intervention in the remaining three patients (25%).