Why we still use intravenous drugs as the basic regimen for neurosurgical anaesthesia Pol Hans and Vincent Bonhomme Purpose of review Evolution of neurosurgery mainly trends towards minimally invasive and functional procedures including endoscopies, small-size craniotomies, intraoperative imaging and stereotactic interventions. Consequently, new adjustments of anaesthesia should aim at providing brain relaxation, minimal interference with electrophysiological monitoring, rapid recovery, patients’ cooperation during surgery and neuroprotection. Recent findings In brain tumour patients undergoing craniotomy, propofol anaesthesia is associated with lower intracranial pressure and cerebral swelling than volatile anaesthesia. Hyperventilation used to improve brain relaxation may decrease jugular venous oxygen saturation below the critical threshold. It decreases the cerebral perfusion pressure in patients receiving sevoflurane, but not in those receiving propofol. The advantage of propofol over volatile agents has also been confirmed regarding interference with somatosensory, auditory and motor evoked potentials. Excellent and predictable recovery conditions as well as minimal postoperative side-effects make propofol particularly suitable in awake craniotomies. Finally, the potential neuroprotective effect of this drug could be mediated by its antioxidant properties which can play a role in apoptosis, ischaemia-reperfusion injury and inflammatoryinduced neuronal damage. Summary Although all the objectives of neurosurgical anaesthesia cannot be met by one single anaesthetic agent or technique, propofol-based intravenous anaesthesia appears as the first choice to challenge the evolution of neurosurgery in the third millennium. Keywords intravenous agents, neurosurgery, neurosurgical anaesthesia, propofol Introduction Five years ago, in the Editorial Review of the Neuroanaesthesia section of Current Opinion in Anaesthesiology, Marcel Durieux highlighted important changes in the practice of neurosurgery, either in current progress or foreseen in the years to come. Those changes mainly trended towards minimally invasive and functional surgery including endoscopic procedures, small size craniotomies, intraoperative magnetic resonance imaging and stereotactic approaches in different pathologies. Neurosurgery in the third millennium should aim at preserving or restoring brain function, achieving immediate and good recovery, and avoiding as far as possible the usual stay in the intensive care unit. Those objectives may require long duration interventions, heavy operative equipment, electrophysiological monitoring and patient’s cooperation during surgery. Such an evolution raises a question to neuroanaesthesiologists. Should new fashion neurosurgery unavoidably cause new trends in anaesthesia practice? The answer, which is a noncommittal one, is probably yes and no. In the textbooks of neurosurgical anaesthesia, the classical criteria which characterize the ideal anaesthetic agent include a list of well-known properties such as smooth induction, haemodynamic stability, no interference with cerebral autoregulation, decrease of intracranial pressure (ICP), brain relaxation, rapid emergence and neuroprotection. None of them can be discarded today, but some should probably draw more attention than others and additional ones should be considered. The new challenge of neurosurgical anaesthesia involves the use of anaesthetic agents and techniques that minimally affect brain function, are devoid of any interference with electrophysiological monitoring, facilitate new neurosurgical procedures, allow patient’s cooperation during surgery, and are associated with rapid and excellent recovery. Curr Opin Anaesthesiol 19:498–503. ß 2006 Lippincott Williams & Wilkins. University Department of Anaesthesia and Intensive Care Medicine, CHR de la Citadelle, Liege University Hospital, Liege, Belgium Correspondence to Pol Hans, University Department of Anaesthesia and ICM, CHR de la Citadelle, Boulevard du 12e de Ligne 1, 4000 Liege, Belgium Tel: +32 4 225 6470; fax: +32 4 225 7308; e-mail: [email protected] Current Opinion in Anaesthesiology 2006, 19:498–503 Abbreviation ICP intracranial pressure ß 2006 Lippincott Williams & Wilkins 0952-7907 Neuroanaesthesia is not blind cooking and believing that one single agent or technique can be applied to all patients whatever the type of surgery would be naive. Nevertheless, we are convinced that intravenous anaesthetic agents are still the basic anaesthetic regimen in the majority of neurosurgical procedures. In so far as synthetic opioids and other intravenous analgesics are commonly used in the operating room whatever the hypnotic agent, this review will essentially compare propofol and 498 Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited. Why we still use intravenous drugs Hans and Bonhomme volatile anaesthetics at the light of the more recent data in the literature. Brain relaxation Brain relaxation appears to be a cornerstone in anaesthesia for intracranial surgery, being mandatory in the case of intracranial hypertension and of great interest for the surgical approach of the base of the skull in the absence of expanding lesions. It can be considered as a neuroprotective measure in so far as it may reduce surgical compression, local hypoperfusion and cerebral ischaemia. It is of outstanding importance in minimally invasive surgery for the removal of brain lesions through small size craniotomies. However, brain relaxation must be balanced according to the degree of intracranial hypertension and the surgical approach of the lesion which relies on preplanned navigation data. In contrast to inhalational agents which may adversely affect ICP, propofol plays a key role in brain relaxation. In a randomized prospective study of patients subjected to craniotomy for cerebral tumours, ICP and cerebral swelling at the opening of the dura have been shown to be lower, and mean arterial blood pressure and cerebral perfusion pressure to be higher in propofol-anaesthetized patients compared to patients anaesthetized with isoflurane or sevoflurane [1]. It was concluded in the same study that during craniotomy for cerebral tumours, operating conditions would be better during propofol than isoflurane or sevoflurane anaesthesia. Indeed, sevoflurane, even when used at subanaesthetic concentrations, increases regional cerebral blood flow and regional cerebral blood volume [2], and may impair dynamic cerebral autoregulation [3]. Propofol is known to reduce regional cerebral blood flow and metabolism comparably, while sevoflurane reduces blood flow to a lesser extent, due to its own vasodilating effect [4]. Indeed sevoflurane, such as the other volatile anaesthetic agents, has both a direct, intrinsic dilating effect on cerebral vessels and an indirect, extrinsic constricting effect related to brain metabolism depression. The opposite effect of propofol and sevoflurane on cerebral vascular tone has been confirmed by Marval et al., using transcranial Doppler ultrasonography [5]. In that study, hypocapnia decreased the estimated cerebral perfusion pressure and increased the zero flow pressure under sevoflurane anaesthesia, but did not change those parameters in propofol-anaesthetized patients. The incidence of low jugular venous bulb oxygen saturation has been reported to be higher during propofol than sevoflurane/nitrous oxide anaesthesia and hyperventilation should be more cautiously applied in propofol anaesthetized patients [6,7]. Recent results, however, indicate that increasing propofol concentrations do not affect jugular venous bulb oxygen saturation in neurosurgical patients [8]. In summary, in case of low intracranial compliance, ICP can be decreased by 499 propofol and increased by volatile anaesthetics. The use of hyperventilation to improve intracranial relaxation, expected to be more frequent in the case of volatile anaesthesia, may compromise cerebral perfusion pressure. Finally, moderately deep sedation with propofol in spontaneously breathing, nonintubated patients with an intracranial expanding lesion does not result in a higher ICP than the use of no sedation [9]. According to another recent study, more episodes of arterial hypotension would be observed with sevoflurane than with propofol anaesthesia during elective intracranial surgery [10]. In our personal practice, total intravenous anaesthesia using propofol and remifentanil for craniotomies is well tolerated in normotensive patients, but may be associated to some degree of arterial hypertension in hypertensive patients. Those episodes of arterial hypertension are usually not resolved by deepening the level of anaesthesia or analgesia, impede the neurosurgeon’s work and may favour brain bulk in case of disturbed autoregulation. They can often be treated by intravenous administration of hypotensive drugs, but may also be successfully controlled by adding sevoflurane at subanaesthetic concentrations to the basic intravenous regimen. Electrophysiological monitoring Electrophysiological monitoring can be used to assess the depth of anaesthesia as well as to localize cortical or subcortical regions and so facilitate the surgical approach of lesions or the placement of deep brain stimulation electrodes. It can also be of interest to control the integrity of neural structures in patients at risk of ischaemia. In so far as electrophysiological effects are concerned, propofol has a considerable advantage over volatile anaesthetics. Propofol is a potent cerebral metabolism depressor and has well established anti-convulsant properties while the epileptogenic effects of high sevoflurane concentrations particularly in the paediatric population are know for several years. Regarding evoked potentials, inhalational agents significantly decrease N2O amplitude and prolong N2O latency of somatosensory evoked potentials in a dose-dependent manner [11]. Propofol, when compared to isoflurane in patients undergoing spine surgery, causes less suppression of the cortical somatosensory evoked potentials with better preservation of somatosensory evoked potential amplitude and less variability at an equivalent depth of anaesthesia [12,13]. Animal data have shown that sevoflurane depresses the middle latency auditory evoked potential waveform and suggest that sevoflurane is not the inhalant agent of choice in a research setting where electroencephalographic measurements are to be recorded during anaesthesia [14]. Regarding motor evoked potentials, Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited. 500 Neuroanaesthesia isoflurane inhibits intraoperative neurophysiological monitoring more than does propofol, which is recommended when motor pathway function is monitored [15]. Good conditions of motor evoked potentials recording have also been reported in a child under ketamine-based anaesthesia [16]. On the other hand, motor evoked potentials can be elicited noninvasively by using transcranial magnetic stimulation, and this technique has been recently shown to be feasible during anaesthesia with propofol and remifentanil [17]. Propofol has also been used successfully in spinal surgery patients subjected to double-train transcranial electrical stimulation [18]. In summary, although intravenous and volatile anaesthetics affect evoked potential characteristics, the significantly lower effect of propofol would incite us to use this drug rather than inhalational agents when electrophysiological monitoring is required. This choice is still reinforced by the ‘anaesthetic fade’, reflecting a progressive depression of motor transcranial motor evoked potentials over time at a constant level of anaesthesia [19]. Recovery and awake craniotomies Neurosurgery is more and more focusing on neurological function, which is best monitored by looking at the patient directly. After classical craniotomies, neurological function is clinically assessed when patients emerge from general anaesthesia and recover consciousness. A rapid emergence will allow immediate neurological examination, early detection and efficacious management of any potential surgical complication. In a study comparing propofol/remifentanil with propofol/sufentanil for supratentorial craniotomy, the propofol/remifentanil regimen was shown to provide quicker recovery [20]. On the other hand, recent studies comparing sevoflurane and propofol combined with either remifentanil or sufentanil in patients undergoing neurosurgical procedures have shown all techniques to be comparable in terms of time to recovery and cognitive functions [10,21,22]. In the absence of any demonstrated difference between propofol and volatile agents regarding the speed and quality of recovery, a lower incidence of nausea and vomiting observed in an ambulatory anaesthesia meta-analysis with propofol is a key factor when patient comfort in the postoperative period is concerned [23]. Referring to ambulatory anaesthesia for neurosurgical patients could sound inappropriate, but outpatient craniotomy for brain tumour has been reported to be feasible [24]. In particular situations, the neurosurgeon may require the patient’s cooperation during surgery, either for the removal of lesions located close to functional areas of the brain, including vision, language and motor areas, or to check the therapeutic efficacy of deep brain stimulations such as in surgery for Parkinson disease. In those cases, anaesthesia relies on the concept of monitoring anaesthesia care and should fulfil the following criteria: sufficient depth of anaesthesia during opening and closure, full consciousness during functional testing, smooth transition between anaesthesia and consciousness, adequate ventilation, and immobility and comfort throughout the entire procedure [25]. The drugs that are most frequently employed for awake craniotomy patients include local anaesthetics, sufentanil or remifentanil, propofol, and the a2 agonists dexmedetomidine and clonidine. Propofol is still the first choice hypnotic in this indication. Its administration can be performed using a target control infusion technique, guided by a depth of anaesthesia monitor and combined to remifentanil infusion [26–28]. During propofol-based anaesthesia for excision of brain tumours located in eloquent brain areas, patients wake up within 5–15 min after stopping propofol infusion, and the laryngeal mask may be temporarily removed and easily replaced [29]. In a retrospective analysis of 98 patients undergoing craniotomy requiring intraoperative awake functional brain mapping, combined infusion of propofol and remifentanil has been recognized to provide satisfactory anaesthetic conditions and allow a wake-up time of 9 min [30]. In epilepsy surgery, propofol stopped to allow patient awakening has not been found to interfere with electrocorticographic recordings, and has been proposed with fentanyl as a safe and useful regimen for awake craniotomy in selected paediatric patients [31]. As a result of its easily titratable sedative effect, rapid recovery with clear headedness and antiemetic properties, propofol is a convenient agent for awake brain surgery [32]. Neuroprotection Neurosurgery can induce ischemic brain damage and trigger neuronal death, the mechanisms of which vary over time and may prolong for several weeks. Excitotoxicity appears to be a critical event in the opening stages of ischaemia. Oxidative stress and inflammatory response initiated afterward directly affect neurons, but also play a key role in triggering delayed apoptotic neuronal death. The capacity of general anaesthesia, as compared to the awake state, to increase neuronal tolerance to hypoxic ischaemic insults has been established for a long time, although this beneficial effect appears to be transient. During the last two decades, the ability of volatile halogenated anaesthetics to reduce ischemic cell death through the anaesthetic preconditioning pathways has been increasingly recognized. Nevertheless, one should keep in mind that Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited. Why we still use intravenous drugs Hans and Bonhomme intravenous agents such as opioids and propofol also have potential neuroprotective properties. Opioids such as morphine, fentanyl and remifentanil have been demonstrated to exert some preconditioning effect on the heart as well as on neuronal cells [11,33,34]. Propofol may affect the biochemical pathway of cell death at different levels. During the last decade, it has been shown to be neuroprotective in vivo, in both focal and global models of cerebral ischaemia. First, this agent has well-established antioxidant properties which are partially attributed to its scavenging effect on peroxynitrite. Propofol has been reported to protect endothelial cells exposed to a peroxynitrite donor and to increase the expression of heme-oxygenase [35,36]. It inhibits the protein nitration induced by activated polymorphonuclear neutrophils [37]. At clinically relevant concentrations, it attenuates the effect of oxidative stress on astrocyte glutamate uptake and retention [38]. Propofol also maintains the capacity of brain cells to extrude protons during oxidative stress [39]. In an experimental model of traumatic brain injury, it has been shown to decrease levels of endogenous indices of oxidative stress [40]. Its neuroprotective effect in models of cerebral ischaemia has been related to its capacity to prevent the increase in neuronal mitochondrial swelling [41]. In addition, one may reasonably assume that the beneficial effect of propofol recently described on lung endothelial injury induced by ischaemia-reperfusion and oxidative stress could be extrapolated to the central nervous system [42]. Second, there is a growing body of evidence suggesting that propofol has antiapoptotic properties. It inhibits neuronal damage after incomplete cerebral ischaemia with reperfusion for at least 28 days after injury [43]. It also reduces spinal cord apoptosis associated with aortic cross-clamping in rabbits [44]. In a rat model of cerebral ischaemia, propofol compared to placebo has been associated with an improvement in neurologic function still observed after 3 weeks, although there was no difference in infarct volume [45]. That antiapoptotic property seems to be partly mediated by its antioxidant effect, i.e. its capacity to inhibit peroxynitrite-mediated apoptosis in astroglia cells [46], but also implies an altered expression of apoptosis-regulating proteins such as Bax and Bcl-2 [43,44,47,48]. Third, several reports suggest that propofol-based anaesthesia favourably influences the pro versus anti-inflammatory cytokine balance when compared to isoflurane [49,50]. This effect is expected to improve neurological outcome since upregulation of pro-inflammatory cytokines such as tumour necrosis factor-a and interleukin-6 is correlated with increased mortality and neurological deterioration [51,52]. Propofol has also been shown to reduce tumour necrosis factor-a-induced human umbilical vein endothelial cell apoptosis [48]. It could therefore be beneficial in so far as inflammation may exacerbate tissue 501 damage by increasing local metabolic demand. Indeed, the increase in local temperature resulting from energetic requirements further activates the inflammatory process and worsens outcome of focal ischaemia [53]. As far as preconditioning is concerned, propofol has no direct preconditioning effect, probably because of its antioxidative properties. Indeed, ischaemic as well as anaesthetic preconditioning involves activation of protein kinase C, mitochondrial potassium ATP channels and the transcription factor NF-kB that is at least partially triggered by activated oxygen species. In contrast, the preconditioning effect of intravenous opioids has already been mentioned above. Nevertheless, propofol could have a beneficial effect upstream and downstream of the preconditioning cascade. Indeed, it inhibits activated oxygen species responsible for damaging lipids, proteins and DNA when produced in excess after ischaemiareperfusion. Its inhibition of NF-kB activation during focal cerebral ischaemia-reperfusion in rats has also been suggested to be one mechanism of neuroprotection [54]. Propofol also increases the ratio of antiapoptotic to proapoptotic proteins which partially mediates the preconditioning effect [43,44,47,48]. Conclusions Although all the objectives of neuroanaesthesia cannot be achieved by using one single pharmacological agent or one single anaesthetic technique, propofol-based intravenous anaesthesia still has a promising future in the field. 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