Many neurosurgical interventions require specific anaesthetic measures to establish optimal surgical conditions, for example craniotomy, brain tumour surgery or aneurysm clipping. Some previous studies tried to identify the optimal anaesthetic drug (volatile or intravenous) that does not interact with cerebral homeostasis. Ideally, this drug should have both neuroprotective and anticonvulsive properties and exert additional general effects on the patient (e.g. antiemetic properties, cardiovascular stability, favourable pharmacokinetics).1,2 However, there are no recent and solid data covering this topic and presenting clear evidence-based results.
All anaesthetic drugs may affect cerebral vasculature, metabolism or autoregulation and, therefore, influence intracranial pressure, cerebral blood flow and cerebral volume. Propofol is known to reduce cerebral metabolism, cerebral blood flow and intracranial pressure,3,4 and thus has been the preferred agent in many neurosurgical patients for some time and is still the first choice for patients with elevated intracranial pressure and low intracranial elastance.2 However, it is still unclear to what extent, or even if, volatile agents may alter intracranial pressure or cerebral blood flow as compared with propofol. In this letter, we review previously published data on variations of cerebral blood flow and intracranial pressure by volatile anaesthetics with regard to minimal alveolar concentration (MAC).
By an extensive analysis of the database PubMed (http://www.pubmed.com), we identified all studies in the English language that presented intraoperative data for different volatile agents [cerebral blood flow, measured as velocity (Vmca; cm s−1); intracranial pressure (mmHg)] in relation to the minimal alveolar concentration used (Table 1). For the search strategy, the terms ‘volatile anaesthesia’, ‘inhalational anaesthesia’, ‘CBF’, ‘cerebral blood flow’, ‘ICP’, ‘intracranial pressure’, ‘neurosurgery’, ‘craniotomy’ and ‘anaesthesiology’ were used in a varying order. Publications were independently screened by two colleagues and relevant articles selected for this analysis.
Eleven studies were identified with patient numbers ranging from n = 6 to 117 (Table 1). Six studies comparing cerebral blood flow were found with a combined patient total of 124 (number of studies using isoflurane = 3, sevoflurane = 4, desflurane = 2). Concerning intracranial pressure, six studies were found with a total of 502 patients (number of studies using isoflurane = 6, sevoflurane = 4, desflurane = 3).
Although there seemed to be a tendency for higher cerebral blood flow in the isoflurane group as compared with the sevoflurane group in most of the studies, no clear-cut differences can be extracted because of the lack of sufficient data. For all MAC values investigated there were no clinically relevant differences for volatile agents as compared with propofol. Concerning intracranial pressure, values were in the physiological range at 0 MAC to 1 MAC (isoflurane, sevoflurane and desflurane), lying between 9 and 12 mmHg. No data were available for intracranial pressure at 1.5 MAC in the previously published studies.
One additional study described the differential effects of propofol vs. isoflurane in 13 ICU patients with subarachnoid haemorrhage. The authors found an increased cerebral blood flow but no changes in intracranial pressure during isoflurane sedation5 – presumably because of disease-related uncoupling of cerebral blood flow and intracranial pressure. In this study, the main observation was that isoflurane safely increased cerebral blood flow without increasing intracranial pressure.
There is very scarce information about the cerebral effects of volatile anaesthetics (i.e. cerebral blood flow and intracranial pressure) in craniotomy patients. In the studies available, the number of patients, underlying pathologies, methods of assessment (central intracranial pressure or lumbar spinal fluid pressure), agents used for the induction and maintenance of anaesthesia (thiopentone, propofol, fentanyl, remifentanil) as well as techniques used to quantify the effects differ greatly. Compared with desflurane, there were more data on isoflurane or sevoflurane, presumably as these agents are older and thus more often investigated (publication dates range back to the early 1990s). Furthermore, from a pathophysiological point of view, increases in cerebral blood flow could even be beneficial and desirable in some conditions whereas they could be potentially deleterious in others. The relevance of cerebral blood flow and intracranial pressure could, therefore, depend on the particular situation.
In neurosurgical patients recent studies have shown no relevant disadvantages of inhalational anaesthesia nor have they shown a clear benefit for propofol anaesthesia compared with inhalational anaesthesia. Because of this lack of adequate data no valid conclusions as to the most appropriate anaesthetic may be drawn at this point. Therefore, further research in this field is urgently required to provide valid data. Only then can a reliable decision be made regarding the pros and cons of inhalational anaesthesia versus propofol for craniotomy patients.
This commentary is intended to underline the necessity for further more standardised studies (especially randomised controlled trials) in neurosurgical patients so as to clearly define the effects of volatile anaesthetics on cerebral blood flow and intracranial pressure and to establish appropriate guidelines with regard to anaesthetic drugs in neuroanaesthesia.
Acknowledgements relating to this article
Assistance with the study: none.
Financial support and sponsorship: none.
Conflicts of interest: none of the authors received funding or honoraria in connection with this letter. JH received honorarium from Baxter GmbH (Munich, Germany).
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