Anesthesia in neurosurgery embodies a vital element in the development of neurosurgical intervention. This undisputed interest has offered surgeons and anesthesiologists an array of anesthetic selections to utilize, though with this allowance comes the equally essential requirement of implementing a maximally appropriate agent. General anesthetic agents can be categorized as those administered intravenously or inhaled, both pairs granting unique advantages. The latter of which can be further subdivided into either volatile or non-volatile agents. As the methods of administration vary, so do the proposed mechanisms of action for these substances.
1. Introduction
As neuroanesthetic induction and maintenance must balance the preservation of stable hemodynamics and perfusion of cerebral tissues with alterations to intracranial pressure that such perfusion may incur, it is pertinent to note the systemic effects of these various anesthetic agents in neurosurgical procedures
[1]. This consideration not only ensures that the metabolic demands of the organs of the central nervous system are met, but also reduces injury and trauma to them. Most anesthetic agents, with the exception of ketamine, reduce the overall global oxygen metabolism of the brain, whereas the cerebral blood flow is increased by volatile anesthetics, nitrous oxide, and ketamine but reduced by other intravenous agents
[1]. Such changes in physiological parameters impinge upon several endogenous mechanisms including cerebral autoregulation, vasomotor reactivity, and neurovascular coupling, of which anesthetic agents have varied effects
[1].
Cerebral autoregulation entails adjustments to vascular tone that are sensitive to both long-term and short-term changes in systemic blood pressure
[2]. These changes are hypothesized to be regulated myogenically or endothelially in response to stress on the vasculature, neurogenically by the autonomic nervous system and neurons, and to some extent metabolically
[3]. Volatile anesthetic agents have been found to reduce cerebral autoregulation at higher doses, and propofol in particular has been found to reduce it at levels >200 mcg/kg/min
[1]. In addition, synthetic opioids have been found to increase cerebral blood flow by vasodilation as a result of such cerebrovascular autoregulation
[4]. The vasomotor reactivity of cerebral arterioles arises from pH changes to vasculature and surrounding smooth muscles, and as such, is largely dependent on the partial pressure of carbon dioxide: increases in PaCO
2 have been shown to have vasodilatory effects, increasing blood flow
[1]. The vasomotor reactivity is reduced at higher concentrations of certain volatile agents and has been shown to be variably affected by propofol
[1]. Neurovascular coupling can be thought of as a combination of phasic and tonic vasomodulation regulated by the production of substances and metabolites by neurons and astrocytes, respectively. The role of these various anesthetic agents on neurovascular coupling has yet to be fully determined but they have been shown to affect neural activity as well as subsequent vasoactive signal transmission, and vasoactive response as well
[1][5]. Most notably, etomidate, propofol, and barbiturates have been found to decrease cerebral blood flow secondary to decreased cerebral metabolic demands
[1].
Intracranial pressure, determined by the pressure within the skull exerted by its content in the form of blood volume, tissue volume, and cerebrospinal fluid volume, is functionally related to cerebral perfusion pressure, and affected by overall cerebral blood flow
[6]. As such, alterations in cerebral blood flow following the use of anesthetic agents subsequently coincide with changes in intracranial pressure, as demonstrated through the Cushing reflex equation:
where CPP is cerebral perfusion pressure, MAP is mean arterial pressure, and ICP is intracranial pressure
[3].
2. Total Intravenous Anesthetics
The first mention of total intravenous anesthesia (TIVA) in the literature dates to 1872 in a report by Pierre-Cyprine describing the use of chloride hydrate
[7]. Following the introduction of propofol in 1977, the archetypal TIVA, TIVA utilization rapidly increased
[8]. Propofol increases GABA-mediated chloride channels in the brain resulting in inhibitory tone in the CNS. Furthermore, the drug increases the duration of the GABA’s effects by decreasing dissociation from its receptor, leading to the hyperpolarization of the cell membrane
[9]. Its effects have a rapid onset and are short-acting, which allows for rapid recovery post-surgery and evaluation of neurological function
[10]. Propofol also decreases intracranial pressure, cerebral blood flow, cerebral metabolism, and edema while supporting cerebral perfusion pressure and mean arterial pressure. This aggregate of effects is neuroprotective during cerebral ischemia
[11]. Furthermore, the administration of propofol with an opioid provides hypnosis, amnesia, and minimizes response and movement to surgical stimulation, achieving all of the factors of a true anesthetic while also decreasing postoperative nausea and vomiting
[8][10]. TIVA is now generally classified as the combined use of a hypnotic agent (e.g., propofol) and an opioid (e.g., fentanyl or remifentanil) without concurrent use of inhaled anesthetics
[12][13]. Additionally, TIVA is conducive to new considerations that have emerged in neurosurgery anesthesia such as minimizing affected brain function and electrophysiological monitoring. Due to propofol’s rapid onset and short duration, infusion rates can be adjusted to allow for patient cooperation when necessary during a procedure
[10]. Current common intravenous agents include thiopentone, propofol, etomidate, ketamine, benzodiazepines, and opioids
[11].
3. Volatile Anesthetics
Diethyl ether was identified centuries ago and may have been originally compounded by an 8th-century Arabian philosopher. However, it was not until 1842 that the first use of ether for surgical anesthesia was documented
[14]. More than 150 years after this discovery of a form of general anesthesia, volatile agents are still in clinical use today
[15]. While the precise mechanism of action of volatile anesthetics remains largely unknown, studies have demonstrated that they are active in the central nervous system, augmenting GABA receptors and stimulating potassium channels
[16]. Additionally, these agents have a role in depressing excitatory pathways including acetylcholine receptors, glutamate receptors, serotonin receptors, muscarinic receptors, and nicotinic receptors
[16]. They are most often administered through a face mask, laryngeal mask, or endotracheal tube. While induction with volatile agents is often preferred in infants and young children because it allows for a needleless experience while awake, adult patients typically undergo an intravenous induction to reduce risks inherent to a full-grown patient. In a randomized double-blind comparison of 8% sevoflurane and propofol as anesthetic agents, Thwaites et al. found that induction with sevoflurane was associated with a lower rate of apnea, decreased time to establish spontaneous ventilation, and a smoother transition to maintenance. Furthermore, emergence was found to be earlier with sevoflurane
[17]. While volatile agents have few absolute contraindications beyond gene variants for malignant hyperthermia, in the field of neurosurgery, these agents have concerns as they have been shown to decrease cerebral perfusion and increase intracerebral pressure
[13][18]. Some of the most common volatile anesthetics in use today include halothane, isoflurane, desflurane, and sevoflurane.
4. Advantages and Disadvantages of Anesthetic Options
When administering anesthesia to neurosurgical patients, the onus is on the anesthetist to gauge whether inhaled or intravenous anesthetics would be more advantageous. Three of the most significant considerations regarding a neurosurgical anesthetic are the effects on hemodynamics, intracranial pressure, and postoperative conditions
[11]. Hemodynamic stability is needed to maintain cerebral autoregulation
[19]. Furthermore, the intrinsic autoregulation of factors (e.g., partial CO
2 pressure and mean arterial pressure) that influence hemodynamic stability may be lost via the circumstances precipitating neurosurgical management
[20][21]. Therefore, hemodynamic consideration is especially important within the context of neurosurgical anesthesia. In a randomized study, Strebel et al. demonstrated that volatile agents can impair autoregulation while propofol is helpful in preserving it
[22]. Furthermore, Van Hemelrijck et al. demonstrated that the use of a propofol-loading infusion did not change the measure of blood pressure or heart rate, demonstrating the benefits of TIVA within the context of hemodynamics
[23]. Low intracranial pressure is a significant factor allowing for optimal operating conditions and can also be neuroprotective. In a randomized prospective study, Petersen et al. found that subdural intracranial pressure was lower and cerebral perfusion pressure was higher in patients anesthetized by propofol compared to sevoflurane- or isoflurane-anesthetized patients
[24]. This finding suggests that TIVA is beneficial in neurological surgery by minimizing local hypoperfusion and cerebral ischemia. Following surgery, rapid recovery from anesthesia is important to allow for neurological examination. Regarding the duration of effect, both TIVA with propofol-remifentanil and volatile agents have short half-lives, allowing for a rapid recovery after surgery
[25]. This is supported in the literature as demonstrated by a randomized sample of patients undergoing craniotomy. Here, Talke et al. demonstrate that there was no difference between early post-operative recovery variables such as recovery time and early cognition
[26]. An appraisal of the present literature of randomized-controlled trials comparing anesthetic agents head-to-head across various peri and postoperative parameters are included in
Table 1 and
Table 2 [23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55][56][57][58][59][60][61][62][63][64].
Table 1. Published trials of general anesthetic agents for cranial procedures.
Table 2. Published trials of general anesthetic agents for spinal procedures.