Neuroanesthesia and Monitoring for Cranial and Complex Spinal Surgery

Preanesthetic Evaluation

The fundamental goals of preanesthetic evaluation are to obtain pertinent patient information, preoperative optimization, risk assessment, and formulation of suitable anesthetic plan. Preanesthetic evaluation is a critical component of the perioperative surgical home model ( Fig. 5.1 ), which incorporates, among other things, efforts to reduce unnecessary interventions that do not have the potential to benefit patients (eg, routine preoperative laboratory studies) as well as efforts to reduce cancellations and postoperative lengths of hospital stay. The preanesthesia evaluation improves perioperative care, prevents delays/last-minute cancellations, and reduces the risk of adverse patient outcomes. It also decreases the cost of unnecessary routine medical consultations.

In addition to the evaluation pertinent to the condition requiring neurosurgery, the preanesthesia evaluation includes eliciting a history of concurrent medical problems (such as hypertension, coronary artery disease, pulmonary disease, endocrinopathy, and renal disease). The severity, degree of control, and potential for preoperative optimization are ascertained. An assessment of the patient's functional capacity is documented. Surgical and anesthetic history is enquired focusing on any difficulties in airway management, allergic reactions to medications (anaphylaxis, latex allergy, contrast dye allergy), postoperative nausea and vomiting, pain control, and any adverse events. Medication history is recorded. Patients on corticosteroids may need supplementation in the perioperative period. Beta-blockers and antihypertensives are usually continued in the preoperative period, although angiotensin-converting enzyme (ACE) inhibitors and anticoagulants may be stopped preoperatively. The patient's social history, including any history of smoking as well as alcohol and illicit drug intake, is documented, given the implications for organ function, drug dosing, and the risk of adverse reactions during surgery (eg, hypertensive crises and myocardial ischemia in patients on cocaine). A family history of malignant hyperthermia and pseudocholinesterase deficiency is critical. Malignant hyperthermia is a rare complication triggered by specific anesthetic agents and could be fatal if encountered; hence avoidance of potential triggering agents is paramount. Pseudocholinesterase deficiency is associated with prolonged recovery from the neuromuscular blocking agents succinylcholine and mivacurium, necessitating avoidance of these muscle relaxants.

The preanesthetic evaluation includes a physical examination to record vital signs (blood pressure, heart rate, respiratory rate, oxygen saturation) and body mass index (BMI). Establishing baseline blood pressure is important in neurosurgical patients to ensure optimal perioperative maintenance of cerebral and spinal perfusion. An increased BMI predicts difficulties with airway management. In addition, obesity is associated with heart disease and diabetes mellitus. Airway examination is a vital component of the physical examination from the anesthesiologist's perspective. Inadequate management of the airway may adversely affect the neurologic outcome. Hence the patient's airway should be assessed carefully for the ease of ventilation and tracheal intubation, in consideration with specific surgical needs such as hemodynamic stability and spinal immobilization. Mallampati scoring, thyromental distance, the presence of overbite or underbite, and neck flexion/extension collectively provide an estimate of the risk of difficult intubation ( Table 5.1 ). Difficult airway should be anticipated in patients with recent supratentorial craniotomy in whom the mouth opening might be significantly reduced secondary to ankylosis of the temporomandibular joint, and in patients with acromegaly or cervical spine lesions. Timely recognition of potential airway difficulty allows proper planning with accessory equipment and resources, as well as formulation of a backup plan, enhancing patient safety.

Review of laboratory tests is important to rule out anemia, thrombocytopenia, renal and electrolyte abnormalities, coagulation abnormalities, and pregnancy (if applicable). Ensuring a current type and screen and antibody screen is invaluable in surgeries associated with major blood loss. According to the American Society of Anesthesiologists Task Force on preanesthesia evaluation, preoperative tests may be ordered, required, or performed selectively on the basis of clinical characteristics for purposes of guiding or optimizing perioperative management. For example, a preoperative resting 12-lead electrocardiogram (ECG) is reasonable for patients with known coronary heart disease, significant arrhythmia, peripheral arterial disease, cerebrovascular disease, or other forms of significant structural heart disease. Hemoglobin or hematocrit, serum glucose and electrolytes, and coagulation studies are indicated in most neurosurgical patients, whereas blood levels of phenytoin may sometimes be required. Patients with dyspnea of unknown origin and patients with heart failure with worsening dyspnea or other change in clinical status may need a preoperative evaluation of left ventricular function.

An important aspect of the preanesthesia evaluation is the identification of high-risk patients in order to provide better perioperative management, inform patients about expected risks, make selective referrals before surgery, order specialized preoperative investigations, initiate preoperative interventions intended to decrease perioperative risk, and arrange for appropriate levels of postoperative care. The American Society of Anesthesiologists (ASA) physical status classification is widely used for this purpose ( Table 5.2 ). The Revised Cardiac Risk Index (RCRI) is a simple and widely used index for predicting major cardiac complications after noncardiac surgery. It incorporates six equally weighted components: coronary artery disease, heart failure, cerebrovascular disease, renal insufficiency, diabetes mellitus, and high-risk surgical procedures.

Patient education is another key component of the preanesthesia evaluation. The ASA guidelines recommend a minimum fasting period of 2 hours for clear liquids and 6 hours for light meals in order to reduce the severity of complications related to pulmonary aspiration of gastric contents. It is now recognized that optimal management of the perioperative period in a nonfragmented fashion producing a continuum of care can enhance recovery, lower medical and surgical complications, and reduce cost and length of hospital stay. This is the concept of enhanced recovery and the perioperative surgical home. The perioperative surgical home is a team-based model of care the goals of which are to guide the patient through the surgical experience and enhance the quality of care, thereby enhancing recovery, improving outcomes, reducing costs, and improving patient satisfaction. Having a systematic preanesthesia evaluation is critical to this model.

Neurophysiology and Anesthetic Neuropharmacology

The major goals in neuroanesthesia are to ensure that the patient is safely anesthetized with adequate analgesia and amnesia, effective control of cardiovascular and respiratory parameters, and maintenance of adequate perfusion to the brain and spinal cord while providing optimal conditions for the surgeons to operate. Anesthetic agents and techniques have different effects on the cerebral circulation, metabolism, and intracranial pressure (ICP) both in normal and pathologic conditions, and they have to be accounted for in successful anesthetic management.

The normal brain receives 14% of the cardiac output while consuming 20% of the oxygen intake. The cerebral blood flow (CBF) is coupled to metabolic needs. The CBF remains relatively constant at approximately 50 mL/100 g/min over a wide range of cerebral perfusion pressures (CPPs), although the actual limits of autoregulation vary. For a given value of blood pressure, the CBF may be either higher or lower than that estimated by the traditional autoregulatory curve. Therefore the management of CBF should be guided by a multifactorial but integrated framework of CBF regulation, especially in patients who are at risk of cerebral ischemia. The CPP is dependent on mean arterial pressure and ICP. The latter is dependent on intracranial blood volume (CBV), brain mass, cerebrospinal fluid volume, and central venous pressure. The cerebral blood flow is profoundly influenced by the PaCO ₂ and, to a smaller extent, the PaO ₂ . Within the physiologic range of 20 to 60 mm Hg, the CBF changes by 3% to 4% per mm Hg change in CO ₂ tension, with an accompanied decrease in CBV within seconds of changing the CO ₂ . Thus acute hyperventilation quickly reduces CBV and ICP. However, excessive hyperventilation may cause iatrogenic ischemia. Prolonged change in systemic CO ₂ tension is accompanied by active transport of bicarbonate in or out of cerebrospinal fluid to restore a normal acid-base balance. Thus the effects of hyperventilation on CBF are not sustained beyond 24 hours. With the onset of hyperventilation, the pH of both CSF and the brain's extracellular fluid space increases, leading to an abrupt decrease in CBF. However, due to alterations in function of the carbonic anhydrase enzyme, there is extrusion of bicarbonate from the CSF, and the pH returns to normal in 8 to 12 hours. Hence only brief duration of mild to moderate hyperventilation should be instituted selectively. Hypoxemia causes vasodilatation of the cerebral vessels and an increase in CBF, but this does not occur until the PaO ₂ is less than 50 mm Hg. These homeostatic mechanisms may be impaired in the neurosurgical patients. Thus cerebral metabolism is depressed in a patient with an altered level of consciousness, ICP may be elevated, flow-metabolism coupling may be lost, autoregulation may be impaired, and the blood-brain barrier may be disrupted. Except in severe injury, CO ₂ reactivity is usually preserved.

In the anesthetized patient, the cerebral circulation is affected by multiple processes: anesthesia causing suppression of cerebral metabolic activity, drug- and dose-related effects of the anesthetic agents on cerebral vasculature, suppression of the sympathetic nervous activity, and disturbance of the systemic hemodynamics. Moreover, cardiac output could alter the cerebral circulation and CBF by its effect on the cerebrovascular resistance. Intravenous anesthetic agents, including thiopental and propofol, are indirect cerebral vasoconstrictors, reducing cerebral metabolism coupled with a corresponding reduction of CBF. Both autoregulation and CO ₂ reactivity are preserved. Ketamine is a weak noncompetitive N-methyl-D-aspartate (NMDA) antagonist that has sympathomimetic properties. Its cerebral effects are complex and are partly dependent on the action of other concurrently administered drugs. Etomidate decreases the cerebral metabolic rate, CBF, and ICP. At the same time, because of minimal cardiovascular effects, CPP is well maintained. Although changes on EEG resemble those associated with barbiturates, etomidate enhances somatosensory evoked potentials and causes less reduction of motor evoked potential amplitudes than thiopental or propofol. However, it may reduce tissue oxygen tension. Dexmedetomidine is a highly selective alpha-2 adrenoreceptor agonist that provides sedation without causing respiratory depression, does not interfere with electrophysiologic mapping except when used in higher doses, and provides hemodynamic stability. It is particularly useful for implantation of deep brain stimulators in patients with Parkinson disease and for awake craniotomies, when sophisticated neurologic testing is required. The cerebral effects of inhaled anesthetics are twofold: they are intrinsic cerebral vasodilators, but their vasodilatory actions are partly opposed by flow-metabolism coupling mediated vasoconstriction secondary to a reduction of the cerebral metabolic rate. The overall effect is unchanged flow during low-dose inhalation anesthesia but increased flow during high doses. Despite the vasodilatory potential of volatile anesthetics, they have been successfully used in neuroanesthesia without any deleterious effects, usually in concentrations less than the minimum alveolar concentration (MAC). However, when the intracranial pressure is raised sufficiently to produce a severe decrease in compliance, omitting them and using a predominantly total intravenous anesthesia (TIVA) may be prudent.

Anesthesia for Craniotomy

The major anesthetic goals for craniotomy are depicted in Fig. 5.2 . Neurosurgical procedures are usually done under general anesthesia with intubation and controlled ventilation (except awake craniotomies). Induction of anesthesia and tracheal intubation are critical periods for patients with compromised intracranial compliance. The goal of the anesthetic technique should be to induce anesthesia and intubate the trachea without increasing ICP or compromising CBF and CPP. Arterial hypertension during induction increases CBV and promotes cerebral edema. Sustained hypertension can lead to marked increases in ICP, decreasing CPP and risking herniation. Excessive decreases in arterial blood pressure can be equally detrimental by compromising CPP. The most common induction technique employs propofol with modest hyperventilation to reduce ICP. A neuromuscular blocker is given to facilitate ventilation and prevent straining or coughing during intubation, both of which can abruptly increase ICP. An intravenous opioid given with propofol blunts the sympathetic response; in addition, a short-acting beta-blocker such as esmolol may be used to prevent tachycardia associated with intubation. Nondepolarizers such as vecuronium or rocuronium are used for intubation. Succinylcholine is the agent of choice for rapid sequence induction or when there are concerns about a potentially difficult airway.

Anesthesia can be maintained with inhalational anesthesia, TIVA, and a combination of an opioid. Even though periods of stimulation are few, neuromuscular blockade is recommended to prevent straining, bucking, or movement while allowing motor evoked potential monitoring. Increased anesthetic requirements can be expected during the most stimulating periods: skin incision, dural opening, periosteal manipulations, including Mayfield pin placement, and closure. Intravenous anesthesia with propofol is commonly used for craniotomy because it produces dose-related decreases in CBF, the cerebral metabolic rate of oxygen (CMRO ₂ ), and ICP; has a rapid onset and offset of action; causes minimal interference with electrophysiologic monitoring; and its metabolic suppressant effect may provide neuroprotection. However, it can decrease the mean arterial blood pressure, and attention should be paid to maintaining CPP. Given the evidence of potential neuroprotective properties, there is resurgence of interest in the use of ketamine for neurosurgery. Opioids are part of the balanced anesthetic technique to provide analgesia in the preoperative period. Traditional opioids such as morphine, meperidine, and hydromorphone are in general not preferred for cranial surgery due to their longer half-life potentially interfering with timely neurologic evaluation. Short-acting agents such as fentanyl and remifentanil are more commonly used for cranial surgery. Remifentanil is a selective mu opioid receptor agonist of high potency, making it a highly capable agent to tackle the variable physiologic stressors that occur during a neurosurgical case. It is rapidly hydrolyzed by nonspecific plasma and tissue esterases: this imparts brevity of action. Its action is precise and is easily titrated. It is noncumulative and offers rapid recovery after administration ceases (attributable to rapid clearance). The context-sensitive half-life is very short (3 to 4 minutes), independent of the duration of infusion. Its unique pharmacokinetic profile makes it highly desirable in neuroanesthesia. Although it can facilitate early neurologic evaluation, its analgesic effect is evanescent and it can cause tolerance and hyperalgesia.

Airway Management in Neuroanesthesia

Careful airway management involving securing and maintaining airway patency and ensuring adequate ventilation and oxygenation is critical for neurosurgical procedures. The American Society of Anesthesiologists provides a practice advisory and a “difficult airway algorithm” that offers guidelines for the evaluation and management of a difficult airway situation. Successful airway management consists of proper preoperative assessment. Making a choice between an awake intubation and an asleep intubation often depends on the anticipated difficulty of mask ventilation. Tables 5.3 and 5.4 list factors that predict difficult mask ventilation and difficult intubation, respectively. The Mallampati score predicts the ability to have a full laryngoscopic view of the airway during intubation. Adequacy of neck extension is important because it enables the patient to be in the sniffing position during intubation, which helps alignment of the long axes of the mouth, the oropharynx, and larynx. An inability to assume the sniffing position is a predictor of difficult intubation. Imaging studies can provide additional information before definitive airway management is decided, depending on the urgency or emergency of the situation. Computed tomography (CT) and magnetic resonance imaging (MRI) may help to determine the nature of a possible difficult airway.

Induction of anesthesia leads to decreases in functional residual capacity (FRC) attributable to the supine position, muscle paralysis, and the direct effects of the anesthetic agents. Preoxygenation is the process of allowing the patient to breathe 100% oxygen through a mask prior to the anesthetic induction in order to replace the nitrogen in the lungs with oxygen, thereby increasing the length of time before hemoglobin desaturation occurs in a patient with apnea. This lengthened apnea time provides a margin of safety while the anesthesiologist secures the airway and resumes ventilation. It is critical to avoid hypoxia and hypercarbia during airway management in neurosurgical patients. The most common technique for the induction of general anesthesia is the standard intravenous induction, which entails the administration of a rapid-acting intravenous anesthetic followed by a muscle relaxant, which improves intubating conditions by facilitating laryngoscopy, preventing both reflex laryngeal closure and coughing after intubation. Rapid sequence induction (RSI) is a variation of the standard intravenous induction commonly used when the risk for gastric regurgitation and pulmonary aspiration of gastric contents is present (eg, in patents with intracranial hypertension). After adequate preoxygenation and while cricoid pressure is applied, and the trachea is intubated without attempts to provide positive pressure ventilation by bag and mask. The goal is to achieve optimal intubating conditions rapidly so as to minimize the length of time between the loss of consciousness and securing the airway with a cuffed tracheal tube. Cricoid pressure involves the application of pressure at the cricoid ring to occlude the upper esophagus, thereby preventing the regurgitation of gastric contents into the pharynx.

In situations of expected difficult mask ventilation and difficult intubation, the safest approach to airway management is to secure the airway while the patient remains awake. Topical application of local anesthetics on the airway is essential in this situation. Lidocaine is the most commonly used local anesthetic for awake airway management because of its rapid onset, high therapeutic index, and availability in a wide variety of preparations and concentrations. Other agents such as benzocaine and cocaine can also be used. Topical anesthesia targets the glossopharyngeal nerve, the superior laryngeal nerve, and the recurrent laryngeal nerve, all of which provide for sensory supply of the oropharynx and larynx. The airway is then secured using fiberoptic intubation. A variety of video laryngoscopes have now become available and have revolutionized airway management. In addition, the supraglottic airway, otherwise called the laryngeal mask airway (LMA), has also enhanced the ability to ventilate the patient. The LMA refers to a diverse family of devices that are blindly inserted into the pharynx to provide a patent conduit for ventilation, oxygenation, and delivery of anesthetic gases without the need for tracheal intubation. The LMA is a pivotal component of the ASA difficult airway algorithm. Fig. 5.3 shows a typical preparation for managing an anticipated difficult airway.

Airway management is critical in cervical spine surgery to avoid injury to the cervical cord. The key in this scenario is maintaining the neck in a neutral position with minimal neck movement during endotracheal intubation. The use of video laryngoscopes is beneficial in this scenario. Studies comparing upper cervical vertebral motion during intubation using direct laryngoscopy, LMA, and fiberoptic intubation show that the fiberoptic produced the least motion in the upper cervical spine. Not surprisingly, fiberoptic intubation is often the preferred method for airway management during cervical spine surgeries.

Osmotherapy and Diuretics

The anesthetic strategies for providing intraoperative brain relaxation and control of ICP are listed in Table 5.5 . Mannitol is the most commonly used osmotic diuretic intraoperatively because of its effectiveness in rapid reduction of brain volume. The doses are usually between 0.25 g/kg and 1 g/kg. Mannitol should be administered by infusion over 10 to 15 minutes and may need to be repeated. If given rapidly, it can increase circulating volume, which although temporary can be dangerous in a patient with poor cardiac function. It can also cause electrolyte imbalance and volume depletion. In the critical care environment, the use of hypertonic saline (HTS) in place of mannitol is increasing. HTS does not cause diuresis and volume depletion and hence may be useful in situations of hypovolemia. A hypertonic saline bolus may be administered in concentrations ranging from 3% to 23.4% with goal of serum sodium between 140 and 150 meq/L. Concentrations > 2% must be given through a central venous catheter. In addition, there are anecdotal reports of HTS being effective in patients who were refractory to mannitol. Loop diuretic (usually furosemide) is sometimes combined with an osmotic diuretic, the rationale being that mannitol establishes an osmotic gradient that draws fluid out of brain parenchyma and furosemide by hastening the excretion of water from the intravascular space, maintaining the gradient. Neurons and glia have homeostatic mechanisms to ensure the regulation of cell volume. Although they shrink in response to the increased osmolarity produced by mannitol, they recover their volume rapidly as a consequence of the accumulation of idiogenic osmoles, which minimizes the gradient between the internal and external environments. One of those idiogenic osmoles is chloride. Loop diuretics inhibit the chloride channel through which this ion must pass and thereby retard the normal volume-restoring mechanism.

Burst Suppression

Pharmacologic burst suppression has been an area of tremendous interest due to its attractive potential to provide neuroprotection in the setting of acute cerebral injury. It has been studied in the setting of both global and focal cerebral injury. Anesthetic agents such as barbiturates, propofol, etomidate, midazolam, and inhalation agents can produce a large reduction (approximately 50%) in the cerebral metabolic rate of oxygen, which could help to protect the brain in situations of CBF compromise. Anesthetic burst suppression has not been found to be of clear benefit in comatose survivors of cardiac arrest and cerebral aneurysm surgery. There is insufficient evidence in the setting of traumatic brain injury to determine benefit or harm. Current indications for pharmacologic burst suppression based on low-level evidence include refractory status epilepticus, refractory intracranial hypertension (such as in traumatic brain injury), and intraoperative neuroprotection during cerebrovascular (such as carotid endarterectomy) surgery. Burst suppression is titrated to electroencephalographic (EEG) monitoring as is often quantified as a burst suppression ratio although an optimal burst suppression ratio is unknown. Maintenance of mean arterial pressure is important during periods of burst suppression, which can result in severe hypotension due to the vasodilatory potential of anesthetics. To ensure collateral flow and perfusion, it is important to keep the systemic pressures elevated with the help of vasopressors and to limit the duration of episodes of temporary occlusion. Burst suppression can often delay the emergence at the end of surgery, and hence reducing its duration to the minimum essential is desirable.

Hemodynamic Management in Cerebrovascular Diseases