Brain capillaries contain tight-junctions that limit the passive diffusion of many substances into the brain tissue. The physiology of the BBB facilitates reduction of brain volume by osmotic agents such as mannitol and hypertonic saline, which are not freely permeable. Many pathologic states, including trauma, sepsis, and hemorrhage disrupt the BBB.

Neuroelectrophysiologic monitoring for surgical procedures on the brain and spine is increasing in scope and usage. The fundamental goal of these techniques is to avoid injury to functional pathways from either direct anatomic disruption or ischemia during surgical resection and manipulation.

Techniques include EEG (Electroencephalography) monitoring, monitoring of descending motor pathways and the corticospinal tract via MEP (motor evoked potentials), ascending sensory pathways and dorsal column system via SSEP (somato sensory evoked potentials), local neuromuscular pathways via EMG (electromyography), near infrared monitoring (NIRS), transcranial doppler (TCD), and cranial nerve function monitoring. The positive and negative predictive value of changes in the monitored parameters varies based on the type and location of signal monitored, the anesthetic agents, blood pressure, temperature, and preoperative neurologic deficits.

It is important to understand the use of neuromonitoring in selected cases, to know the basic tract(s) under surveillance, and to understand the general impact of anesthetic agents on these parameters (see Table 19.1)

Maintenance of intraoperative neuromuscular blockade is contraindicated in any cases in which motor response will be monitored. Cortically generated potentials of any kind are significantly depressed by inhaled potent agents, which should be avoided or used in low concentrations during cortical monitoring. Spinal potentials and deeper brain potentials (e.g., auditory) are substantially more resilient to the effects of anesthetic agents and are compatible with a wider range of anesthetics. Benzodiazepines in anxiolytic doses, and most opioid agents in typical analgesic doses have little impact on monitored potentials. A critical caveat is to avoid bolus delivery of anesthetic agents and provide a relatively stable depth of anesthesia throughout the monitoring period. Maintenance of steady blood pressure and core body temperature also fall under the purview of anesthetic management during neuromonitoring.

Anesthetic agents almost universally decrease brain activity, with the exceptions of ketamine and nitrous oxide when used alone (see Table 19.1). For this reason, ketamine and nitrous oxide are often omitted from anesthetic management in intracranial surgery. The decrease in brain activity with other agents (e.g., propofol) correlates with a decrease in global CMR (cerebral metabolic rate). However, inhaled potent agents such as isoflurane will vasodilate the major intracerebral arteries resulting in an overall increase in cerebral blood flow and intracranial volume, which reflects an uncoupling of CMR with CBF and excess perfusion. Inhaled agents are relatively contraindicated in situations of increased ICP or when increased brain volume impedes surgical access to the anatomy of interest. In contrast, the intravenous agents propofol and thiopental decrease both CMR and CBF (i.e., they maintain the normally coupled relationship). Infusions of these agents may be beneficial in the management of patients with increased ICP or used to facilitate surgical exposure in the “tight,” swollen brain.

Other agents commonly used in balanced anesthesia include opioids and benzodiazepines. Generally speaking, these agents have minimal impact on CMR or CBF and are commonly used as part of a balanced anesthetic.

General endotracheal anesthesia is indicated for most intracranial procedures except for the “awake craniotomy” for epilepsy or resection of a lesion in the motor or speech cortex. Invasive monitoring is indicated for all but the most limited neurosurgical procedures (e.g., stereotactic biopsy or Burr hole drainage). An arterial line will facilitate close management of blood pressure, carbon dioxide, serum osmolality, hemoglobin, and oxygenation. Central venous access should be considered based on likelihood of high volume blood loss (e.g., invasive cancer, AVM resection) or air embolus (sitting position). Maintenance with intravenous or inhaled agents should be individualized to the patient and the proposed surgical approach. Opioids should be used judiciously; fentanyl and hydromorphone are most commonly employed. The most stimulating periods of surgery are head pinning, skin incision, and dural opening. Benzodiazepines should be used sparingly to facilitate rapid emergence and postoperative neurologic evaluation. Some anesthesiologists avoid Lactated Ringers because it is hyponatremic and hypo-osmolar. Large volumes of normal saline, however, may produce a non-anion gap metabolic acidosis, which must be considered in assessment of arterial blood gases.

Rapid emergence and extubation is feasible after most neurosurgical procedures. Exceptions include patients with profoundly decreased mental status prior to surgery, significant intraoperative complications, acute traumatic brain injury, marginal surgical hemostasis with high likelihood for re-exploration, and procedures involving critical neural structures of the posterior fossa.

Arteriovenous malformations are abnormal collections of veins and arteries with convoluted vessel contributions that lack capillaries. These lesions may feed functional cortex, which can be studied prior to surgery by selective barbiturate injection in the awake patient. An AVM may be selectively embolized in the radiology suite preoperatively to reduce bleeding.

These procedures are technically challenging, high-risk interventions with unique considerations for anesthetic management. The complexity of the dissection, the risk of rupture, and the surgeon’s plan for CSF drainage, burst-suppression, deliberate hypotension, deep hypothermic circulatory arrest, or temporary clipping should all be outlined in detail during the preoperative preparations.

Blood pressure control is of central importance. Acute hypertension prior to clipping can lead to catastrophic aneurysmal rupture. AVM’s, by nature of the anatomy involved, are generally much less prone to rupture than aneurysms. Intubation, pinning, and incision are times of high risk for this complication. A smooth induction to a deep plane of anesthesia with complete muscle relaxation, generous narcotic administration, glottic topicalization, and brief laryngoscopy is often desirable. Hypertension should be treated immediately with additional intravenous hypnotic agents, rapidly acting vasodilators (nitroprusside; nicardipine), and prompt cessation of stimulation. Aneurysm rupture is a catastrophic, albeit rare complication. Transient cardiac standstill using IV adenosine is occassionally used after rupture to facilitate surgical exposure and control of bleeding. Blood loss can be substantial and sudden. Hypotension can be problematic during AVM resection due to low flow venous outflow pathway.

For the advanced student these key questions (with no clear answers) serve as excellent starting points for reading on current topics and intraoperative discussion with both residents and faculty. References are provided for further reading and to stimulate discussion.