Chapter 180 Anesthesia
Preoperative Assessment
Laboratory Studies
Routine laboratory screening tests, including coagulation studies, rarely reveal abnormalities that were not apparent from the history and physical examination. It is reasonable to obtain a preoperative hemoglobin and hematocrit in patients with coexisting disease or a history of anemia and serum electrolytes in patients being treated with diuretics. Chronic hypokalemia, to a serum potassium of 3 mmol/L, is not a contraindication to elective surgery in the absence of cardiac comorbidity, symptomatic arrhythmias, or digitalis therapy.1 Symptomatic chronic hypokalemia in the perioperative period requires oral replacement on an outpatient basis because rapid IV supplementation may increase morbidity and mortality.2 Preoperative electrocardiograms and chest radiographs should be limited to elderly patients or to those with known or suspected cardiopulmonary disease.
Considerations in Patients with Spinal Cord Injury
Preoperative considerations in spinal cord injury (SCI) patients vary with the timing of surgery in relation to the time of and the anatomic level of injury. Patients who show symptoms of neurologic deficits secondary to acute SCI are the most challenging. SCI patients must have sufficient respiratory muscle strength to oxygenate and ventilate effectively. They may have impaired coughing ability and significant ventilation-perfusion mismatch.3 Pneumonias are common in patients with acute or chronic SCIs due to the high incidence of aspiration and pulmonary dysfunction with lesions above T7.4 The potential for associated injuries related to trauma, including rib fractures, pneumothoraces, closed-head injuries, and pelvic fractures, must be considered. Most of these patients show symptoms of varying degrees of hypotension as well as impaired myocardial contractility resulting from acute sympathetic denervation. They require judicious volume loading and, often, vasopressors and/or inotropes to maintain adequate organ perfusion pressure. Depolarizing muscle relaxants may result in fatal hyperkalemia.
Considerations in Patients with Scoliosis
It is important to evaluate the degree of preoperative pulmonary compromise in the patient with scoliosis. Pulmonary function tests are quite useful in this patient population. The forced vital capacity (FVC) and forced expiratory volume in 1 second (FEV1) are the best indicators of the extent of restrictive lung disease caused by the thoracic deformity. Baseline arterial blood gas measurement or preoperative oximetry may also be helpful in guiding postoperative care.
Considerations in Patients with Rheumatoid Arthritis
Patients with rheumatoid arthritis must be carefully evaluated for the extent of their systemic disease so that the risks of surgery and anesthesia may be minimized. Deformities produced by articular involvement may make intravascular catheter placement difficult and increase the risk of positioning-related injury. Cervical spine films should be obtained because up to 30% of these patients may have asymptomatic cervical instability.5 Cervical spine instability or significant temporomandibular joint disease may require awake fiberoptic airway management and strict attention to positioning. The electrocardiogram should be examined for the presence of conduction abnormalities, and an echocardiogram should be obtained if there are any history or physical examination findings compatible with valvular dysfunction. The serum blood urea nitrogen (BUN) and creatinine should be checked to assess renal function in patients taking high doses of nonsteroidal anti-inflammatory drugs. Liver function tests are useful in patients taking cytotoxic drugs. Finally, stress-dose steroids should be ordered for all patients with a recent history of steroid use.
Pharmacology
Preoperative medications serve a variety of functions, including sedation, amnesia, anxiolysis, and aspiration prophylaxis. The goal of premedication in the neurosurgical patient is to provide anxiolysis with minimal sedation at the termination of surgery. Benzodiazepines have largely supplanted barbiturates and anticholinergics for this purpose. The reliability of midazolam in the immediate preoperative period has greatly reduced the need for longer-acting premedicants. H2 blockers raise the pH of gastric fluid6 and usually decrease gastric volume in patients at risk for aspiration. However, the routine use of histamine blockers for aspiration prophylaxis in patients not at risk is difficult to justify, given their cost.
Induction
Agents used to induce anesthesia include barbiturates, narcotics, benzodiazepines, and a variety of other unclassified drugs. Since the early 1940s, the barbiturates have been used for this purpose and reliably decrease cerebral blood flow, cerebral metabolic rate of oxygen (CMRO2), and intracranial pressure (ICP). Treatment with barbiturates after a global ischemic event does not appear to provide neuronal protection. Barbiturate administration after focal or partial ischemic events, however, seems to provide some protection from neurologic injury.7 These agents have limited use in maintaining anesthesia secondary to their prolonged effects. Most of them depress cardiac output and systemic vascular resistance, so care must be taken when they are given to a hypovolemic or traumatized patient.
Propofol
Propofol, a sedative-hypnotic agent, possesses all the benefits of the barbiturates with regard to reduction of cerebral blood flow and CMRO28 Propofol is cleared rapidly and produces prompt awakening in patients shortly after an infusion is discontinued. The autoregulatory capacity of the cerebral circulation remains intact during propofol anesthesia.9 To date, there is little experimental evidence indicating that propofol provides a significant degree of neurologic protection in temporary focal ischemia models. The only animal study suggesting a protective benefit of propofol in burst-suppressive doses failed to measure or control cerebral perfusion pressure.10
Ketamine
Ketamine, a phencyclidine derivative, differs from most induction drugs in that it raises CMRO2, blood flow, and ICP.11 It is thus less ideal for neuroanesthesia, but these are desirable properties for use in the hypovolemic patient. Ketamine preserves central circulating volume and afterload in patients with traumatic spinal cord lesions secondary to the release of endogenous catecholamines. However, in severely hypovolemic patients who have exhausted their sympathetic reserve, the bolus administration of ketamine may result in hemodynamic collapse because of its unopposed direct myocardial depressant effects.
Inhalation Agents
Inhalation anesthetics are the agents used most commonly for the maintenance of general anesthesia. Their mode of delivery and pharmacokinetics allow for controlled, predictable action and easy reversal. They are typically mixed with inspired gases via vaporizers, which are devices that make adjustments for temperature, flow rate, and anesthetic vapor pressure so that a known quantity can be delivered over a wide range of conditions. The inhalation agents act on the brain via an unknown mechanism. Hypothesized mechanisms include membrane protein inhibition and membrane depolarization through membrane swelling or carrier protein inhibition.12 Anesthetic potency parallels the lipid solubility of the agent. A standard known as the minimum alveolar concentration (MAC) is used as a guide to compare anesthetics of different potency. One MAC of any anesthetic is the end-tidal concentration that will render 50% of patients immobile to the surgical incision. The MAC for different anesthetic agents is additive; 0.5 MAC of nitrous oxide mixed with 1 MAC of isoflurane yields 1.5 MAC of anesthetic.
A number of factors determine the rate of increase of the partial pressure of an anesthetic in the brain, and hence its speed of onset. These factors include the concentration of the anesthetic delivered, solubility of the anesthetic in both the blood and the brain, alveolar ventilation, cardiac output, and presence of intrapulmonary or intracardiac shunts.13 For example, nitrous oxide is a poorly soluble gas with a MAC of 105% that is routinely delivered in high concentrations (50–70%) and has the most rapid onset of action. Isoflurane has intermediate solubility, a MAC of 1.2%, and a slower onset.
The inhalation anesthetics currently in common use include isoflurane, desflurane, and sevoflurane. They all possess cerebral vasodilator properties and decrease blood pressure by reducing either cardiac output or systemic vascular resistance. The increased cerebral blood flow seen with isoflurane can be attenuated by hyperventilation and a reduction in the partial pressure of carbon dioxide (Pco2).14 Desflurane and sevoflurane are both less soluble in blood than isoflurane and possess the theoretic advantage of more rapid emergence. Their effects on the cerebral vasculature parallel those of isoflurane,15 although sevoflurane appears to preserve the autoregulatory ability of the cerebral vasculature at higher MAC levels than either isoflurane or desflurane. Nitrous oxide is the least potent and most used inhalation agent and exhibits a favorable safety profile in spine surgery. It causes a mild rise in blood pressure and ICP when used alone. It is not clear if any inhalation agent confers specific advantages in spinal cord surgery, and agent choice should be dictated by the overall anesthetic plan.
Narcotics
Remifentanil, one of our newest narcotic agents, is unique in that it does not demonstrate any significant accumulation over prolonged periods of infusion. Recovery from remifentanil is essentially dose-independent because of its rapid esterase metabolism.16 Remifentanil is particularly useful in cases involving somatosensory-evoked potential (SSEP) or motor-evoked potential (MEP) monitoring as well as any case requiring a total intravenous anesthetic (TIVA). Because of the rapid offset of remifentanil, which is faster than the onset of most other analgesics, care must be taken to provide supplemental analgesics prior to stopping remifentanil in cases where substantial postoperative pain is anticipated.17
Muscle Relaxants
The use of muscle relaxants in spine surgery optimizes the conditions for intubation, provides an immobile surgical field, and reduces the risk of patient coughing and straining. Muscle relaxants may be broadly classified into two groups: depolarizing and nondepolarizing. Succinylcholine, the only depolarizing agent approved for use in the United States, has a rapid onset and short duration, qualities that make it useful when rapid intubation conditions are desired. This agent actively depolarizes the muscle at the myoneural junction until it becomes refractory to further stimulation. Typically, the administration of succinylcholine produces a 0.5-mEq/L rise in serum potassium.18 Succinylcholine also depolarizes extrajunctional acetylcholine receptors in patients with burns or denervation injuries. These receptors are more numerous and have a greater ionic permeability, leading to acute, profound hyperkalemia when stimulated.19 The risk of hyperkalemia is greatest after 3 to 7 days after injury and may persist for several years.20 Life-threatening succinylcholine-induced hyperkalemia has been hypothesized but not reported after immobilization or disuse atrophy in the absence of other causal factors.21 Succinylcholine is also a triggering agent for malignant hyperthermia and is contraindicated in any patient with a family history of malignant hyperthermia or a history of degenerative muscular disease. The routine use of succinylcholine is also contraindicated in children based on several reports of postadministration hyperkalemic cardiac arrest presumed secondary to unrecognized or undiagnosed muscular dystrophy.
The nondepolarizing muscle relaxants, including pancuronium, vecuronium, rocuronium, and cisatracurium, differ from one another primarily in onset and duration of action. These agents all bind to the myoneural junction and competitively inhibit the binding of acetylcholine. The extent of neuromuscular blockade is monitored intraoperatively in a number of ways. The most reliable method is with the use of a train-of-four (TOF) monitor. This device allows for subjective or objective comparison of the ratio of the first and fourth muscle stimuli, which correlates well with the density of receptor occupation. A ratio less than 0.25 correlates with dense paralysis, and a ratio greater than 0.75 correlates well with the patient’s ability to maintain protective airway reflexes after extubation.22
Muscle relaxation is reversed by the administration of anticholinesterase agents. These agents reliably reverse a blockade when the effects of the nondepolarizing muscle relaxant have begun to fade. Because these compounds increase acetylcholine levels at all cholinergic receptors, they are usually given in conjunction with a muscarinic anticholinergic drug (e.g., atropine or glycopyrrolate) to prevent unwanted bradycardia, salivation, and bronchial secretions.
The most important factors that affect the ability to reverse muscle relaxation are the depth of block at the time of reversal, choice and method of administration of relaxant, and dose of reversal agent. Other factors that may antagonize the ability to reverse a nondepolarizing blockade include hypothermia, metabolic acidosis, respiratory alkalosis, and the administration of certain antibiotics.23 As previously mentioned, reversal is followed with the TOF monitor. The best clinical assessment of adequate reversal is the ability of the patient to sustain an unassisted head lift for at least 5 seconds. The assessment of less cooperative patients can be carried out by observing the negative inspiratory force generated during spontaneous ventilation. A negative inspiratory force of at least –25 cm H2O correlates well with adequate reversal but not airway protection.24
Monitoring
General Monitoring
Patients with both acute and chronic cervical spine injuries may show symptoms of a variety of specific electrocardiographic abnormalities. These abnormalities have been attributed to the autonomic imbalance created by disruption of sympathetic pathways located in the cervical cord. Severe acute cervical spine injury is frequently associated with marked sinus bradycardia. It also carries an increased incidence of ventricular and supraventricular arrhythmias, as well as cardiac arrest, when compared with injury of the thoracolumbar spine.25 Multilead ST-segment elevation has been noted in a significant percentage of patients with chronic, complete SCI. These alterations in ventricular repolarization are hypothesized to be manifestations of central sympathetic dysfunction and, indeed, resolve with low-dose isoproterenol infusion.26
Central monitoring of venous or pulmonary artery pressure may be indicated in patients with a history of ischemic heart disease or left ventricular dysfunction, particularly in the setting of anticipated large blood loss or fluid shifts. In patients with normal cardiac function, central venous pressures provide an adequate estimate of left ventricular end-diastolic volume. A pulmonary artery catheter, however, may more accurately assess left ventricular volume in patients with ventricular dysfunction. Acute cervical spine injury with spinal shock is associated with substantial hemodynamic lability and a high incidence of left ventricular dysfunction.27 Spinal shock patients are less tolerant of aggressive fluid replacement and more prone to develop pulmonary edema. The acutely quadriplegic patient qualified for surgery should be monitored with both an arterial line and either a central venous or pulmonary artery catheter.
Neurophysiologic Monitoring
Awake Patient
The awake patient is the ultimate spinal cord monitor. Several case reports describe the use of local anesthesia for spine surgery in the awake patient, although it is not a common means of neurologic monitoring. Chang28 and Drummond et al.29 both describe the use of anesthesia by local infiltration for dorsal cervical osteotomy. From these descriptions it appears that at least a short period of unconsciousness may be required because of significant discomfort associated with the fracturing of the anterior longitudinal ligament. Zigler et al.30 presented a series of 34 consecutive cases of dorsal cervical stabilization and fusion in patients with unstable cervical spines and variable degrees of neurologic injury using local anesthesia in conjunction with light sedation. They encountered no untoward complications and found that the technique was well tolerated by patients, although occasionally bone graft harvesting under local anesthesia was uncomfortable.
Wake-Up Test
In 1973, Vauzelle et al.31 described their use of an intraoperative “wake-up” with observation of limb movement for the assessment of spinal cord function. This simple test is an excellent monitor of gross motor function and is used most commonly during surgical procedures involving spinal column instrumentation and distraction. Its use is based on clinical evidence that neural impairment resulting from distraction is reversible when the distracting forces are modified during its early phase.29,32 Currently, an awake patient is the only available monitoring modality to provide unequivocal intraoperative documentation of intact motor function.
An advantage of the wake-up test over more highly technical forms of neurophysiologic monitoring is that specialized equipment or ancillary monitoring personnel are unnecessary. Two limitations are (1) that the patient can only be awakened intermittently and, therefore, the anesthesiologist and surgeon are restricted to a few spot checks of the integrity of motor pathways; and (2) it is possible that neurologic impairment may occur despite a successful wake-up test. Diaz and Lockhart33 reported one case of unresolved paraplegia after a normal wake-up test. This test may be difficult or impossible to perform in young children, patients with cognitive difficulties, and those with significant hearing impairment. A number of complications of this technique have been described, including dislodgement of spinal hardware, displacement of IV lines and monitors, accidental extubation, air embolism, and the possibility of intraoperative recall. These complications appear to be uncommon in the clinical setting, although they are always a reason for concern.32,34