Anesthesia

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Chapter 180 Anesthesia

Historically, spine surgery has played “second fiddle” to intracranial neurosurgery in the minds of many anesthesiologists. With advances in surgical technique and concomitant improvements in anesthetic management, however, surgical spine procedures are performed increasingly on elderly and medically compromised patients. Spine surgeons are now able to perform procedures once considered impossible, providing anesthesiologists with some of their most significant clinical challenges. This chapter acquaints spine surgeons with anesthetic concerns and with techniques used during anesthesia for spine surgery.

Preoperative Assessment

Preoperative evaluation allows the anesthesiologist to become familiar with the spine surgery patient’s functional status, coexisting medical diseases, medications, allergies, and anesthetic concerns. It also provides the opportunity to obtain informed consent for anesthesia. A thorough review of symptoms attributable to cardiac or respiratory dysfunction is especially important during the preoperative interview, given that these two systems contribute most to postoperative morbidity. Anesthesiologists can assess airway anatomy and cervical spine mobility to identify patients who may require specialized airway management.

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.

In the patient with chronic SCI, a history of autonomic hyper-reflexia usually portends an intraoperative exacerbation. Autonomic hyper-reflexia is most often seen in patients with sensory levels at or above T4-7. Distention of the stomach, bladder, or rectum is the most common precipitating factor, and removal of the inciting stimulus is the primary treatment. Both spinal anesthesia and deep general anesthesia assist in blocking this reflex response.

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.

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.

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 relaxants provide both optimal intubation conditions during induction of anesthesia and maintenance of muscle relaxation intraoperatively. Paralysis is preferred by some spine surgeons and ensures a “quiet” operative field. Other surgeons prefer to avoid muscle relaxation so that any direct stimulation of peripheral nerves will be readily apparent (nonparalytic anesthetic intraoperative monitoring).

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

The single most important monitor in the operating room is the anesthesia provider. This person is responsible for collecting and analyzing both subjective and objective data about the patient’s vital organ function. The perioperative use of monitoring equipment greatly enhances the ability to perform this vital function. Routine monitoring during spine surgery includes electrocardiography, noninvasive blood pressure measurement, pulse oximetry, end-tidal CO2 levels, temperature, urine output, neuromuscular blockade, and auscultation of breath and heart sounds. More invasive forms of hemodynamic monitoring may be indicated based on the complexity of the operative procedure or the severity of coexisting disease. Electrocardiographic monitoring is useful in detecting myocardial ischemia and cardiac conduction disturbances and in the analysis of arrhythmias.

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

Systemic blood pressure is used as an indirect monitor of organ perfusion. For the majority of elective spine procedures, noninvasive blood pressure monitoring is adequate and sufficient. Invasive monitoring of arterial blood pressure is recommended for patients with a history of significant cardiopulmonary disease, those with preoperative hemodynamic instability, those at risk for significant blood loss, and those who may require a period of postoperative ventilatory support when frequent blood gas measurements are anticipated.

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.

In the majority of spine surgery patients, intravascular volume status can be monitored without invasive central monitoring techniques. For those patients in whom central monitoring is necessary, two practical points should be considered. First, long-arm placement of central lines is the preferred approach to cervical spine procedures because it allows for optimal field avoidance. Second, the absolute accuracy of central monitoring in general, and pulmonary wedge pressures in particular, is questionable in positions other than supine. Thus, patient position for surgery may influence the decision of whether central monitoring is employed.

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

The most important factors contributing to the successful performance of an intraoperative wake-up include adequate preoperative rehearsal with the patient and good intraoperative communication between the surgeon and anesthesiologist about the timing of the wake-up. A wide variety of anesthetic techniques can provide suitable conditions, namely a patient who is free of discomfort, is able to follow commands, and has amnesia for the event. A common technique is a nitrous oxide/narcotic/relaxant-based anesthetic with the addition of a low-dose volatile agent as needed. Frequently, a narcotic infusion provides better control of analgesia and timing of the wake-up. Other possibilities include the use of a nitrous oxide/narcotic/relaxant technique in conjunction with a propofol infusion or a total IV anesthetic using propofol and remifentanil infusions. The choice of anesthetic may be influenced and/or limited by the concurrent use of evoked-potential monitoring.

The general procedure for an intraoperative wake-up is as follows. The wake-up protocol is reviewed in detail with the patient preoperatively. If a nitrous oxide/narcotic/relaxant technique is used, the narcotics, relaxant, and any background volatile agent are discontinued approximately 30 minutes before the anticipated wake-up. The muscle relaxation is monitored using a nerve stimulator, and, if necessary, reversal agents may be given. As a rule, patients become responsive shortly after the discontinuation of the nitrous oxide. Patients are first asked to grip the anesthesiologist’s hand to assess their ability to respond to commands, and then to flex and extend their feet within the direct vision of the surgical team. It is helpful to provide some gentle restraint of their head and arms in case struggling should occur. One should always be prepared to administer a bolus dose of a sedative-hypnotic agent immediately if struggling becomes problematic. The risk of air embolism may be minimized if the wound is packed and flooded with irrigating solution. Generally, the choice of anesthetic matters far less than the skill and attention with which it is administered.

Nonparalytic Anesthesia and Intraoperative Monitoring

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