3 Anesthetic Considerations for Surgery of the Thoracic Spine Abstract The anesthetic considerations for surgery of the thoracic spine must include a preoperative evaluation concentrating of comorbidities which will impact surgical outcome; an intraoperative anesthetic which permits spinal cord neuromonitoring; techniques for blood conservation and preserving hemodynamic stability; and a plan for postoperative analgesia. A specifically designed total intravenous anesthesia will allow multimodal neuromonitoring such that spinal cord and nerve root injury will be detected before the development of permanent damage. In addition, hemodynamic stability and the prevention of excessive blood loss are required to prevent the devastating complications of complex thoracic surgery, spinal cord ischemia, and perioperative loss of vision. Thoracic surgery is painful, necessitating an intraoperative plan for postoperative pain management. Keywords: total intravenous anesthesia, intraoperative neuromonitoring, postoperative visual loss, antifibrinolytics, noninvasive hemodynamic monitors, blood conservation techniques Clinical Pearls • The evaluation of a patient prior to thoracic spinal surgery should pay particular attention to the patient’s airway, pulmonary, cardiovascular, and neurological status, as they all have the potential to be affected by the patient’s surgical disease. • Cardiovascular dysfunction can be due to a number of factors in this patient population: a direct result of the pathology requiring corrective spinal surgery, pulmonary hypertension and right ventricular hypertrophy, and/or age-related ischemic heart disease. • All patients for thoracic spine surgery should have a neurological examination which assesses and documents any preexisting neurological deficits and under which circumstances they are exacerbated. This is essential in order to be able to identify new neurological deficits that appear postoperatively. • For patients with unstable cervical spines and significant cervicothoracic deformities (rheumatoid arthritis, achondroplasia, ankylosing spondylitis) awake fiberoptic endotracheal intubation is the safest approach for tracheal intubation. • The general anesthetic for these procedures is particularly challenging since it must provide analgesia, amnesia, and hemodynamic stability without compromising intraoperative neuromonitoring (IONM). This monitoring includes somatosensory, motor evoked potential, and electromyography monitoring. • Total intravenous anesthesia is the preferred anesthetic for complex thoracic spine procedures because IONM is less affected than with inhalational anesthetics. • Changes in IONM could be due to factors other than anesthetics, including surgical causes, hypotension, anemia, metabolic acidosis, and hypothermia. • Blood loss and the need for homologous blood transfusions during complex spine surgery can be minimized by proper positioning of the patient to reduce intra-abdominal pressure, surgical hemostasis, deliberate controlled hypotensive anesthesia, reinfusion of salvaged blood, intraoperative normovolemic hemodilution, the use of pharmacological agents which promote clot formation, and the preoperative donation of autologous blood. • The role of the anesthesiologists during complex spine surgery is to maintain end-organ perfusion despite large blood losses in an attempt to prevent devastating complications such as spinal cord ischemia, postoperative visual loss, renal failure, myocardial ischemia, and stroke. • Thoracic spine patients will experience considerable postoperative pain which is best treated with a multimodality approach. Thoracic spinal surgeries are common procedures that are performed for a variety of different pathologies. These can range from surgical decompression for spinal stenosis, urgent procedures for trauma or infection, microsurgical techniques to excise spinal cord tumors, and large corrective procedures for deformities such as scoliosis. The procedures can be simple or involve posterior fusions at multiple levels, as well as anterior thoracotomies that result in considerable blood loss. This review will concentrate on the perioperative anesthetic management of adults undergoing thoracic spinal procedures. Regardless of the indication, all patients presenting for thoracic spinal surgery should receive a thorough preoperative evaluation with particular concern for the patient’s surgical pathology and the urgency of the planned procedure. Complications after spinal surgery are associated with surgical complexity, age of the patient, and preexisting comorbidities.1,2,3 There are specific types of patients who are more likely to have spinal surgery and are more likely to have perioperative complications. Trauma involving the spine will often involve other vital organs which must also be evaluated prior to surgery. Spinal tumors may be metastatic cancers hence compromising the function of other organs or include chemotherapeutic agents which will have an impact on the anesthetic plan. Osteoarthritis, a disease of the aging, is a significant risk factor for degenerative disc disease and spinal stenosis. Hence older patients with multiple comorbidities are more likely to have spine surgery. The evaluation of a patient prior to thoracic spinal surgery should pay particular attention to the patient’s airway, pulmonary, cardiovascular, and neurological status, as they all have the potential to be affected by the patient’s surgical disease. The potential for a difficult tracheal intubation is especially pronounced in patients with cervical and upper thoracic spine disease. The patient’s previous anesthetic records should also be reviewed for any history of a difficult airway. The preoperative airway evaluation involves a general assessment of cervical mobility, examination for masses, tracheal deviation, mouth opening, state of dentition, and thyromental distance. An evaluation of the patient’s ability to flex and extend the neck is also performed. Comorbid conditions that may restrict mouth opening, limit cervical range of motion, and alter airway anatomy include osteoarthritis, rheumatoid arthritis, achondroplasia, ankylosing spondylitis, cerebral palsy, and other neuromuscular disorders.4 Arthritis of the cervical spine which may result in anterior subluxation of C1 on C2 (atlantoaxial subluxation) may occur in up to 40% of patients with rheumatoid arthritis, with symptoms of progressive neck pain, headache, and myelopathy. Less common is posterior and vertical migration of the odontoid process. Flexion of the head in the presence of atlantoaxial instability could result in the displacement of the odontoid process into the cervical spine and medulla with concomitant compression of the vertebral arteries. This could precipitate quadriparesis, spinal shock, and death. In at-risk patients, preoperative cervical flexion–extension radiographs should be evaluated and if the distance from the anterior arch of the atlas to the odontoid process exceeds 3 mm, the patient should undergo awake fiberoptic tracheal intubation and the cervical spine protected with a cervical collar during the procedure. Ankylosing spondylitis is a chronic inflammatory arthritic disease that involves ossification of the axial ligaments progressing from the sacral lumbar region cranially, resulting in significant loss of spinal mobility. These patients are a significant challenge to the anesthesiologist with regard to airway management due to the reduced movement of both the cervical spine and temporomandibular joint. Assessment of the cervical spine is also crucial in trauma patients. These patients should be carefully evaluated for signs and symptoms of cervical cord compression, such as pain and neurological deficits.5 Pulmonary complications continue to remain common after spinal procedures.6 Patients presenting for thoracic spine surgery frequently present with conditions that affect their pulmonary function. Patients with thoracic spinal deformities will have a reduced chest cavity with decreased chest wall compliance and restrictive lung disease. Although exercise tolerance is an important determinant of the effects of the severity of the curve on respiratory function, formal pulmonary function studies will guide decisions regarding the extent of surgery permitted and the requirement for postoperative ventilatory support. A vital capacity of less than 40% of the normal range is predictive for postoperative ventilation. Hypoxemia is a common finding, secondary to ventilation–perfusion inequalities caused by alveolar hypoventilation. This may progress to elevated pulmonary vascular resistance and ultimately cor pulmonale. An echocardiogram should be evaluated for pulmonary hypertension and right ventricular hypertrophy (RVH). Patients with severe pulmonary hypertension may not be candidates for surgical correction of the spinal deformity.7 Cigarette smoking not only increases the risk of postoperative pulmonary complications but has a negative impact on the success of spinal fusions.8 Patients should be encouraged to stop smoking at least 6 to 8 weeks prior to surgery in order to reduce the risk of pulmonary complications to that of nonsmokers.9 Cardiovascular dysfunction can be due to a number of factors in this patient population: a direct result of the pathology requiring corrective spinal surgery, pulmonary hypertension and RVH, and/or age-related ischemic heart disease.7,10 In addition, several studies have established an increased risk of cardiovascular morbidity and mortality in patients with rheumatic and connective diseases.11 Since there is a significant incidence of postoperative cardiac complications after spine surgery and it is difficult to assess these patients’ functional status due to the limitations imposed by their disease, many of these patients will require a preoperative pharmacological stress test.12 There is, however, limited data available that preoperative risk stratification and/or coronary revascularization has an effect on outcome. Numerous studies have indicated that the use of perioperative β-blockers can reduce myocardial ischemia.13,14 Recent reports have suggested that although the perioperative administration of β-blockers may prevent myocardial ischemia, they may increase the incidence of stroke and death by preventing postoperative cardiac complications, particularly in patients at intermediate risk. However, β-blockers should be continued perioperatively in those patients on chronic β-blockers and initiated in those at the highest risk with a target heart rate below 80.15 Patients with diabetes are not only at increased risk for perioperative complications from associated comorbidities (myocardial ischemia, vascular disease), but also have a higher incidence of postoperative infections.16 These patients should have a preoperative HbA1c less than 8% and their perioperative blood glucose levels should be maintained between 150 and 200 mg/dL.17,18 All patients for thoracic spine surgery should have a neurological examination which assesses and documents any preexisting neurological deficits and under which circumstances they are exacerbated. This is essential in order to be able to identify new neurological deficits that appear postoperatively. This information is also valuable in positioning the patient for surgery. Surgical treatment for adult spinal deformities presents multiple challenges for intraoperative management, including ventilation, hemodynamic stability, intraoperative neuromonitoring (IONM), management of blood loss, and a plan for postoperative analgesia. Success requires the collaboration between the anesthesiologist, surgeon, and IONM team. General anesthesia with endotracheal intubation and controlled ventilation is a requirement for adult thoracic spinal surgery. Hence the initial procedure, tracheal intubation, may be a challenge in a population with preexisting arthritic conditions or cervical deformities. For many of these patients, tracheal intubation can be achieved with the aid of video-assisted laryngoscopy. However, for patients with unstable cervical spines and significant cervicothoracic deformities (rheumatoid arthritis, achondroplasia, ankylosing spondylitis) awake fiberoptic endotracheal intubation is the safest approach. In the rheumatoid arthritis patient synovitis of the temporomandibular joint may significantly limit mandibular motion and mouth opening. Arthritic damage to the cricoarytenoid joints may result in diminished movement of the vocal cords, resulting in a narrowed glottic opening which is manifested preoperatively as hoarseness and stridor. During laryngoscopy, the vocal cords may appear erythematous and edematous, and the reduced glottic opening may interfere with passage of the endotracheal tube (ETT). There also is an increased risk of cricoarytenoid dislocation with traumatic endotracheal intubations. Surgical spinal corrections involving high anterior thoracic levels or video-assisted thoracoscopic surgery will require the isolation of one lung ventilation (OLV). OLV has been traditionally achieved with a double-lumen ETT. In single-staged anterior then posterior spinal fusions, before the postoperative procedure the double-lumen ETT should be replaced with a single-lumen ETT, to avoid trauma to the larynx from the larger double-lumen ETT. Alternatively, a single-lumen ETT with an enclosed bronchial blocker can also provide OLV and has the advantage of being left in place as a single-lumen ETT with the blocker deflated at the end of the anterior procedure.19 In patients with restrictive lung disease, adequate oxygenation may be difficult during OLV and may require continuous positive airway pressure to the nonventilated lung and positive end-expiratory pressure (PEEP) to the ventilated lung. This, however, can only be achieved with a double-lumen ETT. Once the airway is secured, ventilation can represent an anesthetic challenge secondary to restrictive lung disease and pulmonary hypertension. These patients should be ventilated with pressure-cycled ventilation, lower than conventional tidal volumes (6–8 mL/kg) and PEEP. Mechanical ventilation with higher, conventional tidal volumes has been shown to be contributing to the development of acute lung injury (ALI).20 However, recently this approach has been questioned in a randomized controlled trial of patients in the prone position undergoing spinal surgery.21 PEEP is used to prevent the collapse of recruited alveoli; however high levels of PEEP can have adverse consequences on hemodynamics and induce alveolar wall stress. Hence, PEEP levels of 5 to 7.5 mm Hg are employed with frequent assessment of oxygenation and ventilation with arterial blood gases. Both hypercarbia and hypoxia can increase pulmonary vascular resistance and exacerbate existing pulmonary hypertension. In addition, careful attention to fluid management is essential to maintain right ventricular preload and cardiac output. Nitrous oxide should be avoided in these patients since it will increase pulmonary vascular resistance in patients with preexisting pulmonary hypertension. The general anesthetic for these procedures is particularly challenging since it must provide analgesia, amnesia, and hemodynamic stability without compromising IONM. Multimodal intraoperative monitoring has become the standard of care for complex reconstructive spinal surgery.22,23,24,25 This monitoring includes somatosensory (SSEP), motor (MEP) evoked potential, and electromyography (EMG) monitoring. The anesthesiologist determines the quality of neuromonitoring during surgery. Successful neuromonitoring which is essential for favorable outcomes after thoracic spinal surgery depends on the careful selection of anesthetic agents, the control of critical systemic variables, and the close cooperation between the anesthesiologist, surgeon, and neuromonitoring team.26 EMGs are used to monitor nerve root injury during pedicle screw placement and nerve decompressions. MEPs assess the integrity of the anterior, motor, spinal cord. There are several potential adverse effects of MEP monitoring, including cognitive deficits, seizures, bite injuries, intraoperative awareness, scalp burns, and cardiac arrhythmias. It is advisable to employ a soft bite block during MEP monitoring to prevent tongue biting and dental damage. MEP monitoring should be avoided in patients with active seizures, vascular clips in the brain, and cochlear implants. The posterior sensory portion of the spinal cord is evaluated using SSEP monitoring. In SSEPs, an impulse is sent from a peripheral nerve and measured centrally. In MEPs, an impulse is triggered in the brain and monitored as movement of a specific muscle group. SSEPs and MEPs are evaluated with regard to amplitude and strength of the signal and latency, time it takes the signal to travel through the spinal cord, compared to the patient’s nonsurgical control values. Although SSEP monitoring is continuous, the assessment requires temporal summation of the signals which may take several minutes to change. Spontaneous patient muscle firing will result in SSEPs which are difficult to interpret (noisy), hence the ideal environment for assessing SSEPs but not MEPs is in patients treated with muscle relaxants. MEPs are assessed intermittently but the findings are real time. A number of physiological factors will attenuate SSEP and MEP monitoring, including hypotension, hypothermia, hypocarbia, hypoxemia, anemia, and anesthetics. The potent inhalational agents produce a dose-dependent attenuation of both SSEP and MEP monitoring. This effect, however, is nonlinear; hence for a specific patient 0.5% isoflurane may have minimal effect on IONM, while at 0.7% it may abolish the signals. The various halogenated agents are similar in action, but the less soluble inhalational agents appear to be more potent with regard to suppression of IONM. In addition, potent inhalational agents reduce systemic vascular resistance and act as negative inotropes, which can contribute to hypotension and reduced tissue perfusion. Hence, for these procedures it is best to either eliminate potent inhalational agents from the anesthetic or utilize them at a low concentration which is maintained at a constant blood concentration throughout the procedure. Nitrous oxide is a commonly used anesthetic and a carrier gas for the more potent inhalational agents in general anesthesia, because it allows for rapid emergence and has both anxiolytic and analgesic properties. Nitrous oxide attenuates MEPs and the cortical components of SSEPs. On an effective dose equivalent basis (minimum alveolar concentration [MAC]) nitrous oxide has a greater suppressant effect on IONM than the volatile agents. In addition, nitrous oxide acts synergistically with potent inhalational agents to suppress IONM. Prolonged exposure to nitrous oxide may also promote postoperative nausea and vomiting, increase cardiovascular morbidity, and have deleterious effects on cognition. This leaves total intravenous anesthesia (TIVA;
3.1 Introduction
3.2 Preoperative Evaluation
3.3 Intraoperative Management
Table 3.1) as the preferred anesthetic for complex thoracic spine procedures. IONM is the least affected by narcotics and benzodiazepines, but propofol will depress MEPs in a dose-dependent manner. Since propofol accumulates in fatty tissue, the infusion rate should be decreased during long procedures. Ketamine, an N-methyl-D-aspartate (NMDA) receptor antagonist, at subanesthetic doses, will reduce the MEP-negative effects of propofol. In addition, ketamine will reduce narcotic requirements and prevent opioid-induced hyperalgesia.27 This is an important consideration when using remifentanil, which because of its very short half-life (metabolized by plasma esterases) and lack of MEP suppression at 20 times the usual doses, has become the opioid of choice for complex spinal procedures. However, remifentanil administration has been strongly associated with opioid-induced hyperalgesia and because of its rapid elimination, it must be supplemented with other opioids to provide analgesia upon emergence. Methadone, administered at the beginning of the procedure has the advantage of long duration of action and has both opioid and NMDA properties. Intraoperative administration of methadone has been shown to reduce postoperative opioid consumption and pain scores.28 Postoperative analgesia can also be achieved with the administration of intraoperative intrathecal morphine.29
• Stable infusion of propofol 25–50 μg/kg/min • Ketamine 2 μg/kg/min (~ 1 mg/kg/h) • Opioid fentanyl (1–2 μg/kg/h) or remifentanil (1–2 μg/kg/min) • Dexmedetomidine 0.5 mcg/kg/h • +/− Lidocaine 1 mg/kg/h • Benzodiazepines: midazolam 5 mg at induction and diazepam 10 mg |
Recently, TIVA anesthesia for complex spine corrections has also included two other intravenous agents: lidocaine and dexmedetomidine. The infusion of intravenous lidocaine at 1 to 2 mg/kg/h during spine operations has been shown to reduce postoperative opioid requirements and the incidence of nausea and vomiting, as well as a faster return of bowel function.30 Dexmedetomidine is a selective α2 agonist with anxiolytic, sedative, and analgesic properties. When infused at 0.5 mcg/kg/h, it has minimal effect on cortical SSEPs or MEPs. During spine operations, when compared to a TIVA infusion of propofol and fentanyl, the addition of dexmedetomidine improved the quality of recovery and decreased the release of cytokines implicated in the systemic immune response syndrome.31
The 5th National Project Audit in Great Britain (NAP5) determined that the sole use of TIVA anesthesia was associated with accidental awareness under general anesthesia (AAGA).32 Since the brain concentration of anesthetics required to produce loss of awareness cannot be predicted and as complex spine procedures progress anesthetic dosing is often reduced to accommodate IONM, the anesthesiologist’s dilemma is how to detect consciousness. Several studies have recommended the use of depth of anesthesia monitors during TIVA anesthesia. The administration of benzodiazepines may also reduce the risk of AAGA.
If the anesthesiologist is able to provide a stable physiological environment in which IONM can be interpreted with minimal influence from the anesthetics, then changes in either the SSEPs or MEPs can be used to assess surgically induced neurological injury. An example of MEP changes during a thoracic scoliosis correction is presented in 
Fig. 3.1, 
Fig. 3.2, 
Fig. 3.3, 
Fig. 3.4. In the presented case, a patient is undergoing a thoracic scoliosis correction under TIVA anesthesia. Nine minutes after the rods are placed and the curve is corrected, there is a loss of MEP signals in specific left-sided muscle groups. The next steps which are undertaken by all of the participants are imperative for a favorable outcome (
Table 3.2). If a stable TIVA anesthetic has been utilized throughout the procedure, then other causes are investigated as the cause for a change in IONM, including hypotension, anemia, metabolic acidosis, and hypothermia. Once nonsurgical causes have been eliminated, the curvature correction is released by the surgeon and the degree to which the deformity can be recorrected without neurological injury is reevaluated. In this example, the rods were replaced with less deformity correction and the procedure proceeded without any further change in IONM. If the IONM deficits had persisted, the TIVA anesthetic would have been eliminated (except possibly the dexmedetomidine) and a “wake-up” test performed. The “wake-up” test involves having the patients move their hands and feet to command during the surgical procedure. It is only feasible under circumstances where the anesthetic has been designed to provide rapid emergence with minimal chance for AAGA. Preoperatively, the potential for a “wake-up” test should be discussed with the patient. Its neurological validity is limited, as it represents one point in time and only gross movement. In addition, the complications associated with this test include AAGA, tracheal extubation, air embolization from a Valsalva maneuver, and gross movement inducing neurological injury.33
Table 3.2 Corrective measure after loss of intraoperative neuromonitoring
• Is the problem technical? Check electrodes and repeat the signals • Is the problem related to the anesthetic? ◦ Stop any inhalational agent ◦ Reduce or eliminate the propofol infusion • Is the problem spinal cord perfusion? ◦ Raise the MAP to 90 mm Hg ◦ Hb level ≥ 8 mg/dL ◦ Check ABG; correct metabolic acidosis • Is the problem the surgical correction? |
Abbreviations: ABG, arterial blood gas; Hb, hemoglobin; MAP, mean arterial pressure. |
Complex spine surgery, particularly corrective thoracic deformity surgery, is often associated with large blood losses. Multiple factors have been suggested to influence the magnitude of this blood loss, including surgical technique, operative time, number of vertebral levels fused, anesthetics, mean arterial blood pressure, platelet abnormalities, dilutional coagulopathy, and primary fibrinolysis.34 Several techniques have been employed to reduce this blood loss and limit the need for homologous blood transfusions (
Table 3.3): proper positioning of the patient to reduce intra-abdominal pressure; surgical hemostasis; deliberate controlled hypotensive anesthesia; reinfusion of salvaged blood; intraoperative normovolemic hemodilution; the use of pharmacological agents which promote clot formation; and the preoperative donation of autologous blood. The predonation of autologous blood for these procedures suffers from several disadvantages: patients often are anemic on the day of surgery; the predonation and storage of autologous blood is expensive; it does not eliminate the risk of a patient receiving the “wrong” unit of blood; blood is stored as packed red blood cells (RBCs), which eliminates coagulation factors; and if the surgery is rescheduled, the stored unit may expire. In patients with normal preoperative hematocrits, whole blood can be removed in the operating room prior to surgery and replaced with colloid or crystalloid such that the patient remains normovolemic.35 This technique permits a reduction in red cell mass intraoperatively and the blood which has been removed contains platelets and coagulation factors not present in stored packed RBCs.
