Anesthetic considerations and intraoperative management during spine surgery

1

What are some key areas of focus in relation to perioperative anesthetic management of spinal surgery patients?

• 1.

  Preanesthetic patient risk assessment and optimization for surgery

• 2.

  Assessment of procedure-specific risk factors

• 3.

  Identification of a potentially difficult airway, cervical instability, or spinal cord compression

• 4.

  Hemodynamic monitoring requirements

• 5.

  Intraoperative neurophysiologic monitoring and anesthetic agent selection

• 6.

  Intraoperative positioning

• 7.

  Maintenance of normothermia

• 8.

  Fluid and blood loss management

• 9.

  Preparation for potential intraoperative crises

• 10.

  Postoperative care coordination including tracheal extubation and pain management

2

How do anesthesiologists assess anesthetic risk for a patient scheduled to undergo spinal surgery?

Anesthesiologists perform a preanesthetic evaluation and assign an American Society of Anesthesiologist (ASA) physical status classification ( Table 21.1 ). Preanesthetic assessment includes a review of medical records, patient interview, and a focused history and physical examination, including assessment of any comorbid conditions. Preoperative testing and subspecialty medical consultations are obtained as indicated. Areas of particular interest include current medications, allergies, anesthetic history, history of surgical procedures, history of bleeding disorder or excessive bleeding, cardiovascular and respiratory risk factors, identification of patients who may pose challenges related to airway management, and assessment of functional capacity (metabolic equivalents of task score [METS]). An informed consent for anesthesia care is obtained. Factors that contribute to development of the anesthetic plan for a specific patient include the surgical site, number of operatively treated spinal levels, surgical approach, anticipated blood loss, surgical duration, and use of intraoperative neurophysiologic monitoring. Standardized high-risk spine protocols have been defined for patients, including those who undergo surgery lasting >6 hours, >6 surgical levels, staged procedures, adult and pediatric spinal deformity procedures, and patients with significant medical comorbidities. Online surgical risk calculators (e.g., ACS-NSQIP, SpineSage) may be used to provide a more detailed estimate of patient and procedural risk.

3

What are some of the major categories of intraoperative adverse events associated with spinal surgery?

Various classification systems have been developed to categorize intraoperative adverse events associated with spine surgery. (1) Some general categories of potential intraoperative adverse events during spinal surgery are listed in Table 21.2 .

4

What types of cervical spine pathologies are at risk for neurologic injury with endotracheal intubation?

Patients with an unstable cervical spine (e.g., fracture, rheumatoid arthritis, odontoid hypoplasia) or severe cervical stenosis are at risk for neurologic injury with endotracheal intubation. Important factors for minimizing neurologic injury include recognizing cord compression and/or spinal instability, performing intubation with care, and avoiding neck movement, especially extension of the cervical spine. Many anesthesiologists prefer fiber-optic intubation in this setting. Monitoring of neurologic function can be performed directly if an awake intubation technique is utilized or indirectly with neurophysiologic monitoring if an unconscious fiber-optic–guided intubation is performed. A variety of alternative intubation methods have been described, including manual inline cervical immobilization and orotracheal intubation, nasal intubation, as well as use of specialized laryngoscope blades, video laryngoscopes, lighted stylets, and bronchoscopes. Intubation technique is guided by the ASA Difficult Airway Algorithm and the skills and preferences of the anesthesiologist.

5

What are some strategies that can be used to avoid operating at the wrong site during spinal procedures?

Wrong site spinal surgery can involve operating at the wrong spinal level, the wrong side of the spine, or even performing surgery on the wrong patient. Various guidelines and safety checklists exist to ensure that wrong site surgery does not occur. The North American Spine Society developed a Sign, Mark, and X-Ray Checklist for safety which includes:

• Preprocedure verification of patient identity, procedure site, and imaging studies. It is important to confirm that level identification on preoperative MRI correlates with level identification based on preoperative radiographs.

• Marking of the surgical site prior to surgery.

• Time-out prior to start of procedure to confirm correct surgical site, surgical side, and patient identity.

• Intraoperative imaging to locate and mark the appropriate vertebral level with a radiopaque marker, preferably at the level of the pedicle.

• Time-out during surgery to confirm radiographic localization of the appropriate surgical level(s).

Intraoperative localizing imaging studies require correlation with preoperative radiographs and MRI studies, as anatomic variations (i.e., Klippel-Feil anomaly, nonstandard number of ribs, transitional lumbosacral junction) may lead to incorrect level identification. Regional anatomic landmarks and visualization of radiologically evident pathology (e.g., vertebral fracture, spondylolisthesis) may be used as a secondary check to confirm appropriate level selection. Localization of thoracic levels during anterior approaches may be especially challenging, and preoperative placement of polymethyl methacrylate impregnated with barium sulfate or radiopaque coils have been described to facilitate identification of the correct thoracic spinal level.

6

What types of spinal procedures benefit from use of single-lung ventilation?

Thoracic spine procedures performed with the assistance of thoracoscopy require single-lung ventilation to maintain a safe working space within the thoracic cavity. Anterior thoracic spine procedures performed through an open thoracotomy approach for exposure of the spine above the level of T8 also benefit from single-lung ventilation. In open anterior thoracic spine procedures, single-lung ventilation decreases the difficulty of retracting the lung from the operative field in the upper thoracic region. For open procedures below the T8 level, the lung can more easily be retracted out of the operative field without the need for single-lung ventilation. Options for single-lung ventilation include use of a double-lumen endotracheal tube or a bronchial blocker inserted into a single-lumen tube. Remember that a double-lumen tube will need to be changed to a single-lumen tube at the end of the procedure if extubation is not planned as it is not suitable for postoperative ventilation.

7

What patient populations are at increased risk of latex allergy?

Prior exposure to latex as a result of medical treatment (e.g., multiple bladder catheterizations, multiple surgical procedures at a young age) or occupational exposure may lead to an IgE-mediated anaphylactic reaction upon subsequent exposure to the latex antigen. In addition to health-care workers, patient populations with an increased risk of latex allergy include those with myelodysplasia, congenital genitourinary tract abnormalities, spinal cord injuries, cerebral palsy, and ventriculoperitoneal shunts. Anaphylaxis secondary to latex allergy may occur intraoperatively (usually 20–60 minutes following induction) and must be included in the differential diagnosis of intraoperative emergencies. A detailed history is the best means of detecting patients at risk. Patients with a history of latex allergy may be treated with pharmacologic prophylaxis (diphenhydramine, ranitidine, prednisone), but this may not prevent an anaphylactic reaction. A latex-safe environment must be provided in the operating room (OR).

8

What types of anesthetic monitoring are recommended during spinal procedures?

Basic intraoperative anesthetic monitoring for spine surgery includes continuous assessment of heart rate, electrocardiogram, blood pressure, oxygen saturation (SaO ₂ ), end tidal carbon dioxide, temperature, fluid intake, and urine output. Additional monitoring modalities include intraoperative neurophysiologic monitoring, neuromuscular blockade monitoring, and depth of anesthesia monitoring. Vascular access for major spine procedures includes two large-bore peripheral intravenous lines, intraarterial blood pressure monitoring, and a central line. Use of minimally invasive cardiac output measurement techniques (e.g., FloTrac/Vigileo system) enables use of goal-directed fluid management to optimize intraoperative hemodynamic management.

9

What are some important considerations regarding management of fluid administration, blood loss, and coagulopathy during major spinal reconstructive procedures?

Detailed recording of fluid administration, hourly estimated blood loss, and laboratory values on a tracking sheet or display board is recommended. Crystalloid solutions are administered initially, followed by infusion of colloid solutions for losses in excess of maintenance fluid requirements. Traditional parameters for guiding adequacy of fluid replacement such as heart rate, blood pressure, and central venous pressure react slowly to changes in intravascular volume, and various goal-directed fluid therapy approaches based on variables such as stroke volume variation, pulse pressure variation, and cardiac filling pressures have been suggested to optimize fluid management. (2) For complex procedures at the spinal cord level, use of intraoperative neurophysiologic monitoring and maintenance of mean arterial pressure (MAP) in a range of 85–90 mm have been recommended to prevent ischemic injury to the spinal cord.

There is no universally accepted threshold at which to transfuse blood during spinal procedures, although a restrictive intraoperative transfusion trigger (hemoglobin [Hgb] <8 g/dL, hematocrit [HCT] <25%), has been associated with lower morbidity and shorter length of stay compared to use of a liberal trigger. An intraoperative Hgb level ranging from 8 to 10 g/dL is often targeted, depending on patient comorbidities and case-specific factors including procedure type, procedure duration, and the rate and volume of blood loss anticipated. Institution-specific transfusion protocols have been described for use in cases of massive, life-threatening intraoperative blood loss.

Options for treatment of coagulopathy include fresh frozen plasma, cryoprecipitate (fibrinogen level <150 mg/dL), platelet infusion (platelet count <100,000 U/μL),), desmopressin (for persistent oozing after correction of fibrinogen and platelet levels), and recombinant factor VIIa (for persistent oozing if international normalized ratio [INR] >2 and desmopressin has been administered). (3) In rare cases, disseminated intravascular coagulation (DIC) may develop due to widespread activation of the coagulation system with excessive consumption of coagulation factors, platelets, fibrinogen, and associated microvascular thrombi formation, and life-threatening bleeding.

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