Anesthesia for Complex Spine Surgery

Authors: Benjamin F. Gruenbaum, MD, PhD, C. Harlow Ladd, DO – Mayo Clinic, Jacksonville FL

Goals:

1. Discuss perioperative pain management in patients undergoing complex spine surgery.
2. Understand the goals of intraoperative neuromonitoring, the influences of anesthesia on these monitors, and how to respond to intraoperative neuromonitoring changes.
3. Recognize positioning concerns in patients undergoing complex spine surgery.
4. Discuss choice of maintenance of general anesthesia for patients undergoing complex spine surgery, and the benefits and limitations of each option.
5. Recognize risk factors for intraoperative bleeding in patients undergoing complex spine surgery, discuss techniques to reduce its magnitude, and discuss approaches for rapid identification and management.

Case: A 67-year-old gentleman is brought to the OR for revision of instrumentation from T10-pelvis, removal of a thoracic (T7) spinal cord stimulator, and extension of fusion up to T6 with additional levels as needed.

Past Medical History: The patient has a past medical history significant for type 2 diabetes (last A1c 7.6 controlled with daily insulin), hypertension, hyperlipidemia, GERD, CKD stage 3a, OSA (non-compliant with CPAP), obesity (BMI 36), and chronic pain with longstanding opioid use. Most recently, the patient suffered a fall while in the shower at home that has resulted in unrelenting low back pain that radiates down the posterior aspect of his lower extremities bilaterally. The patient also endorses new onset right sided foot drop that has impaired his gait and has rendered him immobile for the last 3 months. The patient suffers from chronic pain and has utilized 30 mg of PO oxycodone every 4 hours for the past 5 years. Since his fall, his opioid use has increased to 45 mg PO oxycodone every 4 hours, however he still notes severe pain with movement.

Medications: 45 mg PO oxycodone every 4 hours, Insulin glargine with Novolog SSI, Lisinopril 10 mg, Amlodipine 10 mg, Coreg 12.5 mg, Tylenol 1 g TID, Albuterol PRN, Omeprazole BID, Seroquel 50 mg QHS for sleep, Zofran PRN

Imaging: Full spinal x-ray shows posterior instrumented fusion from L1 to pelvis with sacropelvic fusion, prior ACDF with hardware intact, thoracic stimulator device at T7 level with slight sagittal imbalance noted, and diffuse multilevel disc space narrowing throughout thoracic spine. MR of the lumbar spine shows prior instrumentation hardware and moderate to severe spinal canal stenosis from T12-L4 due to posterior-lateral disc extrusion with accompanied ligamentum flavum hypertrophy.

Past Surgical/Anesthesia History: The patient has an extensive history of spinal surgeries including L1-L5 extreme lateral interbody fusion, L1-pelvis fusion with L5-S2 transforaminal lumbar interbody fusion, ACDF from C3-C6, and a thoracic spinal cord stimulator (T7) which has wires in place but is turned off and nonfunctional. The patient also has a history of delayed emergence from anesthesia that required prolonged ventilation support for 4 hours following his last spinal fusion.

Vitals: Pulse 95bpm, BP: 142/82mm Hg, RR 16, T 37°C. Preop EKG: Sinus with Q waves in leads II, III and aVF.

Key Questions:

1. Do you require further workup prior to proceeding with the surgery? What would you like to know?
2. What monitors would you decide to use on this patient? Would you place an arterial line? Why/why not? Would you add a CVP monitor? What access would you like?
3. How would you like to secure the airway?
4. Are there any concerns of having the patient prone? What if you lose the airway while prone?
5. What is your choice of maintenance of general anesthesia for this patient?
6. What will be your analgesia regimen for this patient?

Case Continued: 4 Hours into surgery, the neuromonitoring team tells you that the SSEP signals have decreased in amplitude by 30-40% and they are noticing an increase in latency.

7. What would you like to do next? What is the purpose of neurophysiological monitoring? How does neuromonitoring affect your anesthetic plan?

Case Continued: The surgeon reports excessive and unexpected bleeding along with oozing in the field. You have maintained him on crystalloids until now. You send a quick ABG, and the H/H is 8.1/26.

8. Would you transfuse?What methods could you have used to reduce blood loss?
9. What are your blood pressure goals? How much fluid will you give the patient?
10. How would you record the patient’s temperature? If noted to be 36.9°C, how would you warm the patient?
11. How often would you monitor the patient’s glucose level? What is your goal?

 Case Continued: During the case, you notice that you are no longer recording end-tidal CO2. You look at the patient’s head and realize that the breathing tube has been dislodged and is no longer endotracheal.

12. How do you re-secure the airway in a prone patient?

 Case Continued: The surgery finishes after 13.5 hours of operating time. As you position the patient supine, you notice swelling around the face, tongue, and eyelids.

13. Why has this happened? Would it influence your decision to extubate?

Case Continued: You decide to extubate the patient now for a postop neurological assessment. You have titrated your narcotics and agents appropriately. End tidal sevo is 0.1, your opioid infusion has been off for 1 hour and you have not paralyzed the patient. However, your patient is still not waking up.

14. What are your differential diagnoses and how would you approach this situation?

Case Continued: You extubate him when criteria are met and bring the patient to the ICU and upon waking, he complains of significant 10/10 pain despite being lethargic and falling asleep in between sentences.

15. How would you manage this patient’s pain?

Case Continued: Upon waking up further he complains of blurry vision in both of his eyes.

16. What do you do next?How do you differentiate the different causes of changes in vision? What are some risk factors? What could you have done to prevent this?

Discussion

Preoperative Assessment

 Specific preoperative assessment for complex spine procedures (question 1)

• A thorough preoperative assessment must be performed to identify significant medical comorbidities.
• The preoperative physical exam should include an airway exam, a neurological exam, assessment of neck mobility, a cardiopulmonary evaluation, and an assessment for potential vascular access. The patient’s age, pertinent comorbidities, surgical plan, and degree of invasiveness should guide the anesthesiologist’s preoperative evaluation.
• Preoperative laboratory work should be performed, including a complete blood count to assess hemoglobin and platelet levels. Since large amounts of blood loss may be expected, a type and screen should be ordered to expedite any intraoperative blood transfusions that may be required.
• Complex spine surgeries are not without risk, and the patient should be consented and made aware of the potential for visual loss, neurological dysfunction, infection, transfusions, and increased post-operative pain.
• Preexisting comorbidities that should be identified and addressed prior to surgery:
  • Neurological deficits: Many patients presenting for complex spine surgery have preexisting neurological deficits. These deficits range from weakness and muscle atrophy to quadriplegia. The extent of neurological dysfunction should be documented thoroughly in the preoperative period as certain neurological conditions may change the anesthetic plan. For example, patients with unstable cervical spines may require specific intubation techniques, and patients with loss of motor function due to spinal cord pathology may not be candidates for succinylcholine.
  • Previous spinal surgeries: The patient should be assessed for any previous history of spine surgeries. Previous spinal surgery or preexisting spinal hardware may result in a more complex surgical approach with a longer operation time and/or increased blood loss.
  • History of spinal cord injury: Patients with a history of spinal cord injury must have the level of the lesion documented. Acute spinal cord injuries may develop spinal shock syndrome, which is defined as the sudden loss of reflexes and muscle tone below the level of injury with or without associated hypotension. Spinal shock may present as loss of spinal cord function caudal to the level of the lesion, with flaccid paralysis, anesthesia, bowel and bladder incontinence, and loss of reflexes. Spinal shock syndrome may also be complicated by neurogenic shock, which refers to hemodynamic instability such as hypotension, bradycardia, and hypothermia.¹ After the resolution of spinal shock, the patient may be at increased risk for autonomic hyperreflexia, especially if the spinal cord injury is above the mid thoracic level (T5-7). Autonomic hyperreflexia is provoked by cutaneous or visceral stimulation below the levels of the spinal cord lesion. It presents as unopposed sympathetic output below the level of the spinal cord lesion, exhibited by severe hypertension, skin pallor, pilomotor erection, spastic muscle contraction and increased muscular tone. The hypertension results in reflex bradycardia from the effects on the carotid sinus. Above the level of the lesion, there is unopposed parasympathetic activity resulting in vasodilation with head/neck flushing, diaphoresis, mydriasis, and lid retraction. Due to the serious nature of autonomic hyperreflexia, the best course of action is to acquire a detailed preoperative evaluation and history so that the provider can prepare appropriately to avoid any triggers of this event.
  • Cardiopulmonary disease and/or spinal cord injuries at or above the level of T7: Patients with preexisting cardiopulmonary disease and/or spinal cord injuries at or above the level of T7 may benefit from preoperative spirometry to better assess their respiratory function. Patients with spinal cord injury above the C5 level may have phrenic nerve damage (innervated by C3-5) which may lead to respiratory compromise due to diaphragm weakness. Lesions above C5 are usually associated with hypoventilation and the patient may require intubation and/or mechanical ventilation. These patients may be unable to clear secretions, placing them at higher risk for aspiration and/or pneumonia.
  • Visual defects: Any preexisting history of visual defects should be documented in the preoperative evaluation. If the patient is presenting with visual defects that have not been evaluated prior to the surgery, it may be prudent to consider an ophthalmology consult. Although post-operative blindness is a rare event occurring in only roughly 0.2% of spinal surgeries, patients should still be informed about the risks during the preoperative evaluation.² During the evaluation, the patient should also be assessed for risk factors for postoperative visual loss (POVL) including hypertension, diabetes, smoking history, peripheral vascular disease, carotid artery disease, glaucoma, obesity, preoperative anemia, and any anomalies of the optic nerve vascular supply. For patients with preexisting risk factors for POVL, it is also important to consider operative factors that put that at higher risk including prolonged operation times, large amounts of blood loss and anemia, intraoperative hypotension, vasopressor use, massive fluid administration, facial edema, and the head down position.² If these risk factors for POVL are present and the anesthesia provider determines that the patient is at high risk, they should have a candid discussion about these risks with the patient.
  • Chronic pain or long-term opioid use: Many patients presenting for complex spine surgery have a history of chronic pain or long-term opioid use. Documenting the distribution and type of pain, and the medications or therapies that the patient uses to control their pain, is advised. Patients with long-term opioid use will likely have higher amounts of opioid requirements and post-operative pain. A detailed history of their outpatient pain regimen should help shape the ideal multimodal anesthetic plan to limit excess post-operative pain.
  • Other: Address dietary concerns for high-risk malnourished patients, and rehabilitation or preoperative physiotherapy for frail patients. Risk factors for postoperative delirium should also be identified and addressed.³

 Intraoperative Assessment

 IV access and monitoring (question 2)

• Establishment of appropriate IV access is prudent. Two large bore intravenous catheters are generally recommended, but a central venous catheter may also be appropriate depending on the clinical situation. The decision to utilize a central line should be based on the nature of the surgical procedure and individual patient comorbidities. For prolonged procedures with high levels of anticipated blood loss in patients who may require vasopressor support or large amounts of transfusions, central venous access may be beneficial. Arterial lines are used to closely monitor blood pressure and levels of glucose, electrolytes, acid/base status, and coagulation parameters.
• Foley catheters, bladder or esophageal temperature monitors, fluid warmers, and upper and lower forced air warmers can help maintain normothermia throughout the case to avoid excess blood loss, infection risk, and associated poor wound healing. If the patient is hypothermic despite these efforts, the surgeon should be notified, and the ambient temperature of the room should be increased.
• Processed EEG monitors may be useful to help monitor and adjust the anesthetic regimen intraoperatively and to avoid over- or under-sedation if a total IV anesthetic is being utilized.

 Airway management (question 3)

• For airway management in complex spine patients, the difficult airway algorithm provided by the American Society of Anesthesiologists Practice Guidelines for Management of the Difficult Airway (2022) should be implemented as needed.
• Patients undergoing complex spine procedures may have cervical myelopathy or instability, which may require techniques such as video laryngoscopy or flexible fiberoptic equipment concurrently with in-line stabilization to ensure the neck maintains a neutral position.⁴
• If motor neuromonitoring is performed throughout the procedure, a bite block should be placed after the airway is secured to avoid accidental tongue lacerations when the muscles of mastication are stimulated. An extra piece to grasp, or a “tail,” on the bite block will ensure that it is not inadvertently swallowed by the patient.
• Properly securing the endotracheal tube is important to avoid losing the secured airway while the patient is in the prone position. While prone, some patients may have copious amount of nasal and oral secretions that could loosen the adhesives used to secure the endotracheal tube.

 Positioning concerns (question 4)

• Spine surgery may be performed in a variety of positions, though most spine surgical procedures are carried out in the prone position. Supine surgical procedures on the spine are associated with risks such as higher likelihood of injury to the trachea, esophagus, recurrent laryngeal nerve, sympathetic chain, carotid artery, and jugular vein. Sitting (cervical) or lateral decubitus (usually lumbar) positions are occasionally used as well.Proper positioning will depend on the anatomic location of the spinal pathology and the surgical approach and should be discussed with the surgeon prior to the procedure.
• For prone positioned cases, the patient will likely be induced on the stretcher on which they are wheeled into the operating room. Intubating on these beds is more difficult, as there is another large piece of equipment that limits space. The anesthesia machine may also be farther from the patient and other equipment may be somewhat inaccessible. Proper set up before inducing the patient is imperative. The bed should be at the correct height and angled in a way that allows for access to both sides of the patient during intubation. The patient’s head should be at the end of the bed and their IV access should be readily accessible as well.
• Turning the patient to the prone position is physically demanding and often requires a coordinated technique with help from the operating team. Extra attention should be paid to the patient’s lines and airway during the flip to the prone position to avoid any accidental removals or obstructions to flow. Having the lines and airway taped properly and within the line of sight during the flip will limit accidents. If the patient loses their airway in the prone position, another bed should be readily available in order to flip the patient back to the supine position.
• There are multiple physiologic changes that occur when the patient is placed into a prone position. Once in the prone position, there is venous pooling, a decrease in cardiac output, and a compensatory increase in systemic vascular resistance and pulmonary vascular resistance to maintain prefusion. There is evidence that these changes are more pronounced in patients being maintained with total IV anesthetic vs inhaled anesthetics. However, it is likely that this difference is insignificant in the absence of severe cardiovascular disease.² If the abdomen is compressed, there may be increased venous pressure which causes engorgement of the epidural veins of the perivertebral venous plexuses which may lead to more bleeding. Shifting the patient’s position while prone also has the potential to compress the great vessels, leading to markedly reduced venous return and resultant hypotension. Pulmonary changes in the prone position include lower diaphragmatic compliance which leads to an increase in the functional residual capacity, which in turn actually improves ventilation/perfusion mismatching.⁵
• Surgical approach, history of carotid artery or vertebrobasilar insufficiency, or cervical spine injury may limit the available choices of head and neck positioning. If possible, avoid excessive neck movements and maintain a neutral position. If the patient’s head is placed in pins, ensure that they are maintained under an adequate depth of anesthesia or have either a paralytic or another agent that ensures immobility (such as remifentanil) to avoid any unintended head movements that could result in cervical spine injury.
• The patient’s arms should not be positioned in any posture that elicits pain or paresthesias when the patient is awake. The arms may either be tucked at the patient’s side or placed over their head. If the arms are placed over the head while prone, they should be rotated cephalad parallel to the sagittal plane of the body. To limit brachial plexus and axillary neurovascular injury, appropriate padding should be used at the elbows and axilla and abduction of the arms should be limited to less than 90 degrees.
• While prone, the patient’s chest should be supported on parallel rolls of foam, gel, or other padding to facilitate ventilation and chest wall expansion. Boney structures such as chest, shoulders, iliac crest, and elbows should all be padded and supported appropriately to reduce peripheral nerve injuries. Male genitalia should be checked to ensure that it is not trapped between the bolsters or the patient’s thighs. Females should have their breasts positioned neutrally or directed medially to avoid compression.
• Avoidance of pressure on the orbit is of extreme importance when patients are in a prone position to avoid POVL. The patient’s eyes should be secured shut with occlusive dressings or tape to reduce the chances of corneal abrasions. With changes in the position of the bed intraoperatively, it is important to periodically monitor and assess that the patient does not have excess pressure on their eyes, chin, nose, ears, and forehead.

 Choice of maintenance (question 5)

• The choice of maintenance anesthetic should consider a multitude of factors, including the expected hemodynamic stability of the patient, expedient emergence for a quick neurological exam, the risks of postoperative nausea and vomiting or cognitive impairment, the risks of intraoperative awareness, and requirements for intraoperative neuromonitoring.
• Total intravenous anesthesia (TIVA) with propofol and opioids is frequently utilized for spinal surgery that requires intraoperative neuromonitoring. However, the choice between TIVA and a regimen based on a volatile anesthetic should be determined on a case-to-case basis.
• If somatosensory evoked potentials (SSEP) are being monitored, volatile agents should be limited to a minimum alveolar concentration values of 0.5 or less to avoid the increase in latency and decrease in amplitude of the SSEP monitors.
• If motor evoked potentials (MEP) or electromyography (EMG) are being monitored throughout the case, then use of neuromuscular blockade drugs should be avoided. Remifentanil or other opioids may be utilized to keep the patient still while allowing for MEP to be monitored.
• Dexmedetomidine at low doses is generally considered to be a safe adjuvant anesthetic,⁶ however higher doses may potentially suppress MEP signal amplitudes and should be avoided.⁷
• With these considerations in mind, it is also worth highlighting the importance of communication between the anesthesiologist and the neuromonitoring technician. To avoid any confusion when dose changes are made to the anesthetic intraoperatively, it is prudent to update the neuromonitoring team, so they may anticipate the resultant changes in their monitoring devices to avoid any false alarms.

 Pain management (question 6)

• Patients undergoing spinal surgery often experience intense post-operative pain that can last for days and have the potential to transition into persistent post-surgical pain; therefore, a multimodal intraoperative pain plan is essential.
• Acetaminophen: Despite widespread use throughout a variety of surgical cases, recent literature on acetaminophen does not show any significant difference in post-operative opioid consumption for patients receiving acetaminophen compared to placebo. Some conflicting evidence observed a small but significant reduction in pain scores, but did not observe reduced post-operative opioid consumption.⁸ In patients with normal liver function, the use of acetaminophen can be a beneficial adjunct for a multimodal approach to pain management.
• Gabapentinoids: Gabapentin has been shown to provide a small yet significant reduction in pain and opioid consumption with the initial 24 hours following spine surgeries. Studies with these results administered the gabapentin preoperatively as a single oral dose or two divided doses 2-24 hours before surgery at a dose ranging from 300 mg to 1,200 mg. Gabapentin also has anti-hyperalgesic effects resulting from its action on the dorsal root ganglia.⁹ It is worth noting that multiple studies did find a significantly increased incident of post-operative somnolence in patients who received gabapentin. This should be factored into the individualized patient assessment, especially when administering a post-operative neurological exam. Due to the possibility of increased post-operative somnolence and a delay in neurological assessment following emergence, the use of gabapentinoid agents should be considered with caution.
• Opioids: Historically, a variety of opioids have been the cornerstone of pain control for complex spine cases. It has been well-established that morphine, hydromorphone, tramadol, methadone, sufentanil, fentanyl, and remifentanil all can reduce acute pain. However, these drugs are not without their adverse effects.⁴ Given the negative effects of respiratory depression, sedation, post-operative nausea and vomiting, and potential for chronic pain development, the use of opioids should be implemented in a multimodal approach to lessen these undesirable side effects.
• Methadone: Methadone is a long acting, mu-opioid receptor agonist with N-methyl-D-aspartate (NMDA) acid receptor antagonist that has a dose dependent half-life. A double-blinded study from 2017 examining intraoperative methadone (0.2 mg/kg) in patients undergoing posterior spinal fusion surgery found that patients who received intraoperative methadone reported decreased pain scores, decreased opioid consumption, and increased satisfaction with pain management up to 3 days post-operatively compared to those patients who received 2 mg of dilaudid at surgical closure.¹⁰ There is also evidence that a single intraoperative dose of methadone (0.2 mg/kg) administered at the beginning of complex spine cases may reduce chronic opioid consumption up to three months post-operatively compared to patients who received 2 mg hydromorphone at surgical closure.¹¹
• Ketamine: Ketamine, NMDA antagonist that has been widely studied for its analgesic effects, may be administered intraoperatively as a low dose infusion (0.1 to 0.2 mg/kg/hr) or as bolus doses through the case. Low dose ketamine infusions may also be continued post-operatively up to 48 hours after the surgery to provide continued analgesia. Recent reviews have demonstrated that in patients undergoing major lumbar spinal surgery, intraoperative use of high dose ketamine (up to 10 μg/kg/min) demonstrated morphine sparing effects with concurrent decreases in pain scores postoperatively and at 6 weeks.¹² It was concluded that intraoperative ketamine has significant opioid sparing effects in complex spine patients, especially chronic pain patients.
• Dexmedetomidine: Dexmedetomidine is an alpha-2 agonist that has sedative, anxiolytic, and analgesic effects. The data behind the analgesic properties of dexmedetomidine in complex spine cases have been mixed, with some studies showing reductions in pain scores and others having inconclusive results. A 2019 narrative review of current anesthetic management techniques for complex spine surgery in adult patients noted that intraoperative dexmedetomidine infusions (0.2–0.6 mg/kg/h) demonstrated inconsistent evidence of reduction in visual analogue score pain scores post-operatively.⁸ A recent meta-analysis from Tsaouisi et al evaluating 913 patients showed that dexmedetomidine was sedative and allowed opioid sparing effects intraoperatively without prolonging emergence. However, a definitive conclusion on its efficacy for analgesia in complex spine cases requires more data.¹³ Despite the inconsistent evidence, dexmedetomidine is still commonly utilized as part of a multimodal approach.
• Magnesium: The data surrounding magnesium as an intraoperative adjunct for analgesia in complex spine cases is not conclusive. Some data indicates a slight reduction in pain scores when used as an adjunct. More research is suggested for ascertaining the efficacy of magnesium, and its use is only cautiously advised.^{12, 14}
• NSAIDS: Multiple studies support the claim that NSAIDS reduce post-operative pain and opioid requirements following spine surgeries.¹⁵ However, there are two major concerns associated with this class of medications. Some data suggests that NSAIDS are implicated in bone non-union following spinal fusion procedures, though data from animal models are contradictory and literature does not yet support a consensus on this topic.¹⁶ The other concern with NSAIDS is the risk of postoperative bleeding, despite a scarcity of evidence. A large retrospective single-institution analysis of 1451 neurosurgical cases did not show a statistically significant association between major bleeding events and perioperative ketorolac use.¹⁷ Due to the need for more research regarding NSAID use and risks of non-union and bleeding, a cautious approach to NSAID utilization in spine surgery is advised. Prolonged use after 3 days post-procedure should be avoided.
• Intravenous lidocaine: While there has been variability in the effectiveness of intravenous lidocaine across different types of surgical procedures, previous studies have shown that perioperative intravenous lidocaine is capable of reducing pain intensity and opioid consumption after spinal surgery.^{18, 19} The optimal dosage for this has not been fully elucidated, with another meta-analysis suggesting that the clinical benefit of intravenous lidocaine might only be significant at infusion doses of greater or equal to 2 mg/kg/hr.²⁰ Further investigation is warranted.
• Regional techniques: The efficacy of epidural infusions has been assessed by several randomized controlled trials in patients undergoing complex spine surgery. Park et al.²¹ and Gessler et al.²² demonstrated that epidural infusions of 0.2% ropivacaine lead to lower pain scores and fewer post-operative opioid requirements compared to IV patient-controlled analgesia opioids. A significant body of literature indicates that epidurals with local anesthetics or local anesthetics and opioids reduces patients’ post-operative pain scores. Published studies support the use of epidural infusions of local anesthetics or both local anesthetics and opioids as part of a multimodal approach to pain management in complex spine cases. There is also a growing body of literature suggesting that local anesthetic peripheral nerve blocks may also be efficacious for pain control. Chen et al. compared pre-operative bilateral single shot ultrasound-guided, lateral thoracolumbar interfascial plane (TLIP) block with a 30 cc bolus of 0.375% ropivacaine with placebo in lumbar fusion patients. Patients in the TLIP group demonstrated significant reductions in their perioperative opioid consumption and decreases in their VAS pain scores at 12, 24, and 36 hours.²³ Erector spinae blocks and transversus abdominis plane blocks have also demonstrated reductions in post-anesthesia care unit opioid consumption, post-operative nausea and vomiting, and hospital length of stay.⁴ It is important to consider possible motor blockade following peripheral nerve blocks, which may complicate post-operative neurological evaluations; therefore, the use of these techniques should be decided on a patient-by-patient basis. Local wound infiltrate with local anesthetic has not been shown to reduce post-operative pain scores or post-operative opioid consumption.¹² Ziegeler et al. compared intrathecal morphine (0.4 mg) to placebo in patients undergoing posterior lumbar interbody surgery and found that the patients in the intrathecal morphine group had less post-operative opioid consumption and lower pain scores at 4 and 8 hours post-operatively compared to placebo.²⁴ However, more data is needed before intrathecal opioid administration can be recommended.

 Intraoperative neuromonitoring and the effects of anesthetics (question 7)

• Intraoperative neuromonitoring (IONM) is utilized to evaluate and monitor the integrity of the spinal cord pathways during spine surgery. Multimodal intraoperative neuromonitoring (MIONM) with somatosensory evoked potentials (SSEPs), motor evoked potentials (MEPs), and/or electromyography (EMG) has been shown to improve detection of neurological injuries in complex spine cases. It is common for more than one monitoring modality to be implemented in complex spine cases. Neurological compromise is implied by a more than 50% decrease in amplitude and/or more than 10% increase in latency of the IONM signal.
• Maintaining open lines of communication between the anesthesiologist, neurophysiologist, and the surgeon while IONM is being utilized reduces the chances of false alarms, allows for faster detection of potential neurological injury, and permits more rapid corrective measures.⁴ Updating the neurophysiologist whenever there is a change in the dose or rate of anesthetic will allow them to anticipate any corresponding neuromonitoring changes, thus reducing false warnings of neurologic compromise.
• These monitoring techniques are affected by certain anesthetic agents that may impair their accuracy; therefore, it is important for the anesthesiologist to be aware of these implications so that they can tailor their anesthetic plan appropriately. Different anesthetic agents have varying effects on specific neuromonitoring modalities.
  • Inhalational anesthetic agents (including nitrous oxide) can interfere with MEPs above MAC values of 0.5 and they also interfere with SSEPs with MAC values above 1.0. Patients who present for surgery with preexisting neurological impairment may require lower doses or the complete exclusion of inhalational anesthetics, in which case total IV anesthesia may be preferred.
  • Propofol, benzodiazepines, and opioids have a smaller effect on evoked potentials, but they can still impair them at high infusion rates or following boluses. Dexmedetomidine at low doses is generally considered safe⁶, however higher doses may potentially suppress MEP signal amplitudes and should be avoided.⁷ Lidocaine does not appear to have a significant effect on neuromonitoring.^{25, 26}
  • Neuromuscular blockade should be generally be avoided if EMGs or MEPs are being monitored, as these agents will impair the monitoring modalities from accurately assessing nerve roots and descending motor pathways in the spinal cord. In addition, if a patient has a spinal cord injury, succinylcholine may still be utilized within the first 48-72 hours and again after 6-12 months have passed. However, there is a significant risk of inducing life-threatening hyperkalemia if succinylcholine is administered after 72 hours or before 6 months following a spinal cord injury due to the increased number of extra-junctional nicotinic acetylcholine receptors.
  • Ketamine is a commonly utilized adjunct anesthetic agent in complex spine cases due to its favorable hemodynamic profile, minimal effect on neuromonitoring, and its NMDA receptor antagonism, which may be beneficial in treating and preventing chronic pain. It is also worth noting that both ketamine and etomidate increase the amplitude of SSEPs.
  • Other physiological factors that can also affect IONM²⁷:
    • Hypocarbia: When CO2 reaches below 20 mmHg, there is a risk of cerebral vasoconstriction leading to neuronal tissue ischemia which may affect cortical MEPs and SSEPs.
    • Hypotension: With decreased blood pressure, there is a corresponding decrease in perfusion of vital tissues. Decreasing the blood supply to the cortical areas of the brain can result in a decreased amplitude.
    • Spinal cord hypoxia is required for accurate assessment of SSEPs. When there is hypoxia, there is a corresponding decrease in the amplitude of SSEPs.
    • Anemia will result in a lack of hemoglobin to carry oxygen to vital organs, and therefore it can create local tissue hypoxia in areas that have high oxygen demand. Anemia can lead to an increase in latency of SSEP.
    • Hypothermia or hyperthermia: both hyper and hypothermia affect evoked potentials. It is recommended to maintain the core body temperature with 2 – 5° C of the baseline temperature. At core temperature below 28° C, both MEPs and SSEPs are inhibited.
  • After all of the factors that may influence the IONM have been checked, it may be prudent to discuss potential surgical etiologies with the surgical team, such as excessive pressure or traction on neurological tissue in the field.
  • If the cause of changes in IONM are still not identified, a wake-up test is recommended, which is the gold standard for motor pathway evaluation.

 Management of intraoperative bleeding and how to manage coagulopathies (question 8)

• Complex spine surgeries are frequently associated with significant intraoperative blood loss. Risk factors for increased blood loss include:²⁸
  • Surgeries involving 3 or more levels, fusions, prolonged surgical time, complex instrumentation, revision surgeries, surgeries for tumor resections, and patients with obesity or increased intraabdominal pressure in the prone position.
  • Surgical misadventure such as trauma to the aorta, vena cava, or iliac vessels
  • Female gender, age over 70 years old, ASA class 3 or greater, and patients with pulmonary disease are associated with increased transfusion rates while undergoing spine surgery.
• Transfusions have consistently been shown to be associated with increased postoperative morbidity, mortality, and increased hospital lengths of stay. Therefore, it is important to have a good understanding of how to manage intraoperative bleeding and coagulopathies.²⁸
• Studies have suggested a more conservative transfusion threshold, with an intraoperative hemoglobin above 9, has been shown to be associated with better outcomes and fewer wound infections than lower thresholds.²⁹ Closely monitoring plasma levels of fibrinogen, platelets and the international normalized ratio is also recommended to guide transfusion therapy. A thromboelastographic-guided approach to resuscitation may reduce overall transfusion requirements.³⁰
• For complex spine cases, invasive hemodynamic monitors, adequate access (usually two large bore IVs or rapid infusion catheter), and a type and screen should be available in preparation for blood loss. If the patient has low hemoglobin at baseline or other comorbidities that may not tolerate low hemoglobin levels, it is advisable to have the blood in the room for immediate access.
• Intraoperative and/or postoperative cell saver can reduce the total amount of donor blood that is required during an operation, but it should be noted that these devices are contraindicated for oncologic surgeries or surgeries associated with an infectious process.
• If the surgery is planned in advance, the patient may be offered to donate blood before surgery that they can then receive during the operation. This method may actually increase the total volume of blood that is transfused during the case and does not entirely eliminate the possibilities for transfusion reactions.³¹
• Intraoperative hemodilution may also be utilized to decrease the amount of allogenic blood transfusions. After induction, the anesthesia provider can remove up to 1L of whole blood from the arterial or central line. This fluid is then replaced with a balanced crystalloid solution or colloid. Near the end of or after the surgery, the blood is then returned to the patient.
• Deliberate induced hypotension has been utilized extensively to limit blood loss in the surgical field. However, there are concerns regarding vital organ ischemia and the risk of post-operative visual loss which should be considered in the decision-making process.
• The use of antifibrinolytic therapy such as aminocaproic acid, tranexamic acid, and aprotinin to limit intraoperative blood loss and to manage coagulopathies has mixed outcomes in the literature. Some studies have demonstrated that these agents decrease perioperative blood loss and blood transfusion requirements without increasing the risk of perioperative deep vein thrombosis.³² These drugs are not without risk. Review of the literature indicates that patients with renal failure, recent pulmonary emboli or deep vein thrombosis, coronary stenting within the last 6 months, active coagulopathies, acute subarachnoid hemorrhage, and patients with previous history of anaphylaxis were excluded from the large tranexamic acid studies. Therefore, the safety profile of this drug in these patient populations is unknown and it should be used with caution.³³

 Fluid management and blood pressure goals (question 9)

• Fluid management for complex spine surgery is a controversial topic, and both hypovolemia and hypervolemia can have significant negative consequences. Hypovolemia may lead to hypotension, hemodynamic instability, and renal injury. Hypervolemia may predispose the patient to congestive heart failure, pulmonary edema, coagulopathy, delayed wound healing, increased hospital length of stay, and facial and/or airway edema that precludes quick extubation.
• The American Society for Enhanced Recovery recommends that as patient risk factors and surgical complexity increase, more advanced monitoring should be implemented to guide fluid management using goal-directed fluid therapy. The goal should be to maintain euvolemia and end organ perfusion throughout the case while avoiding electrolyte derangements and fluid excess. Utilizing routine monitors such as blood pressure, heart rate, or urine output may be fine to use as a surrogate for fluid status in less complex cases, but as the complexity increases, the need for more invasive hemodynamic monitors may arise. Utilizing pulse pressure variability or cardiac output via a pulmonary artery catheter or transesophageal echocardiography can be considered if hemodynamic instability is expected in a patient with significant comorbidities who may not tolerate fluid shifts from euvolemia. These monitors will provide data to determine if the patient would benefit from intravascular fluids. In instances where the patient is hypotensive, but the hemodynamic monitors indicate that the patient would not benefit from more fluids, vasoactive medications should be administered to maintain end organ perfusion.³⁴
• Crystalloids are the first line agents that should be utilized for fluid resuscitation; however, they are not without drawbacks. Administration of large volumes of 0.9% normal saline have been associated with hyperchloremic acidosis due to their very low strong ion difference, which has been shown to result in increased incidence of kidney dysfunction, prolonged hospital stay, and increased 30-day mortality.³⁵ To avoid these concerns, isotonic balanced crystalloid solutions is preferred to 0.9% saline solutions.

Temperature management (question 10)

• Temperature management plays a significant role in large complex spine cases due to the prolonged length of the procedures, the large amount of exposed tissue at the surgical site, and the downstream effects that a low temperature can have on blood loss, wound healing, surgical site infection, and IONM. Core temperature should be maintained above 36 degrees Celsius.
• Core temperature should be monitored throughout the case. Acceptable core monitoring sites include nasopharynx (check after patient has gone prone to confirm positioning), distal esophagus, tympanic membrane, or the pulmonary artery.
• Temperature can be maintained intraoperatively using forced air warming devices, fluid warmers, humidifiers inside of the anesthesia circuit, and through raising the ambient temperature of the room (preferably before the patient arrives in the room).
• Patients who are undergoing complex spine surgery should also be considered for preoperative warming (prewarming) with forced air warming devices to counteract the redistribution heat loss that can occur following induction of general anesthesia.³⁶

 Perioperative glucose management (question 11)

• It has been well established that poorly controlled glucose levels can predispose surgical patients to poor wound healing, infections, increased length of hospital stay and poor outcomes in neurosurgical patients.³⁷
• The Society for Ambulatory Anesthesia and the Endocrine Society both recommend that intraoperative blood glucose should be maintained between 100 and 180 mg/dl.³⁸ Glucose should be serially monitored throughout the case and effort should be taken to maintain it within the 100 to 180 mg/dl range.

 Losing the airway and CPR in the prone position (question 12)

• If the patient loses their secured airway in the prone position, the staff in the room should be notified and help should be called for immediately.
  • The emphasis should be on bringing the bed/stretcher into the room with the intent of immediately flipping the patient into the supine position to re-secure the airway. This must be communicated to the entire operating room staff so that the patient may be turned supine in an efficient and expeditious manner.
    • Several case reports have documented successful securing of the airway in prone patients with video laryngoscope or with supraglottic airway devices; however, the clinician managing the airway in these emergency situations must have significant expertise for these techniques to be implemented successfully. More evidence is needed to determine if there is a safe and effective way to secure the airway prone with video laryngoscope or supraglottic airway devices.³⁹
  • Surgery should be stopped. The wound should have all surgical instruments removed and should be packed and covered.
  • A difficult airway cart that includes a fiberoptic bronchoscope should be readily available.
    • While waiting for the stretcher and difficult airway cart to be brought into the room, anesthesia staff should attempt to oxygenate the patient with 100% FiO2, with or without an oral/nasal airway.
      • Placement of an LMA may be attempted in the prone position to assist with oxygenation and ventilation in patients who are unconscious or sedated.⁴⁰ If an LMA is successfully placed with appropriate ventilation achieved based on your end tidal CO2 monitors and oxygenation levels, the anesthesia and surgical staff may consider using the LMA to complete the surgery or they may intubate through the LMA if appropriate.
    • If intubation is unsuccessful, the anesthesia provider should follow the American Society of Anesthesiologists Practice Guidelines for Management of the Difficult Airway (2022).
  • Cardiorespiratory arrest while in the prone position for a patient who has a secured airway can be a confusing situation, as most providers are unsure of how to perform CPR in this setting. The American Heart Association (AHA) recommends that the patient be flipped to the supine position as soon as possible and to initiate CPR once flipped for optimized results. However, CPR in the prone position may be a viable option if flipping the patient prone is not an immediately possibility.
    • In instances of cardiorespiratory arrest where the patient may not immediately be flipped into the supine position, the following steps should be taken:
      • The situation should be clearly communicated between all staff in the room immediately. Help should be called and the code or emergency light in the OR should be activated.
      • The surgeon should stop the surgery, remove the surgical instruments, and the wound should be packed. If possible, a dressing or occlusive film should be placed over the operation site.
      • Once cardiac arrest has been confirmed, the following should be ensured:
        • The patient is properly attached to the anesthesia machine and is being ventilated.
        • All anesthetic drugs have or are being given at the correct dose and rate.
        • There has not been any sudden unidentified blood loss (IE: injury to major vessels).
      • The stretcher/bed should be brought into the room with the intent of immediately flipping the patient into the supine position. In the event this is not immediately feasible, CPR should continue in the prone position.
      • Standard ALS guidelines should be implemented as if the patient were in the supine position, with the exception of the initial dose of adrenaline. The initial dose of adrenaline should be given in increments of 50-100 micrograms rather than a 1 mg bolus.⁴¹
      • Hand placement in the midline of the thoracic spine between the T7 and T9 vertebrae will allow compressions to take place over the largest transverse section of the left ventricle, optimizing your forward blood flow. It should be noted that compressions in the prone position may be physically more demanding due to the increased stiffness of the costovertebral joints that require more pressure to compress, therefore it is important for the person running the code to pay close attention to the quality of the compressions.
      • Defibrillator pads may be placed in both armpits or one on the left axillary midline and the other above the right scapula.⁴²

 Postoperative Assessment

 Challenges with extubation (question 13)

• Ideally, the anesthetics should be titrated down to allow for a smooth and rapid emergence and an expedited neurological assessment. However, there are certain factors that must be considered upon emergence. Due to the prolonged nature of these surgeries, the commonly used prone positioning, and the fact that they often require large amounts of fluid and blood product replacement, the risk of significant facial and/or airway edema must be strongly considered at the end of the case. Other factors that should be considered prior to extubation include the length of the procedure, the amount of blood loss, the type of surgery performed and how it relates to potential airway compromise (IE anterior-posterior cervical spine surgery), and patient specific factors such as history of pulmonary disease, obstructive sleep apnea, or obesity. Reintubation in these patients may be much more complicated, therefore risk stratification and considering these factors should play a role in your extubation strategy.⁴³
  • For patients with significant facial edema following a return to a supine position, it may be prudent to wait until for some of the edema to recede. The patient should be positioned in the stretcher with the head of the bed elevated at 30 degrees to allow the edema to drain. The anesthesiologist may also consider extubating the patient over a gum elastic Bougie or an airway exchange catheter.
• Extubation criteria following complex spine procedures should be met before the secured airway is removed:
  • The patient should be alert, able to follow commands, and hemodynamically stable without any major metabolic derangements.
  • The patient should also be spontaneously ventilating with vital capacities of more than 15 cc/kg. The PaO2 should also be above 60 with the FiO2 less than 50%. An A-a gradient of less than 350 on 100% FiO2 also indicates that adequate gas exchange is occurring at the alveolar capillary interface and it is another supportive sign that extubation may be appropriate.
  • Dead space ventilation (Vd/Vt) less than or equal to 0.50 is a reliable predictor that extubation will be successful, while a dead space ventilation ratio above 0.60 – 0.65 has been linked to respiratory failure following extubation.⁴⁴
  • The patient should also have a negative inspiratory force greater than –20 mmHg, which indicates that there is adequate diaphragmatic strength to maintain spontaneous ventilation.⁴⁵
• If there is any doubt whether the patient may be safely extubated, a cuff leak test could be implemented where the endotracheal tube cuff is deflated, and the provider listens for the sound of air movement or gurgling. If air is able to move around the tube, the provider may hear it or be able to feel it over the patient’s airway. If there is too much airway edema, when the cuff is let down, the swelling will occlude the opening in the airway and there will be no sound heard upon auscultation. There are other methods to assess the adequacy of a cuff leak test, such as a volume loss around the cuff of 110cc or more (noted as the inspired vs expired volumes on the anesthesia machine), or a volume loss around the cuff greater than 24% of the tidal volume that the patient was pulling on throughout the case.

 Neurological exam (question 14)

• The neurological exam after the conclusion of the surgery should be completed immediately following emergence from anesthesia. The exam is required to assess the functional outcome of the procedure and to ensure that there are not any potential complications such as acute cord compression due to instrumentation or an arterial bleed that results in an expanding hematoma. The timing of the exam requires the anesthesiologist to adequately titrate their anesthetics in order to have a swift emergence. For patients with delayed emergence, the following rapid panel should be assessed:
  • Vital signs such as blood pressure, temperature, and oxygenation should be assessed to look for any reversible causes.
  • A twitch monitor or accelerometer should be applied to assess for any residual effects of paralytic agents if they were utilized during the case. Ensuring that reversal agents have been administered (sugammadex, neostigmine/glycopyrrolate, or edrophonium/atropine) and have been flushed into the patient is important. If pseudocholinesterase deficiency is suspected, the main course of action should be respiratory support with continued mechanical ventilation until the neuromuscular blockade wears off, as the attempts to reverse the drug may not be consistently successful and they do carry their own inherent risks. Continued sedation to ensure the patient does not have awareness while paralyzed is also important in this situation.
  • A neurological exam should also be performed in the event that there is a delayed emergence. The exam should include an assessment of the cranial nerves, pupils, responses to pain, and reflexes. This exam may alert you to potential ischemic or thromboembolic complications that can delay emergence.
  • Metabolic abnormalities such as hyper or hypoglycemia should be tested for with a fingerstick glucose test. If electrolyte abnormalities are suspected, a complete metabolic or renal function panel should be ordered as well. An arterial blood gas is also recommended to help rule out any potential acid-base disorders or hyper/hypocapnia that may contribute to a delayed emergence. It’s also worth noting here that the patient’s pertinent comorbidities should help guide your laboratory evaluation. For instance, in patients with preexisting hepatic dysfunction, evaluating ammonia levels may be prudent in the event of a delayed emergence.
  • Residual anesthetic drugs (volatile, propofol, barbiturates, ketamine, etc.) are another potential complication that can contribute to a delayed emergence. If lingering effects of your anesthetic agents is suspected to be the cause of your delayed emergence, the anesthesiologist should be aware of specific reversal agents.
    • Excessive narcotics may be reversed with 40 mcg boluses of naloxone. However, it should be noted that naloxone is a short acting drug, and the reversal effect may wear off, resulting in the patient becoming re-narcotized.
    • If residual benzodiazepines are suspected, flumazenil 0.2 mg every 1 minute up to a total dose of 3 mg may be implemented for a reversal effect. It should be noted that in patients with long term benzodiazepine or alcohol use, this drug may precipitate seizures.
    • Central anticholinergic syndrome due to a decrease of the inhibitory effect of acetylcholine in the brain should also be on your differential in patients with delayed emergence who have received anticholinergic agents perioperatively. 1.25 mg of IV physostigmine may be used to reverse central anticholinergic syndrome. Physiostigmine is a tertiary amine that competitively inhibits the effects of acetylcholinesterases, meaning that it can cross the blood brain barrier and increase the levels of acetylcholine, therefore antagonizing central anticholinergic effects.⁴⁶
  • If the above causes have been evaluated and ruled out, neurology consult and imaging with a CT scan of the head to evaluate for a possible stroke is warranted. If sedation/coma persists, the patient should be transferred to the ICU for continued mechanical ventilation and monitoring with frequent neurological exams. Imaging to assess for cerebrovascular accidents may be repeated in 6-8 hours if there is no improvement.

 Postoperative pain management (question 15)

• Postoperative pain management is an important consideration after complex spine surgery due to the emerging evidence that acute untreated pain in the immediate postoperative period may predispose the patient to developing chronic pain.⁴⁷ Many patients undergoing complex spine surgery may already have chronic pain and long-term opioid use, and it is important to understand that their opioid requirements may be much higher in the immediate post-op period. Postoperative analgesia is best managed from a multimodal approach. Post-operative analgesia may be achieved with continuous spinal or epidural opioids (often with concurrent local anesthetic use), peripheral nerve blocks, patient-controlled analgesia, ketamine infusions, acetaminophen, or either IV or oral opioids for breakthrough pain. Although more recent literature is needed, methocarbamol has been demonstrated to help with pain related to muscular spasms following cervical and lumbar laminectomies and it is often administered as a post-operative adjunct medication at our institution once the patient has completed their post-operative neuro exam. It should be noted that methocarbamol causes significant sedation and may delay patient cooperation with neurological exams, therefore it should be implemented with caution and on a patient by patient basis.⁴⁸

Postoperative visual loss (POVL) and other potential postoperative complications (question 16)

• Postoperative visual loss is a rare but devastating sequela of spine surgery that has an incidence of roughly 0.0008–1% depending on the specific procedure. The majority of the cases of POVL will present within the first 24-48 hours postoperatively. The most common cause of POVL following complex spine surgeries is posterior ischemic optic neuropathy, followed by central retinal artery occlusion, and cortical blindness.
  • Central retinal artery occlusion (CRAO) occurs when there is direct pressure on the globe of the eye or an embolic event that causes a sudden blockage of the central retinal artery, resulting in retinal hypoperfusion and visual loss. Since CRAO is likely to be the result of a cerebral vascular accident involving the retina, the workup should resemble that of a stroke/TIA workup and imaging such as a CT of the head may be indicated. A fundoscopic exam and an ophthalmology consult is crucial for accurate and timely diagnosis. On fundoscopy the retina will appear pale with a cherry-red central spot.
  • Ischemic optic neuropathy (ION) is caused by inadequate oxygen delivery to the optic nerve, resulting in sudden loss of vision in one or both of the eyes. The diagnosis can be subdivided into anterior ischemic optic neuropathy (AION), which is the most common cause of POVL following cardiac surgery, and posterior ischemic optic neuropathy (PION), which is the most common cause of POVL following spine surgery. ION usually presents within the first 24-48 hours following surgery and patients can exhibit decreased visual acuity to complete blindness. On fundoscopic exam, PION will have a normal optic disc, while AION will have a swollen optic disc.
• The updated 2019 ASA practice advisory for perioperative visual loss associated with spine surgery have compiled the following risk factors that may be associated with POVL:⁴⁹ preoperative anemia, vascular risk factors (such as hypertension, diabetes, peripheral vascular disease, coronary artery disease, previous stroke, or carotid artery stenosis), obesity, tobacco use, age, male sex, and presence of diabetic retinopathy (level of evidence B2-H). Other surgical risk factors for POVL include major blood loss (>45% of blood volume), prone positioning, intraoperative hypotension, and prolonged surgical duration beyond 6 hours.
• Perfusion to the optic nerve can be derived from mean arterial pressure (MAP) – intraocular pressure (or venous pressure depending on whichever is greater). This has implications during prone spine cases because in the prone position there is increased intraocular pressure and increased venous pressure from pressure on the epigastric vessels in the abdomen. Coupled with the fact that MAP may be decreased from blood loss/hypovolemia or from decreased cardiac output due to increased abdominal pressure in the prone position, this position sets the patient up to a low optic nerve perfusion pressure and thus an increased risk for POVL. Therefore, it is important to maintain perfusion to the optic nerve by avoiding hypotension, anemia, and increased venous pressures.
  • Management of suspected ION involved an immediate ophthalmology consult. Other interventions that can be performed immediately include raising the head of the bed to help lower venous pressures, optimizing physiologic conditions by correcting metabolic disturbances, maintaining a stable MAP above > 65 at the minimum to avoid hypoperfusion, correcting anemia with blood transfusions, optimizing O2 to avoid hypoxia, and ensuring that cardiac output is appropriate.
• Due to the serious and often irreversible nature of these adverse effects, it is imperative that the anesthesiologist identify patients who are at high risk for POVL and to inform them that these risk factors may increase their risk of perioperative vision loss.

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