What the Anesthesiologist Should Know before the Operative Procedure
Spinal fusion and spinal deformity surgery appear at first to be the simple placement of screws with rods, cages and or spacers. However, these procedures are made up of a family of complex procedures that can be surgically and technically challenging and that can lead to extensive bony work in vascularly rich areas.
These factors make effective and safe anesthesia care challenging: blood loss can be rapid and profuse, and frequently utilized neurophysiologic monitoring requires adjustments in anesthetic technique, all of which usually occur during a variety of positions, usually the prone position. The complexity of these surgeries, as well as their number, continues to increase as more and more are being done each year.
1. What is the urgency of the surgery?
What is the risk of delay in order to obtain additional preoperative information?
The underlying pathology and thus the indication for surgery will dictate the urgency of proceeding to the operating room. The usual indications for spinal fusion or spinal deformity surgery include neurologic compromise or pain arising from trauma, scoliosis, malignancy, or infection. Acute causes such as trauma (impact) or pathologic fracture that cause neurologic compromise are considered urgent/emergent and usually necessitate immediate operative correction. Most other deformity surgery arising from scoliosis is elective surgery, based on the progression of the curvature of the spine, pain, and functional limitation. Given the risk of corrective surgery and challenges inevitable to the cardiopulmonary and hematologic systems, rigorous preoperative preparation is warranted.
Emergent surgery is usually carried out when an acute change has manifested either through trauma or pathologic fracture. Transgression of the spinal canal and neural impingement dictates expeditious corrective surgery to alleviate current compression and limit further neural damage. In these circumstances, the balance of risk and benefit may favor immediate surgery even in the face of incomplete preoperative work-up. If the inciting factor is trauma, a multitude of other conditions such as hypovolemia and anemia from hemorrhage and other associated injuries should not be overlooked.
Rapidly progressing instability of the spine may require urgent surgery. These patients usually are stable enough to complete at least a cursory preparation for major spine surgery, including a detailed history and physical exam, complimented by baseline measures of coagulation and hemoglobin, type and cross for blood, and, if possible, an indication of cardiopulmonary function and appropriate functional testing if indicated.
A progression in symptoms from neural impingement or pain that is not responsive to maximal nonoperative therapy usually results in the patient presenting for elective major spine surgery. These patients should be prepared by ensuring that a detailed history and physical exam is performed and all chronic conditions have been maximally optimized prior to surgery.
Cardiopulmonary function should be evaluated if indicated. Psychological preparedness, including a mutidisciplinary approach to the preoperative preparation including primary care, physical therapy, neurosurgery, and anesthesiology, may allow the most informed and prepared patient.
2. Preoperative evaluation
The patient’s status of both the central nervous system (CNS) and peripheral nervous systems should be evaluated. A history of stroke or transient ischemic attack and any residual deficits and elevated risk of stroke should be evaluated and maximally, medically managed.
Some patients may be on platelet inhibitors, including aspirin or clopidogrel, and it is unlikely that the surgeon would proceed with surgery while a patient is on these agents. Consultation with a neurologist may assist in therapeutic recommendations in light of the limitation of their use with imminent surgery, as well as to allow an adequate and informed risk benefit discussion with the patient.
Any visual deficits should be documented, given the increased risk of postoperative visual loss (follows) in these patients.
A detailed neurological assessment of the patient should be performed and documented to allow comparison in the postoperative period.
Most patients presenting for spine surgery have underlying pain, usually treated with enteral opioids. The preoperative regimen should be documented and detailed instructions to the patient should be given, specifically to avoid cessation of opioids to prevent immediate postoperative acute withdrawal, happening at the time of acute increase in pain.
There are several reasons why patients with spinal deformity can present with cardiac disease. Long-standing scoliotic patients may have restrictive lung disease and chronic hypoxia, which may lead to increased pulmonary vascular resistance with ensuing right ventricular hypertrophy and right atrial enlargement.
The age of presentation of senile scoliosis may predispose patients to age-related coronary disease. Furthermore, the functional limitations of mobility and pain secondary to the spinal deformity may make assessing functional capacity difficult. The American College of Cardiology/American Heart Association (ACC/AHA) 2007 Guidelines suggest that unless functional capacity is more than or equal to 4 metabolic equivalents (walking up a flight of stairs), noninvasive testing could be performed if it will change management.
Consideration for the magnitude of surgery along with the potential for major blood loss and high level of postoperative pain needs to be made in conjunction with the urgency and indication for surgery. Many spinal deformity patients preoperatively have a functional limitation secondary to pain or neurogenic claudication and should have a functional stress test prior to major spine surgery. Patients with preexisting coronary disease should be seen in consultation by the managing cardiologist to ensure that maximal optimization of medical therapy has been accomplished.
The degree of curvature of the spine is associated with a decrease in pulmonary function, which usually manifest as restrictive lung disease. Cobb angles wider than 40 degrees correlate to a higher incidence. Pulmonary function testing (PFT), while informative and may aid in risk stratification, may not alter management. Patients with underlying COPD may benefit from PFTs at baseline and bronchodilator therapy to help optimize any treatable reactive component. Baseline arterial blood gas measurement, especially noting the level of hypoxemia and hypercarbia, may aid in postoperative care considerations involving ventilator management.
Pulmonary function is not expected to immediately improve post operatively; in fact, a temporary worsening in function (up to 60%) should be expected during the first postoperative week.
Smoking cessation should occur prior to major spine surgery. Aside from the benefits implicit with smoking cessation for any person undergoing anesthesia (e.g., decreased carboxyhemoglobin, improvement in oxygenation, and, if long enough prior to surgery, decreased airway reactivity), nicotine use has been linked to poor bone healing. Many spine surgeons will wait until a patient has completely ceased using nicotine products, and may perform a urine check to screen for tobacco use prior to proceeding with surgery.
Osteoporosis will affect bone quality, which can lead to increased rates of surgical complications, the most immediate of which is an increased technical difficulty in achieving adequate fixation during instrumentation, which may prolong surgery and lead to increased blood loss. In the postoperative period, an increased rate of proximal junctional kyphosis, implant/screw loosening and pseudarthrosis is present in this population.
Muscular dystrophy and related illnesses that may lead to scoliosis has anesthetic considerations inherent to these diseases. Specifically, the use of succinylcholine may lead to hyperkalemia and cardiac arrest. Volatile anesthetics have been associated with rhabdomyolysis, and these patients may be at a higher risk for malignant hyperthermia, necessitating immediate availability of dantrolene. Both cardiac and respiratory muscles can also be afflicted and thus a thorough preoperative assessment and optimization is important.
Spinal deformity surgery should be delayed for adequate preoperative assessment and optimization. Unless neural compromise is acute and progressive, rigorous preparation is imperative to improve the likelihood of successful major spinal deformity surgery.
What are the implications of co-existing disease on perioperative care?
b. Cardiovascular system
Acute/unstable conditions: Acute and/or unstable conditions need to be stabilized and optimized prior to major spine surgery. Trauma or an acute change in previously tenuous neurologic condition is the most common settings in which neurologic compromise and cardiovascular instability may present together. In these instances, a multidisciplinary team consisting of a cardiologist, anesthesiologist, and surgeon should evaluate risks of proceeding with major spinal corrective surgery in the face of coronary instability. Usually, this would be a very rare situation, as most cases of degenerative spine disease are elective in nature and allow for full work-up and optimization.
Baseline coronary artery disease or cardiac dysfunction – goals of management: Patients should be seen by their cardiologist and unless their physical activity (>4 metabolic equivalents) is reassuring, should undergo functional cardiac testing and subsequent optimization. Placement of coronary stents may alter the patient’s optimal timeline for surgery, as placement of both bare metal and drug-eluting stents carry the risk of in stent thrombosis perioperatively. The need for neurologic surgery needs to be weighed against the need for an immediate stent. Cardiac function should be medically optimized if necessary and medications is continued perioperatively. Hematologic medications – such as aspirin, platelet inhibitors, coumadin, or other anticoagulants – are usually discontinued prior to surgery. The underlying condition necessitating these medicines (e.g., coronary artery disease, in dwelling coronary or other vascular stents, arrhythmias) needs careful consideration and may warrant bridging therapy. The physician managing these conditions should have a good understanding of the risk of discontinuation and should be consulted prior to surgery.
Hypertension: Hypertension, especially long standing, will result in the patient’s vital organs adapting to higher systemic pressures. A careful history of the normal blood pressure of every patient is helpful in establishing parameters to keep the blood pressure within the normal range. Often, the blood pressure on the morning of surgery in the preparatory area is acutely elevated and thus should not be used in figuring the baseline blood pressure. No firm data exist as to what is acceptable blood pressure during surgery. We prefer to keep blood pressure within 20% of normal, as lowering it further likely would increase the risk of stroke and postoperative visual loss. High blood pressures during surgery may lead to more surgical blood loss and make visualization of the surgical site more difficult.
COPD, asthma, and restrictive lung disease should be medically optimized prior to surgery. Pulmonary function testing may help characterize the underlying pulmonary state.
Smoking cessation prior to surgery is important for optimizing pulmonary function, as well as increasing the chances of adequate bone healing.
An optimized regimen should be guided by the patient’s primary care physician or pulmonologist and may include bronchodilators (both short and long acting), inhaled anticholinergic drugs, and steroids (inhaled or enteral). It is best to defer surgery in the face of an acute exacerbation of COPD or asthma.
Patients with renal impairment need to be considered carefully. Borderline renal failure patients may be reliant on adequate hydration to avoid worsening of renal failure. End-stage renal failure patients present a difficult scenario because often with major spine surgery comes the need to administer volume as crystalloid as well as blood products. Volume overload is possible, and having a nephrologist on the team of managing physicians preoperatively will allow for expeditious care, including dialysis post operatively.
Potassium levels need to be optimized preoperatively and monitored intraoperatively as these may rise precipitously with the amount of blood product administered during surgery.
A baseline neurologic exam should be performed and documented so as to not confuse new deficits with old ones. Patients with carotid stenosis and at risk for further strokes should be managed, based on the current guidelines directing medical and surgical therapy.
Patients who have had prior strokes need to have baseline physical exams and documentation again. These patients may be at higher risks for developing further neurologic decline perioperatively.
Diabetic patients should be medically managed and optimized prior to surgery. Patients should refrain from taking metformin 12 to 24 hours prior to surgery due to an increased risk of lactic acidosis during major surgery.
Knowing the patient’s insulin regimen prior to the surgery will help in determining his or her level of insulin tolerance and aid in directing blood glucose during surgery. As hyperglycemia has been implicated in both increasing the risk of wound infection, as well as worsening neurologic function following insult, it is prudent to control blood sugars perioperatively.
g. Additional systems/conditions which may be of concern in a patient undergoing this procedure and are relevant for the anesthetic plan (e.g., musculoskeletal in orthopedic procedures, hematologic in a cancer patient)
4. What are the patient's medications and how should they be managed in the perioperative period?
h. Are there medications commonly seen in patients undergoing this procedure and for which should there be greater concern?
i. What should be recommended with regard to continuation of medications taken chronically?
Cardiac: In general, medications should be continued. Debate exists as to whether antihypertensives (nonbeta blocker) should be continued; their ongoing action may make management of hypotension difficult during the surgery, as much blood loss and hypovolemia are expected. Medications to treat poor myocardial function, such as low ejection fraction/systolic dysfunction, should be continued. If a patient is reliant on a medication for preserving cardiac function (e.g., an ACE inhibitor prescribed for afterload reduction), then it should be continued. If the medication is for chronic hypertension, it may be advantageous to hold it during the acute perioperative period. Beta-blockers should not be withheld perioperatively, as acute cessation may cause a withdrawal syndrome with associated tachycardia.
Pulmonary: Inhalers and steroids should be continued if required to achieve optimal control of chronic pulmonary conditions.
Renal: Common medications ito support renal failure patients may include an ACE inhibitor, statin, erythropoetin, diuretic(s), calcium, vitamin D, phosphate binder. All of these medications may be continued until the day of surgery. Consideration for temporary cessation of the ACE inhibitor and diuretic may be warranted, secondary to anticipated hemodynamic compromise intraoperatively.
Neurologic: Medication should be continued perioperatively, anticholinesterase inhibitors (donepezil) can be continued but need to be considered when using neuromuscular blocking agents.
Antiplatelet: Antiplatelet medications should be discontinued prior to surgery. Management of antiplatelet medications requires a review of the indication for use, risk of discontinuation, and discussion with prescribing physician and surgeon.
Psychiatric: Selective serotonin reuptake inhibitors (SSRIs) are usually safe. Tricyclic antidepressants (TCAs) can have anticholinergic effects as well as interact with vasopressors and, if possible, should be withheld. Monoamin oxidase inhibitors (MAOIs) may be associated with life-threatening hypertensive crisis with certain medications given intraoperatively (e.g. ephedrine, demerol). The use of MAOIs has dramatically been reduced and it is uncommon to find patients presenting for surgery still on MAOIs. In a preoperative work up, if a patient presents on an MAOI, this may be an indication that his or her psychiatric disorder may be complex and enlisting the help of a psychiatrist to ensure mental preparedness for such a major surgery as well as to comment on perioperative medication management may be helpful.
NSIADS: These should be held preoperatively for at least 5 to 7 days to avoid interactions with platelets, making hemostasis more difficult
Neuroleptic agents: Neuroleptic agents such as gabapentin should be continued perioperatively. These may decrease the levels of postoperative pain but may lead to somnolence, especially in the elderly.
Opioids: Opioids should not be stopped preoperatively to avoid acute withdrawal at a time of increased pain from surgery.
j. How to modify care for patients with known allergies
Avoid known causes of allergy and use suitable alternatives.
k. Latex allergy – If the patient has a sensitivity to latex (e.g., rash from gloves, underwear, etc.) versus anaphylactic reaction, prepare the operating room with latex-free products.
Avoid contact with latex during the perioperative period.
l. Does the patient have any antibiotic allergies – Common antibiotic allergies and alternative antibiotics
Antibiotic allergies may necessitate a change to a different class of antibiotics as deemed appropriate.
m. Does the patient have a history of allergy to anesthesia?
Documented: Avoid all trigger agents such as succinylcholine and inhalational agents.
Proposed general anesthetic plan: Total intravenous anesthesia with propofol and narcotic.
Insure MH cart availability.
Local anesthetics/muscle relaxants
Local anesthetic allergies should be evaluated as to which class the offending agent belongs to. The most common allergies are to ester local anesthetics due to metabolism to para-amino benzoic acid. Utilization of the amides should be safe in these patients. A reaction to the epinephrine in the local anesthetic should be considered, as many individuals report the epinephrine effect as an allergy to local anesthetics.
Muscle relaxants that cause histamine release may be misinterpreted as an allergy. If a true allergy to a muscle relaxant is present, use of a different class may be warranted. Often, the only time a muscle relaxation is warranted is during tracheal intubation. As such, the use of succinylcholine or deep anesthesia may allow avoidance of a specific muscle relaxant.
The anesthesiologist could consider evaluation by an allergist to help clarify confusing allergies.
5. What laboratory tests should be obtained and has everything been reviewed?
The following baseline labs/tests should be obtained:
Hemoglobin level and platelet count.
Type and cross for blood products (type and amount will depend on complexity and duration of surgery, we typically cross match 6 units of pRBCs and 2 units of thawed plasma).
Coagulation panel, including prothrombin time, partial thromboplastin time.
Imaging: Spine films (plain and advanced CT or MRI) will be present; evaluate these to look for angle of curvature and pulmonary pathology.
Other tests: Cardiac studies should include a baseline EKG; functional testing (stress echocardiograph), unless the patient presents with more than 4 metabolic equivalents; and PFTs if clinically apparent pulmonary disease is present. The helpful aspect of PFTs is for prognostication as well as evaluating the reversibility of defects with bronchodilators.
Intraoperative Management: What are the options for anesthetic management and how to determine the best technique?
General anesthesia is indicated for major spine deformity surgery. The procedure requires significant exposure and duration, such that regional anesthesia is not feasible as a sole modality.
6. What is the author's preferred method of anesthesia technique and why?
The most common approach for spinal deformity surgery is placing the patient the prone position. If the surgery is confined to the lumbar area, the patient’s arms are usually placed in the upward position, toward the anesthesiologist, so arms are available for further cannulation and monitor application.
The patient’s head, in this case, may be placed on a foam head rest or other supportive frame. Care must be taken to ensure that the endotracheal tube does not kink; the long surgical time and body warmth will soften its walls and pressure from the face support (if a foam rest is used) can exacerbate kinking. Also, eyes must be free of pressure and checked routinely for compressions that could lead to postoperative visual loss (discussed below).
If the surgery is on the thoracic spine and definitely cervical surgery, a Mayfield head holder is employed and the patient’s arms are tucked at the sides, which may impede monitor or intravenous line function. The Mayfield head holder does allow for the endotracheal tube to rest in a natural position and makes external kinking much less likely. Also, there is a lesser likelihood of globe compression, as no supportive structure should be near the eyes.
It is important to note that there have been patients who have slipped out of the Mayfield either due to equipment malfunction or operator error, so continued vigilance regarding head position and especially globe compression is required despite use of the Mayfield. Care must be taken to position the rest of the body correctly on the frame selected for surgery.
A Jackson frame usually is used for more complex spine surgery, as it allows the surgeon ease of access for operating, as well as C-arm radiography during the surgery. If the prone position is used, care must be taken to avoid compression injuries to the patient’s breasts, genitals, knees, and toes. Abdominal and transmitted thoracic pressure must also be checked for and avoided after turning prone.
Vascular access and fluid administration. Blood loss can be rapid and preparation with at least two large bore intravenous catheters is necessary. Cannulation of a large central vein with a large bore high flow catheter makes fluid resuscitation as well as vasoactive drug administration more reliable. Rapid fluid resuscitation with peripheral intravenous lines that cannot be observed at skin entry may lead to delayed detection of infiltration. A rapid transfuser (Level 1 or Belmont) can make administering multiple units of blood products more efficient and allow the anesthesiologist free hands to tend to other duties.
Standard ASA monitors should be utilized with care to secure ECG electrodes, as there is very limited access to the patient after the drapes are placed and all the surgical equipment and C-arm are in place. An arterial catheter should be placed for monitoring blood pressure, with anticipated acute changes in intravascular volume, as well as for frequent lab draws to monitor hemoglobin levels and coagulation status.
During the preparation and proning stages, the patient is frequently left uncovered for an extended time, and heat loss can be rapid and profound. Prewarming the patient, as well as warming the room prior to induction, is beneficial.
Depending on the style of the surgical bed, the underside of the patient may also be left exposed (Jackson frame), and correction of hypothermia may be difficult. Effort should be made to place an upper-body as well as lower-body or an under-table warmer on the patient to allow aggressive warming back to normothermia. These efforts are important to avoid a hypothermic exacerbation in coagulopathy and the potential for increased wound infection with hypothermia.
(See discussion below.)
Blood pressure management
Some studies suggest that controlled hypotension with either vasoactive drugs or anesthetics can result in decreased blood loss during surgery. The use of controlled hypotension has seen a recent decline, most probably as the awareness of postoperative visual loss has increased. It is impossible to currently ensure the adequacy of perfusion to vital organs (heart and brain) and eyes, and the risks of reducing blood loss need to be weighed against the risk of ischemia to these organs.
We usually try to maintain normotension (as defined by a review of at least three prior blood pressures documented in the chart) during prolonged and complex spine surgery. Periodic, brief (minutes) episodes of mild hypotension (a decrease of <20% baseline pressures) should be well tolerated by most patients.
The mechanism used to lower blood pressure needs to be taken into account: (1) patient comorbidities and ability to tolerate method – myocardial depression of anesthetics; and (2) monitoring modalities being employed, as increasing depth of anesthesia will lead to a degradation of neurophysiologic monitoring signals.
What are the most common intraoperative complications and how can they be avoided/treated?
Cardiac complications: Hypovolemia from acute blood loss is common during major spine surgery. Blood loss during dissection on approach, osteotomy creation, and pedicle screw placement can be rapid and profused. Maintaining intravascular volume is important to ensure adequate coronary and cerebral perfusion. Maintenance of an adequate hemoglobin level is important in those patients with underlying coronary disease.
Pulmonary: Given the rigid and vascular structure of the bones and propensity for low central venous pressure secondary to hypovolemia, the risk of venous air embolism is always present during major spinal surgery. Although we do not routinely monitor for this complication with TEE or precordial Doppler, a hypotensive event associated with low CO2 return should trigger a heightened suspicion for venous air embolism. Transfusion related lung injury (TRALI) should be suspected when oxygenation deteriorates in the setting of transfusion. Risk ranges from 1:1200 to 1:5000 per unit transfused. Treatment includes stopping the offending transfusion and supportive measures.
Neurologic: Neural compromise can result from direct trauma from the surgical procedure or can include cerebral injury, most likely from hypoperfusion during low perfusion states that may occur should the intravascular volume be depleted. Every effort should be made to keep up with both volume and coagulation status to avoid this complication.
Hematologic: Blood loss and coagulopathy are perhaps the most common and most challenging aspects surrounding the anesthetic management of major spine surgery. Blood loss can be rapid and profuse, and managing the ongoing coagulopathy that develops can be challenging. Monitoring for blood loss can be difficult, as the surgical site is constantly oozing, and blood can be suctioned with copious amounts of irrigation, it can travel down the drapes or soaked up into sponges. Monitoring hematocrit and coagulation at regular intervals aids in detecting irregularities that can arise at any time during the surgery and that are otherwise difficult to diagnose. We monitor hematocrit; blood gas; coagulation, including prothrombin time and partial thromboplastin time; platelets; and fibrinogen on an hourly basis. The triggers to transfuse red cells and plasma are much different than in most surgeries, and early transfusion is warranted. In general, we aim to keep hematocrit higher than 30% and INR below 1.3. The anesthesiologist must remember that when replacing red blood cells, unless coagulation factors are administered, a dilutional coagulopathy will occur. The ratio of red cells to plasma usually ends up being about 1 to 1 or 1 to 2.
Cell saver can be used to try to reduce allogeneic blood use. Care must be used to not suction antibiotic irrigation or gel foam into the cell saver suction. Data on its success in reducing allogeneic blood usage is limited.
Normovolemic hemodilution has also been used to reduce the loss of high hemoglobin containing blood and thus to try to reduce the need for allogeneic blood. There are case series that show promise, but evidence is lacking in both efficacy and safety of this technique.
Antifibrinolytics have shown some promise in reducing blood loss during major spine surgery; however, safety data is lacking in this population.
Intraoperative neurophysiologic monitoring may include motor evoked potential (MEP), somatosensory evoked potential (SSEP), as well as electromyographic (EMG) monitoring. Monitoring any or all of these modalities usually entails the use of a dedicated neurophysiologic technician as well as specialized recording apparatus.
Monitoring does add to set up time (lead application may be started prior to induction and/or completed after induction) and to complexity in line management, as the numerous leads involved in monitoring can add challenge to adequate line management during positioning. Care must be taken to ensure occupational safety when needle electrodes are used, as they are easily and frequently dislodged from the patient and may present a needle stick risk.
MEP monitoring consists of applying a stimulus over the motor cortex which travels down predominantly in the anterior portion of the spinal cord, which then causes measureable responses in distal muscles in the arms or legs. The stimulus is applied at a discrete time and leads to an abrupt and short patient movement. MEP signals are usually requested by the surgeon after placement of a pedicle screw or distraction of the spine. The stimulus frequently activates the masseter muscles, requiring prior placement of a soft bite block between the molars and confirming that the tongue is not protruding to prevent lingual injury.
MEP recording is affected by inhaled anesthetics in a dose-dependent manner. There is some evidence that desflurane may be the most amenable to allowing MEP recording during the administration of volatile anesthetics. The quality and reliability of recording most often is the best with a total intravenous anesthetic (TIVA) consisting of propofol and an opioid infusion. Use of TIVA to facilitate the best MEP signal needs to be considered with inherent risk of TIVA, that is, ensuring continuous anesthetic administration.
With complete neuromuscular blockade, no signal can be obtained. Some studies have shown that MEP monitoring can be successful with partial but constant neuromuscular blockade. Unless a stable infusion of a neuromuscular blocking drug can be administered with a quantitative train of four monitoring, MEP monitoring is most easily and reliably done in the absence of neuromuscular blockade.
The initial administration of neuromuscular blockade to facilitate tracheal intubation deserves special mention, as many tenets of care may be competing. While surgeon- and technician-dependent, baseline MEP recordings may be warranted to (1) establish a baseline to make further comparisons and (2) ensure that induction, intubation, and positioning have not led to any damage of the motor tracts.
A single intubating dose of a nondepolarizing neuromuscular blocker may not allow timely recovery of neuromuscular function to allow a baseline recording. This needs to be weighed against the risks of using alternatives: succinylcholine (risk for hyperkalemia in the setting of potentially progressive upper motor neuron deficit) or no neuromuscular blocker (risk for difficult, traumatic intubation or effects of deep anesthesia from inhaled agents and propofol – hypotension and myocardial depression and from remifentanil – hypotension and bradycardia).
SSEP monitoring is very different from MEP in direction, location, and appearance, as well as sensitivity to anesthetics. SSEPs are generated by stimulating peripherally, which then propagates a signal toward in peripheral nerves towards the CNS, up the spinal cord via the dorsal columns, and finally synapsing over the sensory cortex, where recording electrodes are placed.
SSEP monitoring requires frequent and repetitive stimulation, as a number of stimuli are administered and averaged over time to give the best-quality recording. In an unparalyzed patient, this will have the effect of causing hands and feet to twitch as the stimulation results in neural transmission both toward the peripheral (evoking a motor response that is a by-product of stimulation and unnecessary for SSEP recording – this response is not an MEP), as well as toward the CNS. This movement can interfere with monitors, such as arterial lines and pulse oximeters.
SSEP monitoring also are affected in a dose-dependent fashion by inhaled anesthetics as well as propofol infusions but, in general, is more robust than MEPs. Typically, as long as the anesthetic is stable, quality recordings can be obtained with either volatile agents or propofol. Muscle relaxation can aid in the quality of SSEP recording by reducing background noise, but cannot be used if both MEPs and SSEPs are being recorded.
EMG monitoring usually consists of electrodes over myotomes of interest and either spontaneous discharge is observed or stimulation at the surgical site is monitored. EMG is fairly robust against anesthetic interference, but because it relies on muscle activity, neuromuscular blocking agents are contraindicated.
See Table I (Anesthesia for neurologic intraoperative monitoring recommendations).
The choice of opioid for infusion depends on perceived risks and benefits associated with each agent. The two most commonly used are remifentanil and fentanyl.
Remifentanil has the benefit of end-organ independent and rapid metabolism so that there is no concern with duration of administration and off set of action. It is rapidly titratable, which lends itself to neurophysiologic monitoring, as it would be the portion of the anesthetic that can be titrated readily to vary the level of anesthesia, while minimizing impact on recorded signals. Spinal deformity surgery is a very painful surgery in the postoperative period. Most patients present with some element of opioid tolerance and when using remifentanil, the anesthesiologist must be cognizant of its short duration of action and potential to contribute to acute opioid tolerance.
Fentanyl is longer acting in the postoperative period due to its context-sensitive half life, which may afford better analgesia once the intraoperative anesthetic wears off; however, it also confers the disadvantages of slower offset with ensuing respiratory depression, as well as obscures postoperative neurological assessment.
It is important to remember that hypothermia and hypotension can also lead to degradation of signals, independent of neurologic compromise or anesthetic levels.
Postoperative visual loss after major spine surgery has received increasing attention. The most common cause is posterior ischemic optic neuropathy (PION), the etiology of which remains largely unknown and likely represents interplay between duration of surgery, amount of venous pooling and pressure near/in the orbit, surgical position, and blood pressure management. The association of PION with longer surgeries (>6 hours) or higher blood loss surgeries (>1000 mL) has led to consideration of staging surgeries when these end points are near.
b. If the patient is intubated, are there any special criteria for extubation?
The magnitude of blood loss requiring massive resuscitative efforts coupled with the gravitational disadvantage of the prone position can lead to swelling in the airway. The challenge to the anesthesiologist will be in quantifying the swelling and deciding to proceed with extubation or proceed to the ICU with endotracheal tube in place. This decision can be very difficult to make and the consequence of extubating a patient with a swollen airway can be disastrous, as it may not be possible to mask ventilate and very difficult to reintubate. Keeping a patient intubated is not without consequence: a neurologic exam will likely need to be deferred until after extubation, and the inherent risks to remaining mechanically ventilated, including the risk of ventilator-associated pneumonias, are present.
One method for helping parse out the patient who will be able to maintain patency of his or her airway post extubation is to assess the patient’s ability to breath with an endotracheal tube with the cuff deflated. After suctioning the patient’s oropharynx with the patient spontaneously ventilating, disconnect the endotracheal tube from the circuit and occlude the end of the endotracheal tube. If air movement is detected during normal quiet respiration, it is likely that there is significant enough tracheal calibre to allow removal of the endotracheal tube and have the trachea remain patent. If no air movement is detected, caution must be exercised as there may be significant swelling. Care must be maintained while attempting this maneuver, as a vigorous inspiratory effort against an occluded tube and swollen trachea may lean to negative-pressure pulmonary edema.
c. Postoperative management
Patients who have undergone major spine surgery have multiple systems with sufficient derangement such that monitoring initially in an intensive care setting is indicated.
Even patients with no underlying cardiovascular comorbidity and especially those who present with preexisting risks should be supported hemodynamically with volume or vasopressors. After closure, continued bleeding may occur and intravascular volume can be depleted rapidly. Observing the output from any drains left in place can help guide therapy.
Patients are likely to have worsening of pulmonary function post operatively after spine surgery, especially if the surgery involves transthoracic access. While volume status may be low, it is not uncommon for patients to have pulmonary edema and pleural effusions, perhaps related to the massive blood loss, fluid shifts, and transfusion/resuscitation.
Managing pain post operatively in major spine surgery is difficult. The goals of quick emergence for neurologic examination counter those of smooth emergence with sufficient analgesic medication appropriate for the complexity of the surgery.
Compounding this problem is the high doses of narcotic administered to facilitate general anesthesia, where modern neuromonitoring limits the use of higher doses of inhaled anesthetics and propofol, and which usually limits the use of any neuromuscular blockade. High-dose narcotic facilitates optimum neuromonitoring, while helping to limit patient movement. This intraoperative narcotic use may predispose the patient to opioid-induced hyperalgesia and acute opioid tolerance. Use of ketamine (0.5 mg/kg) preemptively may decrease the risk of this opioid-related tolerance. A ketamine infusion (0.1 mg/kg/hour) can be utilized intraoperatively and postoperatively if deemed necessary. The baseline narcotic usage of the patient should be documented and maintained.
Multimodal therapy, including acetaminophen (oral or intravenous), neuropathic agents such as gabapentin, and ketamine have shown promise in aiding analgesia, while sparing the dose of opioids. Epidural administration of opioids and/or local anesthetics have shown some promise and may be considered in patients that present with significant preoperative opioids. Epidural use of local anesthetics and indwelling catheters need to be considered with regard to the risk of obscuring the neurological exam (local anesthetics) as well as of infection risk, in light of the large amount of hardware that is usually placed during these surgeries.
Ongoing blood loss from a newly corrected spine is common. Blood loss continues for several days post operation, and careful attention to coagulation levels, as well as blood counts, is important. These patients are also at high risk for developing deep vein thrombosis and pulmonary embolism, likely from the derangement in the coagulation system during surgery, as well as the limited mobility post operatively. Prophylaxis, including sequential compression devices, should be used perioperatively and into the postoperative period with chemical prophylaxis, including subcutaneous heparin, which should commence when deemed safe from a blood loss perspective, usually around post-operative day 2.
What's the Evidence?
“Practice advisory for perioperative visual loss associated with spine surgery: a report by the American Society of Anesthesiologists Task Force on Perioperative Blindness”. Anesthesiology. vol. 104. 2006. pp. 1319-28. (Reviews case series of POVL and addresses possible associations with causation/prevention.)
Halpin, RJ, Sugrue, PA, Gould, RW, Kallas, PG, Schafer, MF, Ondra, SL, Koski, TR. “Standardizing care for high-risk patients in spine surgery: the Northwestern high-risk spine protocol”. Spine. vol. 35. 2010. pp. 2232-8. (Excellent review article, giving one organization's approach to these high risk surgeries from preoparative work-up, intraoperative managment, and postoperative care.)
Elgafy, H, Bransford, RJ, McGuire, RA, Dettori, JR, Fischer, D. “Blood loss in major spine surgery: are there effective measures to decrease massive hemorrhage in major spine fusion surgery?”. Spine. vol. 35. 2010. pp. S47-56. (Review article evaluating the literature on possible ways to reduce blood loss during major spine surgery.)
Lo, YL, Dan, YF, Tan, YE, Nurjannah, S, Tan, SB, Tan, CT, Raman, S. “Intraoperative motor-evoked potential monitoring in scoliosis surgery: comparison of desflurane/nitrous oxide with propofol total intravenous anesthetic regimens”. J Neurosurg Anesthesiol. vol. 18. 2006. pp. 211-4. (Describes and tests the use of desflurane versus TIVA for MEP monitoring.)
Sloan, TB, Heyer, E. “Anesthesia for intraoperative neurophysiologic monitoring of the spinal cord”. J Clin Neurophysiol. vol. 19. 2002. pp. 430-43. (Excellent review for understanding the basics of neurophysiologic monitoring.)
Milbrandt, TA, Singhal, M, Minter, C, McClung, A, Talwalkar, VR, Iwinski, HJ, Walker, J, Beimesch, C, Montgomery, C, Sucato, DJ. “A comparison of three methods of pain control for posterior spinal fusions in adolescent idiopathic scoliosis”. Spine. vol. 34. 2009. pp. 1499-503. (Looks at intrathecal opioids or epidural with opioid and local anesthetic versus PCA.)
Zhang, JG, Wang, W, Qiu, GX, Wang, YP, Weng, XS, Xu, HG. “The role of preoperative pulmonary function tests in the surgical treatment of scoliosis”. Spine. vol. 30. 2005. pp. 218-21.
Bowen, RE, Gardner, S, Scaduto, AA, Eagan, M, Beckstead, J. “Efficacy of intraoperative cell salvage systems in pediatric idiopathic scoliosis patients undergoing posterior spinal fusion with segmental spinal instrumentation”. Spine. vol. 35. 2010. pp. 246-51.
Emery, SE, Akhavan, S, Miller, P, Furey, CG, Yoo, JU, Rowbottom, JR, Bohlman, HH. “Steroids and risk factors for airway compromise in multilevel cervical corpectomy patients: a prospective, randomized, double-blind study”. Spine. vol. 34. 2009. pp. 229-32. (A look at extubation in anterior cervical spine surgery patients. Similar leak test can be used in prone spine surgery.)
Guignard, B, Coste, C, Costes, H, Sessler, DI, Lebrault, C, Morris, W, Simonnet, G, Chauvin, M. “Supplementing desflurane-remifentanil anesthesia with small-dose ketamine reduces perioperative opioid analgesic requirements”. Anesth Analg. vol. 95. 2002. pp. 103-8. (Potential benefit of ketamine use intraoperatively to decrease postoperative hyperalgesia and narcotic use.)
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- What the Anesthesiologist Should Know before the Operative Procedure
- 1. What is the urgency of the surgery?
- What is the risk of delay in order to obtain additional preoperative information?
- 2. Preoperative evaluation
- What are the implications of co-existing disease on perioperative care?
- b. Cardiovascular system
- c. Pulmonary
- d. Renal-GI:
- e. Neurologic:
- f. Endocrine:
- g. Additional systems/conditions which may be of concern in a patient undergoing this procedure and are relevant for the anesthetic plan (e.g., musculoskeletal in orthopedic procedures, hematologic in a cancer patient)
- 4. What are the patient's medications and how should they be managed in the perioperative period?
- h. Are there medications commonly seen in patients undergoing this procedure and for which should there be greater concern?
- i. What should be recommended with regard to continuation of medications taken chronically?
- j. How to modify care for patients with known allergies
- k. Latex allergy - If the patient has a sensitivity to latex (e.g., rash from gloves, underwear, etc.) versus anaphylactic reaction, prepare the operating room with latex-free products.
- l. Does the patient have any antibiotic allergies - Common antibiotic allergies and alternative antibiotics
- m. Does the patient have a history of allergy to anesthesia?
- 5. What laboratory tests should be obtained and has everything been reviewed?
- Intraoperative Management: What are the options for anesthetic management and how to determine the best technique?
- 6. What is the author's preferred method of anesthesia technique and why?
- a. Neurologic:
- b. If the patient is intubated, are there any special criteria for extubation?
- c. Postoperative management