Chapter 181 Advances in Anesthesia for Spine Surgery and the Prevention of Complications
Anesthesia for Cervical Spine Surgery
Movement of Cervical Spine with Intubation
The primary force applied by the laryngoscopist is upward lift with a little bit of angular force. This force can be as high as 50 to 70 N (40 N is enough to lift 10 pounds). The more difficult the exposure, the greater the force usually applied. Intubation needs extension at occiput-C1, combined with flexion at lower vertebrae (the fulcrum is probably at C7-T1).1 Direct laryngoscopy with a Macintosh size 3 blade results in near-maximal extension at occiput-C1 (with the posterior arch of C1 touching the skull).1,2
To identify an ideal method for intubation,3 Sahin et al. studied upper cervical vertebral motion with three intubation devices. Comparisons were made between direct laryngoscopy, the intubating laryngeal mask airway (ILMA), and fiberoptic intubation. In their study, the mean motion at the C1-2 level was 10.2 ± 7.3, 5.0 ± 6.3, and 1.6 ± 3.2 degrees, respectively. The fiberoptic method was found to produce the least motion in the upper cervical spine. The authors concluded that fiberoptic laryngoscopy is the most suitable intubation technique when cervical spine movement is not desired.
There are a few degrees less extension at C1-2 with use of a straight blade.4 It is unlikely that this difference is clinically meaningful. During intubation under general anesthesia with neuromuscular blockade and manual in-line stabilization, the use of the GlideScope video laryngoscope (Verathon, Inc., Bothell, WA) produced better glottic visualization, but did not significantly decrease movement of the nonpathologic cervical spine compared with direct laryngoscopy.5
Cervical Spine Movement and Laryngeal Mask Airways
Keller et al.6 implanted microchip sensors into the pharyngeal surfaces of C2 and C3 in 20 cadavers to determine the pressures exerted against the cervical spine by the laryngeal mask airway (LMA) and ILMA. The authors concluded that these devices exert high pressures against the upper cervical vertebrae during insertion, during inflation, and in situ. These pressures could produce posterior displacement of the upper cervical spine.
Manual In-line Stabilization and Cricoid Pressure
The goal of manual in-line stabilization is to apply forces to the head and neck equal in magnitude and opposite in direction to those generated by the laryngoscopist so as to limit the movement that might result during airway management; traction forces should be avoided. Manual in-line stabilization failed to reduce movement at the site of instability in cadaver models.7,8 Cricoid pressure (as long as it is not excessive) did not result in movement in a cadaver model of an injured upper cervical spine.9
Maintaining the head in neutral or near-neutral position can be very important in maintaining proper cervical cord blood supply. Flexion of the spine causes elongation of the cord with narrowing of the diameter of the longitudinal vessels.10 Extension causes an increase in diameter of the cervical cord and folding of the ligamentum flavum, which may exert pressure on the cord and posterior longitudinal vessels.11 Rau et al.12 described a case of quadriplegia in a patient who underwent posterior fossa surgery in the prone position. The authors state that during a prolonged period in which the neck was in hyperflexion, overstretching of the cervical spinal cord and compromise to its blood supply likely caused this devastating complication.
Practical Points to Remember
• Awake fiberoptic intubation is the gold standard for patients with unstable neck injuries.
• Surveys indicate that the majority of U.S. anesthesiologists would prefer to use a fiberoptic bronchoscope to intubate at-risk patients and to do so with the patient awake.13
• Induction of anesthesia diminishes the protective stabilization of the neck musculature. Neck motion during this phase can be substantial, sometimes producing dynamic cord compression that could result in cervical cord injury.
• If the cervical spine is grossly unstable, consideration should be given to both intubating the trachea and positioning the patient while he or she is still awake. If new neurologic symptoms developed during positioning, repositioning should be attempted. During that period, tight control of the patient’s blood pressure and even inducing hypertension can help resolve these new symptoms.
• Airway complications are common after anterior cervical spine surgery and may range from acute airway obstruction to chronic vocal cord dysfunction. Recurrent laryngeal nerve injury after anterior cervical spine surgery could be due to direct nerve injury at the time of neck dissection, surgical retractor placement, and endotracheal balloon insufflation pressure.14,15
• Cervical spine surgery in the prone position could result in laryngeal edema and macroglossia.7,16
• The use of fiberoptic intubation was shown to result in fewer airway complications after cervical spine surgery, thought to be due to a reduction in soft tissue trauma.17
• For patients with subaxial spondylotic myelopathy, neck extension can narrow the diameter of the spinal cord, whereas in patients with atlantoaxial subluxation, such as those with rheumatoid arthritis or Down syndrome, neck flexion will widen the atlantodental interval, narrowing the spinal canal.
• Maintaining adequate spinal cord perfusion is crucial during cervical spine surgery in patients with spondylitic myelopathy. Chronic mechanical compression inhibiting the spinal cord blood supply leads to gradual, intermittent microinfarction of the cord. In this setting, invasive blood pressure monitoring using an arterial line and close attention to maintaining adequate perfusion pressure are very important.14
Physiologic Changes in the Prone Position
Cardiovascular Changes
Using a noninvasive cardiac output monitor, both cardiac index (CI) and venous return18 decreased in unanesthetized, healthy volunteers in the prone position. CI decreased compared with the supine position as follows: knee-chest position (20%); on pelvic props from a modified Relton-Hall frame under the anterior superior iliac spines and padded support under the chest (17%); on an evacuatable mattress (11%); and on pillows (3%; one pillow under the thorax and one under the abdomen, leaving the abdomen free to move). Toyota and Amaki19 studied transesophageal echocardiograms in 15 healthy patients undergoing prone-position lumbar laminectomy. The prone position caused left ventricular volume and compliance to decrease. These changes were attributed to a decrease in the venous return due to inferior vena caval compression, and decreased left ventricular compliance due to increased intrathoracic pressure in the prone position. These results had been confirmed by other studies using thermodilution pulmonary artery catheters to measure the cardiac index when transferring from the supine to the prone position. Cardiac output in these studies decreased by 17% to 24%.14 The reduction in cardiac output in the prone position also leads to a decrease in the metabolism of propofol.20 A reduction in propofol metabolism while in the prone position could also explain the results of Sudheer et al.,21 who showed a significant reduction in cardiac output in the prone position during maintenance of anesthesia using propofol compared with isoflurane. Pearce22 observed vena caval pressures to be 0 to 40 mm H2O in patients in the prone position with the abdomen hanging free. In contrast, patients with abdominal compression had vena caval pressures greater than 300 mm H2O. Increased venous pressure not only increases bleeding during spine surgery owing to congestion of vertebral veins but can impair spinal cord perfusion.
The use of the prone position with abdominal compression was identified as a plausible cause of spinal cord ischemia leading to neurologic deficits after cervical laminectomy. The authors of this case series recommended the avoidance of abdominal compression and hypotension, especially in myelopathic patients for whom maintenance of spinal cord perfusion pressure is of paramount importance.23
Changes in Respiratory Physiology
In an elegant study, Nyren et al.24 studied the regional distribution of pulmonary blood flow in 10 healthy volunteers. The subjects were studied in both prone and supine positions with and without lung distention caused by 10 cm H2O of continuous positive airway pressure. The results demonstrated that ventilation-perfusion matching during both normal breathing and positive pressure is more favorable in the prone than in the supine position. Because perfusion is more evenly distributed in the prone position, the recruitment of dorsal airways results in an increase in lung units and consequently increased functional residual capacity, with near-normal ventilation-perfusion matching and a reduction in shunt.25 By turning the patient prone and recruiting airways in the dorsal lung, prone positioning achieves similar beneficial effects as positive end-expiratory pressure ventilation but without the risks of barotrauma or interference with cardiac function. Of note, the prone position is sometimes used in patients with acute respiratory distress syndrome to improve oxygenation and decrease shunt.25 Similar findings were confirmed by Pelosi et al.26 during general anesthesia. Prone positioning during general anesthesia did not negatively affect respiratory mechanisms, and it improved lung volumes and oxygenation.
Estimating Intravascular Volume Status and Predicting Fluid Responsiveness
Although unquestionably useful, the traditional clinical findings in hypovolemia often lack sensitivity and specificity. This fact has led to a decades-long program of ongoing research to improve the clinical monitoring of volume status. Earlier expectations that cardiac filling pressure data (e.g., central venous pressure, pulmonary capillary wedge pressure) would be helpful in guiding fluid therapy were not fully realized when it became apparent that cardiac filling pressures are often influenced by factors other than blood volume. Later experience with transesophageal echocardiography showed that this technique can be especially helpful in assessing right- and left-sided ventricular filling, but the technique is intermittent in nature, requires extensive training to use and interpret, and entails a very high equipment cost. Consequently, the search for practical and reliable means of monitoring blood volume and related parameters continues. Approaches to this search fall into one of three broad areas: identifying occult hypoperfusion (e.g., by tissue oxygen and carbon dioxide levels), assessing preload responsiveness (e.g., by systolic pressure variation, pulse pressure variation, or stroke volume variation), and identifying end points for fluid resuscitation (e.g., seeking a specific cardiac output or aortic flow velocity value rather than a specific blood pressure). The following is a brief description of some of the methods that show particular promise in this regard.
Arterial pressure variation as a consequence of respiration has been advocated as a relatively simple technique that requires only an arterial line. Clinically, one can “eyeball” the degree of pressure variation with mechanical ventilation to get a rough indication of the degree of hypovolemia. However, more quantitative measures such as measurements of pulse pressure (systolic−diastolic) variation, systolic blood pressure variation, and delta down (decrease in systolic blood pressure from the value during apnea) have all been advocated as more appropriate alternatives to simple visual inspection.27–29
In a similar manner, respiratory variation of stroke volume has been demonstrated to predict fluid responsiveness in mechanically ventilated patients.28,30,31 Methods using either pulse pressure variation or aortic blood flow variation have been advocated. The mechanisms involved in producing this variation have been explained by Mahjoub et al.32
Developments in Pulse Oximetry Technology
Recent advances in pulse oximetry technology have led to developments that should be of special interest to clinicians dealing with complex surgical cases such as major spine surgery. For example, existing and pending developments from Masimo Corporation (Irvine, CA), a major vendor of pulse oximeters, will allow clinicians continuously to measure oxyhemoglobin, oxygen content, carboxyhemoglobin, methemoglobin, and total hemoglobin by simple, noninvasive means. Perhaps even more important, respiratory variations in the pulse oximeter waveform can be used to predict fluid responsiveness in mechanically ventilated patients. As noted earlier, this can be helpful in discerning which hypotensive patients will respond to a fluid challenge. Masimo makes a pulse oximeter that provides a measurement known as the Pleth Variability Index (PVI) that has been shown to be useful in predicting fluid responsiveness. In a 25-patient study by Cannesson et al.,33 a ventilated patient was defined as a “responder” if his or her cardiac output increased by 15% or more after administration of 500 mL of hetastarch 6%; otherwise, the patient was considered to be a “nonresponder.” Of the patients evaluated, 16 were responders and 9 were nonresponders. In this study, a PVI greater than 14% before volume expansion discriminated between responders and nonresponders with 81% sensitivity and 100% specificity. In addition, the noninvasive PVI was as accurate at predicting fluid responsiveness as was analysis of pulse pressure variation from an invasive arterial catheter. Finally, the PVI had superior predictive accuracy compared with traditional central venous pressure and pulmonary capillary wedge pressure indicators of intravascular volume. Figure 181-1 shows sample waveforms from a responder and a nonresponder.
