The Use of Remifentanil Infusion to Allow Intraoperative Awakening for Intentional Fracturing of the Anterior Cervical Spine

We describe the anesthetic management of two patients with chin-on-chest ankylosing spondylitis. They underwent C6,7 and T1 laminectomies, with posterior cervical osteotomies to release the posterior cervical spine, and were awakened intraoperatively for manual, intentional fracturing of the anterior cervical spine, which was followed by C5-T2 posterolateral fusion to correct their gaze to horizontal. Somatosensory evoked potentials (SSEPs) and intraoperative awakening with appropriate motor response to command were used to monitor neurologic function. We chose to use the opioid remifentanil, which does not interfere with SSEPs in smaller doses (1), provides rapidly titratable analgesia, and has a rapid elimination that is independent of the dose or infusion duration (2). Case Report A 43-yr-old, 81-kg man (Patient 1), who was otherwise healthy, and a 47-yr-old, 113-kg, obese, diabetic woman (Patient 2) presented with chin-on-chest ankylosing spondylitis for posterior cervical-thoracic osteotomies and intraoperative intentional fracturing of the anterior cervical spine. After premedication with 2 mg of IV lorazepam and topical anesthesia of the airway, awake fiberoptic endotracheal intubation was performed. General anesthesia was induced with propofol and maintained with isoflurane (0.5%) and a cisatracurium and remifentanil (0.25–0.7 μg · kg−1 · min−1) infusion. The remifentanil infusion was titrated to keep the heart rate and blood pressure within 20% of the baseline. A radial arterial and multi-orificed central venous catheter were inserted, and a precordial Doppler was positioned. The patients, with cranial halo vests in place for later stabilization of the cervical spine, were placed in the sitting position. SSEPs were monitored by a neurologist and were of excellent quality throughout surgery. The cisatracurium infusion was discontinued 30 (Patient 1) to 60 (Patient 2) min before the wake-up test. The isoflurane concentration was decreased to 0.3% 10 min before the wake-up test. Remifentanil, which had been continuously infusing for 5 h in Patient 1 and 2.5 h in Patient 2, was maintained at 0.25–0.375 μg · kg−1 · min−1. With the surgeon in place to fracture and straighten the anterior spine and adjust the halo, the isoflurane and remifentanil administration was discontinued and lack of neuromuscular blockade confirmed. The patients’ names were called every 10 s until they opened their eyes. Patient 1 opened his eyes in 300 s, and Patient 2 opened her eyes 175 s after discontinuation of the anesthetic. The remifentanil infusion was immediately restarted at 0.125 μg · kg−1 · min−1 in Patient 1 and 0.25 μg · kg−1 · min−1 in Patient 2, to provide analgesia. Both patients demonstrated intact motor function when asked to move their extremities. The surgeon placed one hand on the occiput and one hand on the halo (Figure 1) and, with no resistance from either patient, manually extended the neck, fracturing and releasing the anterior cervical spine. The arterial blood pressure increased from 100/62 to 115/82 mm Hg, and the heart rate increased from 78 to 101 bpm in Patient 1. The arterial blood pressure increased from 122/76 to 142/85 mm Hg, and the heart rate increased from 82 to 91 bpm in Patient 2. There was no significant change in the amplitude or latency of the SSEP signal, and intact motor function was reconfirmed by having both patients move their extremities. General anesthesia was then reinduced with propofol and cisatracurium and maintained as before. At the end of surgery, with the patients’ cervical spine stabilized by the cranial halo vest, a wake-up test was performed again as previously described. Both patients followed commands in <300 s. Intact motor function and lack of any significant changes in the SSEP signal were again confirmed. Morphine sulfate was administered for postoperative pain control. Both patients were endotracheally extubated on the first postoperative day. Patient 1 remembered the wake-up test, but did not find it distressing. He rated pain during the procedure as no different than his every day discomfort. He had no neurological deficits. Patient 2 had no intraoperative recall. She had mild bilateral upper extremity weakness, which resolved by postoperative Day 2. Figure 1: Patient in the halo, showing the surgeon’s hands on the halo and the occiput.Discussion Patients who undergo posterior cervical-thoracic osteotomy spinal surgery with fracturing of the anterior cervical spine are at risk of neurological injury. In a series of 120 awake, sedated, sitting patients, 1 patient developed quadriplegia, requiring immediate surgical intervention, and 12 patients developed transient C8 neurological symptoms (3). In another series of 15 patients, under general anesthesia in the prone position, 1 patient developed quadriplegia 1 wk after surgery, and 2 patients had unilateral C8 root palsies (4). Monitoring for trespass on the anterior or posterior spinal cord with appropriate interventions during spinal surgery has been associated with improved neurological outcome (5). SSEPs may be used to monitor posterior spinal cord (sensory) function during general anesthesia. The amplitude or latency of the SSEPs may be altered by inhaled anesthetics and narcotics in large doses, among other factors (5). Motor function is often assessed with a wake-up test (3). Successful anesthetic techniques for wake-up tests have included fentanyl, sufentanil, and morphine sulfate infusions (6). Bolus narcotics interfere with SSEPs and may require narcotic antagonists (5). In these two cases, the anesthetic management not only had to take into account continuous SSEP monitoring and a wake-up test but also provide titratable analgesia for a painful procedure (fracturing of the anterior cervical spine) in an awake, cooperative, intubated patient in the sitting position. The pharmacology of remifentanil made it ideal for this procedure. Remifentanil is a potent μ-opioid agonist with fast onset and peak effect caused by a pKa of 7.07, with more than half of the drug in the uncharged, lipid soluble form that rapidly crosses the blood-brain barrier. Remifentanil has a methyl ester group with an increased susceptibility of hydrolytic metabolism by nonspecific blood and tissue esterases, thereby leading to a rapid termination of effect. This ultra-short duration of action is independent of infusion time. Fentanyl and its derivatives can accumulate to varying degrees after prolonged infusion. The context sensitive half-time, the time to a 50% decrease of an effective site concentration after an infusion is stopped, of remifentanil is three to five minutes (2). These factors make remifentanil a rapidly titratable anesthetic that may be used at larger doses and often combined with an inhaled anesthetic to provide surgical anesthesia or in lower doses in awake postoperative patients to provide rapidly titratable analgesia (2). In both patients, when MAC- (minimum alveolar anesthetic concentration) Awake values of isoflurane were discontinued and remifentanil infusion stopped, the patients awoke in three to five minutes. The rapid metabolism and short-context sensitive half-time of remifentanil might be considered a disadvantage in patients who are awakened intraoperatively for intentional fracture of the cervical spine. The sudden onset of pain resulting in an uncooperative patient before or during cervical stabilization with the halo was a potential hazard when considering the anesthetic technique for this procedure. For this reason, when the patients first became responsive to verbal commands, the remifentanil infusion was immediately restarted and adjusted according to the change in heart rate and blood pressure, as well as facial expression. The dose was expected to provide analgesia and, yet, maintain responsiveness. Remifentanil has no significant effect on SSEPs at smaller doses (0.125–0.4 μg · kg−1 · min−1) and stays consistent when combined with isoflurane (0.4 MAC); SSEPs may be adequate at larger doses (0.5–1.0 μg · kg−1 · min−1) but with slightly reduced amplitude (4). We had excellent quality SSEPs, monitored by a neurologist throughout surgery in both cases. In summary, remifentanil provided rapidly titratable analgesia to match the level of surgical stimulation without interfering with SSEP monitoring. There was rapid awakening for the wake-up test. Surgical conditions and analgesia were excellent for fracturing of the anterior cervical spine in two endotracheally intubated awake patients in the sitting position with a large open posterior cervical incision.