Australasian Anaesthesia 2011 - Australian and New Zealand ...

50 Australasian Anaesthesia 2011Anaesthesia for instrumented spinal surgery 51Anaesthetic management:A history of connective tissue disease (rheumatoid arthritis) or previous cervical spine surgery is important to theplanning of airway management and intubation. A wire re-inforced (armoured) tube can be positioned to optimisesurgical access. Also, it withstands compression from retractors. Intra-arterial blood pressure monitoring is oftenused. This allows for careful titration of the mean arterial blood pressure and provides early information on autonomicstimulation during surgery. Finally, arterial blood gas analysis may assist in the assessment of potential postoperativeairway compromise.Postoperative airway compromise is not common but may have several causes. Recurrent laryngeal nervedysfunction is usually unilateral. It may result in vocal cord palsy and cause postoperative hoarseness. Althoughanterior fusion may include several levels, artificial disc insertion usually only involves one level. Postoperativebleeding arising from the spine may form a retropharyngeal haematoma displacing and narrowing the trachea. Therisk of bleeding increases with the number of levels fused. Patients having undergone fusion of three or morevertebrae generally require close postoperative observation in the intensive care or high-dependency unit.INSTRUMENTED POSTERIOR LATERAL MASS AND CRANIO-CERVICAL FUSIONSThe patient is in the prone position, the head fixed in Mayfield skull pins, and the neck is flexed. The arms rest bythe patient’s sides and are inaccessible to the anaesthetist.Anaesthetic management:This procedure may involve C 1 and C 2 . In the event of odontoid process pathology, an unstable cervical spine shouldbe anticipated. Awake fibreoptic intubation may be the preferred method of airway management. This may dictatethe tube selection. If conventional intubation is possible, a wire re-inforced tube can be useful as the tube lumenis generally preserved and patent for ventilation and suction in the prone position with a flexed neck. Care shouldbe taken to protect the patient’s eyes from the disinfectant solution used for the preparation of the neck. Excessiveflexion of the neck in combination with the prone position may result in swelling and oedema of the tongue, whichmay become apparent following extubation.Intra-arterial blood pressure measurement allows for careful monitoring of the mean arterial blood pressure andfacilitates blood pressure control. Arterial blood gas analysis may assist in the assessment of the severity of anypostoperative airway compromise. Intensive care admission may be considered following surgical proceduresinvolving C 1 and C 2 .LUMBAR SPINE SURGERYInstrumented lumbar spine surgery includes anterior lumbar interbody fusion (ALIF), artificial lumbar disc insertion,posterior lumbar interbody fusion (PLIF) and posterior lumbar decompression and fixation.LUMBAR SPINE ANATOMYThe distal abdominal aorta lies to the left of and anterior to the L 4 vertebral body where it divides into the commoniliac arteries. Above the L 4 body the inferior vena cava lies slightly to the right. The internal and external iliac veinsform the common iliac veins anterior to the sacroiliac joints. The common iliac veins unite on the right hand sideof the L 5 vertebral body. Hence, the iliac vessels must be pushed aside using retractors to expose the L 4 /L 5 and L 5 /S 1 discs through an anterior (abdominal) approach and so may be vulnerable during L 4 / 5 and L 5 /S 1 surgery. Conversely,during a PLIF procedure the postero-medial walls of the inferior vena cava and the aorta may be visible throughthe L 4 /L 5 interbody space.ANTERIOR LUMBAR INTERBODY FUSION (ALIF) AND ARTIFICIAL DISC INSERTIONThe patient is placed in the lithotomy position with the arms abducted. The operating table is placed in a slighthead-down position to facilitate surgical access and the surgeon stands between the patient’s legs. Anterior lumbarspine procedures most frequently involve the L 4 /L 5 and L 5 /S 1 levels and are performed through an infra-umbilicalmidline incision. A hybrid operation indicates a combination of an artificial disc insertion at one level and an ALIFat the adjacent level. The bladder is drained by the insertion of an indwelling urinary catheter.Anaesthetic management:Reliable large-bore intravenous access is essential. Following induction and insertion of the appropriate lines thepatient is positioned in the lithotomy position with both arms abducted to nearly 90°. In the presence of peripheralvascular disease, perfusion of the lower legs may be borderline in this position. It has been suggested that thepresence of a satisfactory pulse oximetry signal from a toe is evidence of adequate perfusion. 2 Monitoring is appliedas determined by the patient’s co-morbidities. Invasive arterial blood pressure monitoring may be useful in theevent of a major bleeding. Central venous access and central venous pressure monitoring are generally not requiredfor ALIF procedures. Bleeding may potentially occur from the iliac vessels during the initial exposure of the surgicalsite or towards the end of surgery when the retractor pins are removed. Cell salvage may be employed in the eventof major blood loss. Postoperative intravenous fluid management is continued until bowel function has returned.POSTERIOR LUMBAR INTERBODY FUSION (PLIF)PLIF should be considered as a major procedure. Although most PLIF procedures involve one level, they mayinvolve from one to three or more levels. The procedures are lengthy; a one-level PLIF generally takes around3.5-4 hrs. A three-level procedure may take around 6 hours. Posterior lumbar fusions of more than 3 levels aremost frequently performed without insertion of interbody cages. There is generally a considerable and predictableblood loss often up to 500mL per vertebral level.PLIF generally involves• Posterior decompression of the spinal cord and nerve roots (extended laminectomy),• Bilateral insertion of pedicle screws above and below the level(s) involved,• Connection of the screws across the intervertebral space with “rods”,• Insertion of a “cage” to replace the intervertebral disc and• Fitting of cross-links between the rods.• Bone growth is promoted by application of bone tissue (from the laminectomy) and bone growth stimulatingagents (Infuse®, Medtronic International Ltd., Singapore, or I-Factor®, Life Healthcare, North Ryde, NSW,Australia) into the interbody cage and along the rods in the interpedicular space.The anaesthetic assessment of the patient should elicit information about issues that may add to patient morbidity.A history of diabetes mellitus has several implications in the setting of PLIF surgery. The risk of visual disturbancesmay be increased because of the diabetic retinopathy. Autonomic neuropathy may impair blood pressure regulationin the prone position. A history of previous shoulder surgery should prompt a careful examination of shouldermovement to ensure that the arms can be abducted to 90°. Preoperative investigations should include ECG, bloodgroup and antibody screen, haemoglobin, coagulation profile, blood glucose level, electrolytes and S-creatinine.THEATRE SETUPMost surgeons prefer to use a Jackson table (Orthopedic Systems Inc. Union City, California, USA) for extensivelumbar surgery. The Jackson table reduces the intra-abdominal pressure and hence the peri-spinal venous pressureby supporting the patient’s chest, bony pelvis and thighs. 3 The patient is at high risk of hypothermia in the proneposition due to increased convection and radiation from the anterior surface of the body. Although forced air warmerblankets are available that may be positioned between the patient and the operating table, most forced air warmerblankets are applied on top of the patient. The use of an upper body blanket covering the arms and upper backcombined with a lower body blanket covering the legs requires two heat generators but greatly facilitates bodytemperature control.ANAESTHETIC MANAGEMENTThe use of a wire reinforced tracheal tube allows for easy positioning of the tube when the patient is prone andavoids kinking. Invasive monitoring of the arterial blood pressure and central-venous pressure (CVP) is generallyindicated. The use of a central-venous catheter facilitates the use of inotropes and vasopressors. CVP monitoringmay also be of value in the assessment of spinal cord and ocular perfusion pressures and may be used to guidefluid and blood replacement therapy. Monitoring should further include neuromuscular function as a deep level ofneuromuscular blockade facilitates the retraction of the paraspinal muscles. Hourly urine production and bodytemperature may be monitored using a combined urinary catheter and temperature sensor (“Foley-Temp”, TycoHealthcare Group LP, California USA). Once the patient is turned prone onto the operating table, positioning shouldensure that the head in placed in a neutral position. Most hospitals use disposable foam head rests with cut-outsfor eyes, nose and tracheal tube. The absence of external pressure on the eyes should be confirmed regularlyduring the anaesthetic. The table may be slightly tilted in the head-up position to reduce cerebral and ocular venousstasis. The prone position may cause a reduction in cardiac output 4 and if the patient is already on an ACE inhibitoror ATII converting enzyme inhibitor, hypotension may be troublesome. Because of the vasodilatation caused bythe anaesthetic agents, a low-dose infusion of an alpha-agonist (metaraminol or phenylephrine) may be useful toensure the adequate perfusion of the spinal cord and the ocular structures.Blood loss during PLIF may arise from decorticated bone. Troublesome venous bleeding may also occur as aresult of an increased blood flow in de-compressed epidural veins. A dural tear may cause collapse of the duralsac and further bleeding from the epidural veins. Repair of a dural tear may extend the surgical time by around 30min. The CSF leak may result in postoperative dural puncture headache and even cranial nerve palsies. 5