Management Of Anaesthesia for Spinal Surgery
by Mr K. Ismail, MB ChB, MD, FRCA
Consultant in Anaesthesia & Intensive Care Medicine
The Mid Yorkshire Hospitals, Halifax Road, Dewsbury, UK
The scope of spinal surgery is considerable. Both adult and paediatric patients present for surgery, which may be elective or urgent. They mainly present with one of five pathologies: trauma, for example an unstable vertebral fracture; infection, for example vertebral abscess; malignancy (metastatic or primary disease with spinal instability, pain, and neurological compromise); congenital/idiopathic, for example scoliosis; or degenerative disease. In excess of 25 000 spinal operations were performed in the UK in 2001-2.
Surgery may be required at any site in the spine from cervical to lumbosacral. Procedures range from minimally invasive microdiscectomy, to prolonged operations involving multiple spinal levels and significant blood loss. An osteotomy is a decompressive procedure, which releases compressive forces at a localized site. Stabilization of the spine involves instrumentation above and below the unstable spinal level. Distractive forces may also be applied to the spine, for example in surgery for scoliosis, with instrumentation placed over multiple spinal levels. Insertion of such devices may be through a posterior, anterior, or a combined approach involving repositioning of the patient part way through the procedure and major blood loss.
The challenge to the anaesthetist is to provide optimal surgical conditions whilst ensuring adequate oxygenation to the brain and spinal cord, and facilitating the use of intraoperative spinal cord monitoring techniques if appropriate.
Preoperative assessment
When assessing patients for spinal surgery, particular care should be given to the respiratory, cardiovascular, and neurological systems; all may be affected by the pathology for which the spinal surgery is proposed.
Airway assessment
The potential for difficulty in airway management should always be considered, particularly in those patients presenting for surgery of the upper thoracic or cervical spine. A careful assessment should be made for previous difficulty in intubation, restriction of neck movement, and the stability or otherwise of the cervical spine. Stability is defined as the ability of the spine, under physiological loads, to resist displacement, which causes neurological injury.
Respiratory system
Patients presenting for spinal surgery frequently have impaired respiratory function. Others have recurrent chest infections.
Preoperatively, respiratory function should be assessed by a thorough history, focusing on functional impairment, physical examination, and appropriate investigations. Scoliosis causes a restrictive pulmonary deficit, with reduced vital capacity and reduced total lung capacity (TLC). Respiratory function should be optimized by treating any reversible cause of pulmonary dysfunction, including infection, with physiotherapy and nebulized bronchodilators as indicated.
Cardiovascular system
Cardiac compromise may be a direct result of the underlying pathology, for example in patients with muscular dystrophies. Cardiac dysfunction may also occur secondary to scoliosis, which causes distortion of the mediastinum, and cor pulmonale secondary to chronic hypoxaemia and pulmonary hypertension. Assessment of functional cardiovascular impairment is difficult in patients who are wheelchair-bound. Minimum investigations should include an electrocardiograph, and echocardiography to assess left ventricular function and pulmonary arterial pressures. Dobutamine stress echocardiography may be used to assess cardiac function in patients with a limited exercise tolerance.
Thromboembolic prophylaxis
Patients undergoing spinal surgery may be at increased risk of thromboembolic disease as a result of prolonged surgery, prone positioning, malignancy, and extended periods of postoperative recumbency. The use of compression stockings and/or pneumatic boots is recommended.95 Many surgeons prefer not to administer anticoagulants because their use may be associated with haemorrhagic complications, including increased blood loss and epidural haematoma.
Neurological system
A full neurological assessment of the patient should be made preoperatively. This should be documented for three reasons. First, in patients undergoing cervical spine surgery, the anaesthetist has a responsibility to avoid further neurological deterioration during manoeuvres such as tracheal intubation and patient positioning. Secondly, muscular dystrophies may involve the bulbar muscles, increasing the risk of postoperative aspiration. Thirdly, the level of injury and the time elapsed since the insult is predictors of the physiological derangements of the cardiovascular and respiratory systems which occur peri operatively. If surgery is contemplated within 3 weeks of the injury, spinal shock may still be present. After this time, autonomic dysreflexia may occur.
Intraoperative monitoring and positioning of the patient
Cardiovascular monitoring
Prolonged anaesthesia in unusual positions, combined with significant blood loss, the haemodynamic effects of thoracic surgery, and where appropriate controlled hypotension, necessitates detailed monitoring of the cardiovascular system. Invasive arterial pressure monitoring is mandatory.
In the prone position, central venous pressure (CVP) may be a misleading indicator of right and left ventricular end-diastolic volume.
Respiratory monitoring
Respiratory system monitoring should always include end-tidal carbon dioxide concentration and peak airway pressure. In major surgery, serial measurements of arterial oxygen tension are recommended.
Temperature monitoring
Thermoregulation may already be impaired in patients who have spinal cord lesions before surgery. Prolonged anaesthesia causes significant heat loss. The use of temperature monitoring, warming of all i.v. fluids and a warm air mattress device is recommended.
Positioning
Patient position for spinal surgery varies depending on the level of the spine to be operated upon and the nature of the proposed surgery.
Prone position is the commonest and great care should be given for protection of the head and neck, avoid any pressure on the eyes, make sure that they are padded and taped. Avoid any nerve compression and avoid any pressure on the abdomen that might impair the ventilation. Patients may be repositioned intraoperatively. It is important that venous pressures at the surgical site are kept low to reduce bleeding (reverse Trendelenburg tilt and a free abdomen), and peripheral nerves, bony prominences, and the eyes are protected. It is also important to avoid displacement of unstable fractures during patient positioning. Intraoperative x-ray imaging is frequently required. The relevant spinal level must, therefore, be placed away from the central support of a radiolucent operating table.
Postoperative care:
Patients who undergo major spinal surgery will be monitored in the High Dependency Unit (HDU). During their stay in HDU, close cardiovascular, respiratory, renal and neurological monitoring will be provided. Postoperative analgesia is can be challenging specially in patients with chronic back pain who are already on different analgesia prior to surgery.
Postoperative analgesia.
Parenteral opioids
The use of parenteral opioids has been the mainstay of analgesia for all patients undergoing spinal surgery. Opioids can be administered via i.m., i.v. (continuous infusion and patient-controlled analgesia devices with or without background infusions), intrapleural, epidural, and intrathecal routes.
Non-steroidal anti-inflammatory drugs
Simple analgesics alone afford inadequate analgesia even for relatively minor spinal surgery. Non-steroidal anti-inflammatory drugs (NSAIDs), both non-selective cyclo-oxygenase inhibitors, and selective cyclo-oxygenase 2 (COX 2) inhibitors, have however been used successfully after spinal surgery.
Epidural analgesia
The use of local anaesthetic agents, alone or in combination with opioids by the epidural route after spinal surgery has been described, the epidural catheter being placed intraoperatively by the surgeon.
Conclusions
Patients are now undergoing major spinal surgery for conditions such as malignancy, scoliosis, and trauma, which would not have been contemplated 20 yr ago. Despite this, postoperative neurological morbidity has been reduced by advances in spinal cord monitoring techniques. The anaesthetist has an important role to play in facilitating the use of these new techniques. The choice of the appropriate post operative analgesia has facilitated the early recovery and reduced postoperative complications.
REFERENCES
Faciszewski T, Winter RB, Lonstein JE, et al. The surgical and medical perioperative complications of anterior spinal fusion surgery in the thoracic and lumbar spine in adults: A review of 1223 procedures. Spine 1995; 20: 1592-9
Hambly PR, Martin B. Anaesthesia for chronic spinal cord lesions. Anaesthesia 1998; 53: 273-89
Park CK. The effect of patient positioning on intraabdominal pressure and blood loss in spinal surgery. Anesth Analg 2000; 91: 552-7
