Anesthesia for Orthopedic Surgery
Many orthopedic surgical procedures lend themselves to the use of regional anesthesia (intraoperative anesthesia and postoperative analgesia) (Horlocker TT, Wedel DJ. Anesthesia for orthopaedic surgery. In: Barash PG, Cullen BF, Stoelting RK, Cahalan MK, Ortega R, Stock MC, eds. Clinical Anesthesia. Philadelphia: Lippincott Williams & Wilkins; 2013:1440–1458). Anesthesia for orthopedic surgery requires an understanding of special positioning requirements (risk of peripheral nerve injury), appreciation of the possibility of large intraoperative blood loss and techniques to limit the impact of this occurrence (intraoperative hypotension, salvage techniques), and the risk of venous thromboembolism (knowledge of the current pharmacologic and mechanical methods of thromboprophylaxis to prevent thromboembolic complications). Potential interactions between anticoagulants and anesthetic drugs or regional anesthetic techniques must be understood to reduce the risk of perioperative bleeding and neurologic injury from expanding hematomas.
III. Surgery to the Spine
Spinal Cord Injuries. Spinal cord injuries must be considered in any patient who has experienced trauma. Cervical spine injuries are associated with head and thoracic injuries, and lumbar spine injuries are associated with abdominal injuries and long bone fractures.
Tracheal Intubation
Airway management is critical because the most common cause of death with acute cervical spinal cord injury is respiratory failure.
All patients with severe trauma or head injuries should be assumed to have an unstable cervical fracture until proven otherwise radiographically.
Table 50-1 Preoperative Assessment of Orthopedic Surgical Patients
Pre-existing Medical Problems
Coronary artery disease (perioperative β-blockade in selected patients)
Rheumatoid arthritis (steroid therapy, limited range of cervical spine movement)
Continue antihypertensive medication and chronic opioid therapy
Continue antiplatelet medications in patients with cardiac stents
Physical Examination
Mouth opening or neck extension
Evidence of infection and anatomic abnormalities at proposed sites for introduction of regional anesthesia (peripheral techniques may be acceptable if a regional technique is contraindicated)
Neurologic examination and documentation of any pre-existing deficits
Arthritic changes and limitations to positioning
Awake fiberoptic-assisted intubation may be necessary, with general anesthesia induced only after voluntary upper and lower extremity movement is confirmed.
In a truly emergent situation, oral intubation of the trachea with direct laryngoscopy (with minimal flexion or extension of the neck) is the usual approach.
Respiratory considerations include an inability to cough and clear secretions, which may result in atelectasis and infection.
Table 50-2 Advantages of Regional Versus General Anesthesia for Orthopedic Surgical Procedures
Improved postoperative analgesia
Decreased incidence of nausea and vomiting
Less respiratory and cardiac depression
Improved perfusion because of sympathetic nervous system block
Decreased intraoperative blood loss
Decreased blood pressure
Blood flow redistribution to large-caliber vessels
Locally decreased venous pressure
Cardiovascular considerations are based on loss of sympathetic nervous system innervation (“spinal shock”) below the level of spinal cord transection. Cardioaccelerator fiber (T1–T4) loss results in bradycardia and possible absence of compensatory tachycardia if blood loss occurs.
Succinylcholine-induced Hyperkalemia. It is usually safe to administer succinylcholine (SCh) within the first 48 hours after spinal cord injury. It should be avoided after 48 hours in all patients with spinal cord injuries.
Temperature Control. Loss of vasoconstriction below the level of spinal cord transection causes patients to become poikilothermic. Body temperature should be maintained by increasing ambient air temperature and warming intravenous (IV) fluids and inhaled gases.
Maintaining Spinal Cord Integrity. An important component of anesthetic management is preservation of spinal cord blood flow. Perfusion pressure should be maintained, and extreme hyperventilation of the lungs should be avoided. Neurophysiologic monitoring (somatosensory- or motor-evoked potentials), a “wake-up test,” or both are used to recognize neurologic ischemia before it becomes irreversible.
Autonomic Hyperreflexia (Table 50-3)
Scoliosis
Pulmonary Considerations. Postoperative ventilation of the patient’s lungs is likely to be necessary if the vital capacity is below 40% of the predicted value. Prolonged arterial hypoxemia, hypercapnia, and pulmonary vascular constriction may result in right ventricular hypertrophy and irreversible pulmonary hypertension.
Table 50-3 Characteristics of Autonomic Hyperreflexia
Occurs in 85% of patients with spinal cord transection above T5
Paroxysmal hypertension with bradycardia (baroreceptor reflex)
Cardiac dysrhythmias
Cutaneous vasoconstriction below and vasodilation above the level of transaction
Precipitated by any noxious stimulus (distention of a hollow viscus)
Treatment is removal of stimulus, deepening of anesthesia, and administration of a vasodilator
Cardiovascular Considerations. Prolonged alveolar hypoxia caused by hypoventilation and ventilation–perfusion mismatch eventually causes irreversible vasoconstriction and pulmonary hypertension.
Surgical Approach and Positioning.
The prone position is used for the posterior approach to the spine. The hazards of the prone position, including brachial plexus stretch injury (the head should be rotated toward the abducted arm and the eyes taped closed), should be considered.
The anterior approach is achieved with the patient in the lateral position, usually with the convexity of the curve uppermost. Removal of a rib may be necessary. A double-lumen endotracheal tube is used to collapse the lung on the operative side.
A combined anterior and posterior approach in one or two stages yields higher union rates but is associated with increased morbidity, including blood loss and nutritional deficits.
Anesthetic Management. The primary aim of preoperative evaluation of patients with scoliosis is to detect the presence and extent of cardiac or pulmonary compromise.
Respiratory reserve is assessed by exercise tolerance, vital capacity measurement, and arterial blood gas analysis.
There are specific anesthetic considerations for surgical correction of scoliosis by spinal fusion and instrumentation (Table 50-4).
Adequate hemodynamic monitoring and venous access are essential in the management of patients undergoing spinal fusion and instrumentation (Table 50-5).
Table 50-4 Anesthetic Considerations for Surgical Correction of Scoliosis
Management of the prone position
Hypothermia (long procedure and extensive exposed area)
Extensive blood and fluid losses
Maintenance of spinal cord integrity
Prevention and treatment of venous air embolism
Reduction of blood loss through hypotensive anesthetic techniques
Table 50-5 Monitoring for Patients Undergoing Scoliosis Surgery
Cannulation of radial artery (direct blood pressure measurement and assessment of blood gases)
Central venous catheter (evaluates blood and fluid management and aspirate air if venous air embolism occurs)
Pulmonary artery catheter (pulmonary hypertension)
Neurophysiologic monitoring (prompt diagnosis of neurologic changes and early intervention)
Somatosensory-evoked potentials
Motor-evoked potentials
Wake-up test
Degenerative Vertebral Column Disease. Spinal stenosis, spondylosis, and spondylolisthesis are forms of degenerative vertebral column disease that may lead to neurologic deficits necessitating surgical intervention.
Surgical Approach and Positioning
Whereas cervical laminectomy is performed in the prone, lateral, or sitting position, thoracolumbar laminectomy is usually performed prone (Fig. 50-1).
The anterior approach places the surgical incision (anterior border of the sternocleidomastoid muscle) near critical structures (carotid artery, esophagus, trachea); edema and recurrent nerve injury are possible.
The use of the sitting position for cervical laminectomy allows a more blood-free surgical field but introduces the risk of venous air embolism.
Figure 50-1. Prone position with the patient’s head turned and the dependent ear and eye protected from pressure. Chest rolls are in place, the arms are extended forward without hyperextension, and the knees are flexed.
Minimally invasive surgery techniques have not been shown to be justified by superior clinical benefits.
Anesthetic Management. General anesthesia is preferred for essentially all thoracic and lumbar spine surgery (airway access, less nausea and vomiting, greater patient acceptance). SCh should be avoided if the patient has progressive neurologic deficits.
Spinal Cord Monitoring. Paraplegia is a feared complication of major spine surgery. The incidence of neurologic injuries associated with scoliosis correction is 1.2%. When patients awaken with paraplegia, neurologic recovery is unlikely, although immediate removal of instrumentation improves the prognosis. It is therefore essential that any intraoperative compromise of spinal cord function be detected as early as possible and reversed immediately. The two methods for detecting intraoperative compromise of spinal cord function are the “wake-up test” and neurophysiologic monitoring.
The wake-up test consists of intraoperative awakening of patients after completion of spinal instrumentation. Surgical anesthesia (often including opioids) and neuromuscular blockers are allowed to dissipate, and the patient is asked to move his or her hands and feet before anesthesia is re-established. Recall may occur but is rarely viewed as unpleasant, especially if the patient is fully informed before surgery.
Neurophysiologic monitoring (as an adjunct or an alternative to the wake-up test) includes somatosensory-evoked potentials (SSEPs) (waveforms may be altered by volatile anesthetics, hypotension, hypothermia, hypercarbia), motor-evoked potentials (MEPs) (neuromuscular blocking drugs cannot be used), and electromyography.
SSEPs reflect the dorsal columns of the spinal cord (proprioception and vibration) supplied by the posterior spinal artery.
MEPs reflect the motor pathways and the portion of the spinal cord supplied by the anterior spinal artery.
The combined use of SSEPs and MEPs may increase the early detection of intraoperative spinal cord ischemia.
Blood Loss. Most of the blood loss in spinal instrumentation and fusion occurs with decortication and is proportional to the number of vertebral levels decorticated.
Measures to decrease blood loss and transfusion requirements include preoperative autologous donations, proper positioning, intraoperative blood salvage, induced hypotension, intraoperative normovolemic hemodilution, and administration of tranexamic acid. (Aprotinin use has been suspended owing to cardiac safety questions.)
Perioperative coagulopathy from dilution of coagulation factors, platelets, or fibrinolysis may be predicted from measurement of either the prothrombin time or activated partial thromboplastin time.
Visual Loss after Spine Surgery. The American Society of Anesthesiologists Visual Loss Study Group has identified risk factors for ischemic optic neuropathy after spinal surgery to include male gender, obesity, use of the Wilson frame, anesthetic duration, increased blood loss, and the use of noncolloid for fluid replacement.
Venous Air Embolus. Venous air embolism can occur in all positions used for laminectomies because the operative site is above the heart level. Presenting signs are usually unexplained hypotension and an increase in the end-tidal nitrogen concentration.
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