Posterior spinal reconstruction

Preoperative Considerations for Posterior Spinal Reconstructive Surgery

• Thorough preoperative evaluation crucial for personalized care plan.
• Complete patient history, including review of associated conditions like muscular dystrophy, cerebral palsy, spina bifida, congenital heart disease, gastroesophageal reflux, dwarfing syndrome, and myasthenia gravis, essential.
• Preoperative assessment must consider potential respiratory complications, vital capacity, ventilation-perfusion mismatch, altered cardiac and respiratory status, and scoliosis severity.
• Surgical correction of scoliosis optimal before complete skeletal development.
• Neurologic: Thorough neurologic exam crucial to assess limitations post-operatively and from prone positioning. This patient has no motor or sensory deficits.
• Respiratory: PFT reveals restrictive lung disease (intrinsic/extrinsic). FEV1 60% predicted, while FEV1/FVC ratio 90%. Low FEV1 and normal/high FEV1/FVC ratio indicative. Decreased FRC and V/Q mismatch elevate risks for intraoperative desaturation. Peak airway pressures demand vigilant monitoring to prevent barotrauma. Postoperative respiratory distress/failure possible. ABG results suggest compensated respiratory alkalosis (low PaCO2, compensatory increase in minute ventilation). Severe Cobb angle causing extrinsic restrictive respiratory defect.
• Cardiovascular: ECG shows normal sinus rhythm. Near-normal ejection fraction (echo). No evidence of cardiac hypertrophy/cardiomegaly (CXR).
• Hematologic: Polycythemia related to chronic hypoxia. Hemoglobin and hematocrit levels are high. Hemoglobin decreased from 28% to 24%. Patient may need blood transfusion.
• Endocrine: Patient on prednisone for bronchial asthma. Hydrocortisone (100 mg IV) pre-op advised to avoid adrenal crisis. Hyperreactive airways possible peri-operatively. Preoperative lung auscultation and bronchodilators as needed.
• Musculoskeletal: No musculoskeletal pathology. However, scoliosis associated with Duchenne muscular dystrophy and spina bifida. Malignant hyperthermia risk with Duchenne muscular dystrophy. Latex allergy possible in patients with spina bifida and self-catheterization.

Intraoperative Considerations

• Correct prone positioning minimizes nerve damage, intraoperative blood loss, and postoperative visual problems. All joints must be padded and free of continual pressure, including the fingers, hands,elbows, knees, ankles, toes, and genitalia. Axillary rolls must not impinge on the axillary arteries, and bilateral radial pulses must be palpable. Arms must not be abducted greater than 90 degrees.
• Serial hemoglobin and hematocrit monitoring essential due to potential for significant hemorrhage. Intraoperative blood salvage techniques readily available. (60-80% of blood loss can be replaced by cell saver systems).
• Careful consideration of cerebral and coronary autoregulation needed when using deliberate hypotensive technique.
• Venous air embolism (VAE) possible during surgery, as air can be drawn into the venous network around the spinal cord. During posterior surgery, the venous network may be above the heart level, potentially entraining air; this creates an airlock, decreasing blood flow into the right heart, impacting left ventricular preload, leading to hypotension and decreased ETCO2.
• Cobb angle measurement critical intraoperatively; it is used to assess surgical success.
• Pedicle screws associated with improved surgical correction and better postoperative pulmonary function.
• Proper prone positioning is critical to minimize pressure on the vena cava to prevent venous stasis in the lower body, and to prevent thrombus formation. Venous compression decreases venous return to the heart and causes engorgement of the epidural venous plexus, increasing intraoperative blood loss. Patients frequently develop periorbital, facial, and airway edema.
• Chest excursion decreases, increasing peak inspiratory pressures, decreasing FRC, increasing ventilation-perfusion mismatch, and decreasing oxygen saturation. Chest rolls must be correctly positioned to minimize pressure on the anterior chest. Abdominal compression displaces the diaphragm cephalad, further reducing FRC and thoracic cage expansion. The anesthetist should recheck for equal, bilateral, clear breath sounds after the prone position is achieved.
• Thoracic outlet syndrome - Preoperative assessment. Arms above head. Paresthesias or decreased radial pulses, keep arms at side during surgery.
• Intraoperative blindness a risk, and possible causes include extra-ocular compression, impaired ocular nerve perfusion (microemboli), anemia, and edema formation. Ways to decrease POVL include maintaining neutral head position and no pressure on eyes.
• Maintenance of adequate mean arterial pressure (MAP), as coronary and cerebral autoregulatory pressures are increased in patients with cardiovascular pathology. Hypotension below 60 mm Hg impacts blood flow to heart and brain, increasing risk for ischemia Patients with cardiac pathology need higher MAPs for adequate perfusion. Dangers of hypotension include potential for cerebral, cardiac, spinal cord, optic nerve, and renal ischemia.
• Maintaining adequate hemoglobin levels is important for oxygen delivery to peripheral tissues. Strategies to preserve hemoglobin include deliberate hypotension, normovolemic hemodilution, intraoperative blood salvage, and autologous blood transfusions.
• Crystalloids used cautiously in volume replacement therapy due to potential for edema formation and optic nerve involvement. Avoid phenylephrine to prevent peripheral vasoconstriction, which could further reduce optic nerve perfusion.

Monitoring During Surgery

• Continuous assessment of spinal cord sensory integrity using somatosensory evoked potentials (SSEPs). SSEPs are affected by anesthetic agents, increasing latency & reducing amplitude. SSEP waveform latency and amplitude are affected by stretching, torsion, retraction, decreased blood flow, hypoxia, hypercarbia, anemia, anesthetic medications and electrical interference (electrocautery), as well as hypothermia. Potential spinal cord damage indicated by increased latency and decreased amplitude.
• Respiratory function monitoring crucial; hypoxemia and hypercarbia are potential complications.
• Patients with a Cobb angle ≥30° are at risk for postoperative respiratory insufficiency.
• Careful attention to potential thoracic cage involvement, congenital scoliosis, and pulmonary hypertension.
• Minimally invasive procedures emerging, potentially decreasing surgical trauma and blood loss with equivalent/superior outcomes.

Surgical Procedure Considerations

• Goal of spinal reconstruction surgery is to straighten the spine, improve torso-pelvis balance, and maintain correction.
• Disadvantages of traditional methods include decreased mobility due to vertebral fusion, inhibition of normal growth, leading to truncal deformity and decreased pulmonary maturation, and chronic pain.
• Posterior approach involves a midline incision from midthoracic to lower lumbar vertebrae. Exposure of vertebrae followed by securing pedicle hooks or screws to lateral aspects.
• Instrumentation causes vertebral fusion within 1 year.
• Hardware can be removed later if chronic back pain develops.
• Rods placed on both sides of the vertebral column are inserted and manipulated to cause tension or distract vertebrae, stimulating and expanding the growth plate (Hueter-Volkmann principle).
• Anterior spinal fusion involves a transthoracic and potentially retroperitoneal approach, removing discs and implanting instrumentation.
• Requires preferential ventilation of one lung, needing a double-lumen endotracheal tube.
• Anterior approach may be associated with significant hemorrhage.
• No definitive evidence of superior results compared to posterior fusion for idiopathic scoliosis in adolescents.
• Emerging minimally invasive techniques include placement of large nitinol staples using video-assisted thoracoscopic instruments, insertion of vertebral bone anchors with flexible ligament tethers, and implantation of "growing rods" (elongatable and distactable with external remote control).
• extubation is individualized; but if compromised lung function, long procedure, anterior approach or large volume of fluids, patient may need postoperative ventilation and weaning over 12 hours.

Intraoperative Monitoring Equipment

• Pulse oximetry
• Blood pressure
• End-tidal carbon dioxide (ETCO2) analysis
• Five-lead ECG
• Esophageal stethoscope (continuous breath sound auscultation)
• Core temperature monitor
• Urinary catheter (bladder drainage and volume assessment; ideal urine output: 1-2 mL/kg/hr)
• Arterial line (continuous blood pressure assessment; changes in anesthetic depth, deliberate hypotension, and volume loss)
• Serial ABG samples
• Central venous pressure monitoring (optional based on patient status)
• Pulmonary artery catheter placement and monitoring (not routine)

Strategies for Maintaining Adequate Hemoglobin Levels

• Deliberate hypotension
• Normovolemic hemodilution (preoperative blood draw)
• Intraoperative blood salvage (cell saver system)
• Autologous or directed donor blood (collected 2-3 weeks prior)
• Blood transfusion (as needed; patient must be typed and crossmatched)

Additional Considerations (from the provided text)

• VAE: Air can entrain into venous network during prone positioning. Blood flow to the right heart is impacted, decreased preload and hypotension can occur. Signs include decreased ETCO2. Close monitoring is critical.
• Deliberate Hypotension: Used to decrease blood loss. Various medications can induce this, including inhalation agents, narcotics, vascular dilators (nitroglycerin, nitroprusside), calcium channel blockers, and beta-blockers. Careful consideration for appropriate candidates and monitoring for possible ischemia (cerebral, cardiac, spinal cord, optic nerve, renal).
• SSEP Monitoring: Detects spinal cord sensory function. A major disadvantage is potential failure to detect motor function issues. May not always prevent postoperative motor paralysis.
• Wake-up Test: Allows assessment of motor function. Risks include dislodgement of hardware, potential VAE and extubation.

SSEP Waveform Considerations

• Wave latency: Time between stimulus and response.
• Wave amplitude: Size of neurologic response.
• Pathological waveforms: Increased latency, decreased amplitude, indicative of impending spinal cord impairment.
• Interventional strategies: Notify surgeon, increase oxygen, decrease anesthetic, ensure adequate blood pressure, assess anemia.
• Anesthetic effects: Inhalation agents affect SSEP waveforms. Proper titration essential.