POSITIONING IN ANAESTHESIA

INTRODUCTION

GENERAL PRINCIPLES

1. Pre-operative assessment: Identify patients with limited joint mobility (rheumatoid arthritis, ankylosing spondylitis, obesity), vascular disease, diabetes, and neuropathy - these increase positional injury risk.
2. Awake positioning test: When feasible, position the awake patient first to assess tolerance and identify discomfort.
3. Padding: All bony prominences, peripheral nerves, and contact points with the table or its attachments must be padded.
4. Monitoring during repositioning: Monitors may be disconnected during repositioning - this is a period of higher risk for unrecognised hemodynamic derangement or hypoventilation.
5. Neutral position: Head and neck should generally be kept in a neutral position. Shoulder abduction should not exceed 90 degrees.
6. Documentation: Record the position used, padding applied, and any special precautions taken.

CLASSIFICATION OF SURGICAL POSITIONS

1. SUPINE (DORSAL RECUMBENT) POSITION

• Respiratory: FRC increases slightly compared to standing (gravity-dependent lung zones shift); however, general anaesthesia reduces FRC by ~20% regardless.
• Cardiovascular: Relatively stable hemodynamics; venous return is well maintained.

• Brachial plexus: From arm board extension >90° or excessive shoulder abduction/lateral rotation.
• Ulnar nerve: From sustained pressure at the medial epicondyle (cubital tunnel).
• Radial nerve: Compression at the lateral humerus.
• Peroneal nerve: Pressure at the fibular head if leg falls laterally.

• Alopecia (pressure-related hair loss): Avoid prolonged hypotension, pad occiput, turn head occasionally.
• Backache: Use lumbar support, slight hip flexion.
• Skin necrosis over bony prominences: Adequate padding.

• Trendelenburg (head-down): Used for lower abdominal and pelvic surgery. Improves venous access. Increases venous return and preload. Causes cephalad diaphragm displacement, decreasing FRC and worsening V/Q mismatch. Risk of airway edema with prolonged use. Shoulder braces (to prevent patient sliding) should be avoided if possible as they compress the brachial plexus.
• Reverse Trendelenburg (head-up): Used for upper abdominal and laparoscopic bariatric surgery. Causes venous pooling in lower limbs, reducing preload and risking hypotension and DVT.

2. LITHOTOMY POSITION

• Respiratory: Elevation of legs shifts abdominal contents cephalad, further decreasing FRC. Worsens V/Q mismatch.
• Cardiovascular: Elevation of legs causes sudden increase in venous return and preload (auto-transfusion of ~500 mL). Lowering legs at end of surgery can cause a sudden drop in venous return and hypotension - should be done slowly, one leg at a time.

• Common peroneal nerve (most common): Compression at the fibular head by the stirrup support. Presents as foot drop and sensory loss over dorsum of foot.
• Femoral nerve: Hyperflexion of the hip compresses the nerve against the inguinal ligament.
• Saphenous nerve: Compression medially at the knee.
• Obturator nerve: Excessive hip flexion or abduction.
• Sciatic nerve: Hip hyperflexion with knee extension stretches the nerve.

• Compartment syndrome: Prolonged venous outflow obstruction causes oedema and ischemia in lower leg compartments. Elevated compartment pressure requires urgent fasciotomy. May lead to myoglobinuria and acute kidney injury.
• Pressure necrosis over the sacrum and heels.
• Backache from loss of lumbar lordosis.

3. LATERAL DECUBITUS POSITION

• Respiratory (critical in thoracic surgery): In the awake spontaneously breathing patient, the dependent (down) lung receives more blood flow (gravity) and more ventilation (better diaphragm mechanics) - V/Q ratio is well matched.
• Under general anaesthesia + muscle relaxation: The dependent lung receives more perfusion but less ventilation (mediastinal weight compresses it, abdominal contents push up diaphragm). This creates a significant V/Q mismatch and hypoxaemia risk, particularly during one-lung ventilation.
• One-lung ventilation (OLV): Required for thoracic surgery. Produces true shunt (HPV mitigates this). PaO2 falls; FiO2 should be 1.0 initially.
• Cardiovascular: Relatively stable but cardiac output may fall slightly.

• Brachial plexus (lower arm): Most important. Prevented by placing an "axillary roll" (chest roll) - positioned caudal to the axilla (NOT in the axilla) to lift the chest off the mattress and decompress the neurovascular bundle.
• Peroneal nerve: Compression at the fibular head of the lower leg.
• Lateral femoral cutaneous nerve: Compression at the ASIS.

• Axillary roll is mandatory.
• "Bean-bag" or tape is used to secure the patient.
• Check all peripheral pulses and pressure points after positioning.
• Beware of contralateral eye pressure (corneal abrasion, retinal ischemia).

4. PRONE POSITION

• Respiratory: FRC may increase (gravity shifts abdominal contents away from diaphragm), reducing atelectasis compared to supine. Airway pressures may increase if abdomen is compressed. Prone positioning is beneficial in ARDS (improves oxygenation).
• Cardiovascular: Abdominal compression increases intra-abdominal pressure, which reduces venous return by compressing the IVC, leading to hypotension and increased bleeding (epidural venous plexus engorgement). To prevent this, proper framing (chest and iliac crest support) must leave the abdomen free.

• Airway: ETT kinking or displacement during turning. Facial and airway edema with prolonged prone. Tongue swelling.
• Eye injuries: Most feared. Pressure on the globe causes retinal ischemia/blindness (ischemic optic neuropathy). Corneal abrasion from contact with headrest. The eyes must be taped, padded, and checked frequently. The head should not be positioned with direct pressure on the orbit.
• Brachial plexus: Arm positioning - "swimmer's position" (one arm up, one down) vs. both arms tucked. Avoid hyperabduction.
• Peripheral nerve: Ulnar nerve (elbow), peroneal nerve (fibular head).
• VAE (venous air embolism): Can occur, especially during posterior fossa or posterior cervical procedures.
• Ischemic optic neuropathy (ION): Most feared complication. Associated with prolonged surgery (>6 hours), large blood loss, hypotension, anemia, and prone position. Presents as painless postoperative visual loss. No proven prevention except minimising risk factors.

• Requires co-ordinated team effort. The anaesthesiologist controls the head.
• Check ETT position (bilateral auscultation) after turning - cephalad shift of the carina in prone can cause endobronchial intubation.
• Re-check all IV lines, arterial lines, monitoring, pressure points.

5. SITTING (BEACH CHAIR / FOWLER'S) POSITION

• Excellent surgical access to posterior fossa and cervical spine.
• Reduced brain swelling (gravity drains cerebral venous blood).
• Reduced bleeding in the surgical field.
• Allows spontaneous ventilation monitoring for neurological procedures.

• Cardiovascular: Most significant concern. The head is elevated above the heart, requiring the heart to pump blood "uphill." This reduces cerebral perfusion pressure. Under anaesthesia, sympathetic reflexes are blunted, making hypotension severe. The mean BP should be measured at the level of the circle of Willis (not the cuff on the arm). Position the BP cuff at the ear level or apply a correction (+/- 0.77 mmHg per cm height difference from the heart).
• Cerebral perfusion: CPP = MAP (at brain level) - ICP. MAP at the brain level = MAP (measured at heart) - (height difference × 0.77 mmHg/cm).

• Precordial Doppler (most sensitive non-invasive method) - placed over the right heart (right parasternal, 3rd-4th intercostal space).
• Transoesophageal echocardiography (TEE) - most sensitive overall.
• ETCO2 monitoring (most practical): A sudden decrease in ETCO2 indicates VAE; as air enters the pulmonary vasculature, there is increased dead space ventilation.
• Central venous catheter (right atrial catheter) - placed at the SVC-RA junction for air aspiration.
• Pulmonary artery pressure monitoring.
• ECG changes (arrhythmias, ST changes).

• Occurs in 20-35% of the population who have a Patent Foramen Ovale (PFO).
• Air crosses from right to left atrium through the PFO and enters the systemic circulation, potentially causing stroke or coronary air embolism.
• Pre-operative bubble contrast echocardiography is used to screen for PFO. If PFO is confirmed, sitting position is contraindicated or the risk-benefit ratio must be weighed.

1. Immediately inform the surgeon - flood the field with saline to prevent further air entry.
2. Compress the jugular veins (to raise venous pressure in the head).
3. Aspirate air through the right atrial catheter.
4. Discontinue N2O immediately (N2O expands the air bubble).
5. Administer 100% oxygen (FiO2 = 1.0).
6. Lower the surgical site or place patient head-down (Durant's manoeuvre - left lateral decubitus position to trap air in the RV apex away from the pulmonary outflow tract).
7. Vasopressors (phenylephrine, norepinephrine) to maintain MAP.
8. CPR if cardiac arrest occurs.

• Known PFO (patent foramen ovale) - risk of paradoxical embolism.
• Severe cardiovascular disease.
• Severe cerebrovascular disease.

NERVE INJURIES RELATED TO POSITIONING

1. Stretch - excessive joint movement elongates the nerve beyond its tolerance.
2. Compression - direct pressure impairs intraneural blood flow, causing ischaemia.

• Limit shoulder abduction to <90 degrees.
• Avoid extreme joint flexion/extension.
• Pad all bony prominences.
• Avoid shoulder braces in steep Trendelenburg if possible.
• Check position after final draping.
• Document position and padding in the anaesthetic record.

HEMODYNAMIC EFFECTS: SUMMARY TABLE

RESPIRATORY EFFECTS: SUMMARY

• FRC reduction is the primary respiratory concern in most positions under anaesthesia.
• Lateral decubitus under GA: Dependent lung is better perfused but worse ventilated - V/Q mismatch.
• Steep Trendelenburg: Diaphragm shifts cephalad - decreased FRC, increased airway pressures, decreased lung compliance, worsened V/Q, risk of endobronchial intubation.
• Prone: Can improve FRC (gravity helps) - used therapeutically in ARDS.
• Sitting: FRC improved, V/Q most favourable; respiratory mechanics best in this position.

EYE PROTECTION IN ALL POSITIONS

• Prevention: Tape eyelids closed with non-allergenic tape, apply lubricating ointment, use protective eye pads.
• Retinal ischemia: From direct pressure on the globe in prone position. Must be prevented by using horseshoe headrests or Mayfield pins that spare the orbit.
• Ischemic Optic Neuropathy (ION): Posterior ION is the leading cause of permanent vision loss after prone spinal surgery. Risk factors: prolonged surgery, massive blood transfusion, hypotension, anaemia, obesity, male sex. No definitive prevention strategy.

SPECIAL CONSIDERATIONS

Obesity

• Standard positions may require additional wide or bariatric tables.
• FRC is significantly reduced even in supine; prone and lateral further compromise respiratory mechanics.
• Lateral decubitus: excess axillary tissue may make traditional axillary roll unnecessary.
• Reverse Trendelenburg (ramp position) is preferred for intubation to optimise FRC.

Pregnancy

• Supine position causes aortocaval compression (inferior vena cava syndrome) due to the gravid uterus, reducing venous return and cardiac output, and causing supine hypotensive syndrome.
• Prevention: 15-degree left lateral tilt (using wedge under right hip) until after delivery.

Pediatric

• Greater risk of nerve and skin injury due to smaller body habitus.
• More prone to hypothermia from exposure during positioning.
• Endotracheal tube position changes significantly with neck flexion/extension and body turning - always re-verify after turning.

Robotic Surgery

• Steep Trendelenburg (often 20-30 degrees) for robotic urological/gynaecological procedures.
• Significant: decreased FRC, increased airway pressures, V/Q mismatch, intracranial hypertension (raised ICP).
• Risk of endobronchial intubation as ETT moves cephalad during Trendelenburg.
• Pressure control ventilation (PCV) is preferred to volume control - studies show lower peak airway pressures and improved compliance with PCV.
• Prolonged duration increases risk of facial/airway edema, corneal abrasion, and brachial plexus injury from shoulder braces.

COMPARTMENT SYNDROME

• Results from haemorrhage into or venous outflow obstruction of a closed fascial compartment.
• Most commonly occurs with lithotomy position of prolonged duration (>2-4 hours) or associated with hypotension.
• Can affect calf, thigh, or gluteal compartment.
• Causes: muscle ischaemia, necrosis, myoglobinuria, acute kidney injury.
• Diagnosis: Compartment pressure >30 mmHg or within 30 mmHg of diastolic BP.
• Treatment: Immediate fasciotomy.

ANAESTHETIC RECORD AND DOCUMENTATION

• Patient position used
• Table modifications (breaks, reverse/Trendelenburg angle)
• Padding and protection measures applied
• Duration of surgery
• Intraoperative position checks performed
• Any concerns noted

CONCLUSION