How does prone positioning affect anesthesia management?

Effects of Prone Positioning on Anesthesia Management

Prone positioning during general anesthesia improves oxygenation and functional residual capacity while minimally affecting respiratory mechanics, but requires careful attention to airway management, hemodynamic monitoring, and prevention of position-related complications. 1, 2

Respiratory Mechanics and Gas Exchange

Beneficial Effects

• Functional residual capacity (FRC) increases significantly from supine to prone position (1.9 ± 0.6 L vs 2.9 ± 0.7 L), with corresponding improvement in PaO₂ (160 ± 37 vs 199 ± 16 mmHg) in anesthetized, paralyzed patients 2
• Lung compliance improves, particularly in obese patients (91.4 ± 55.2 mL/cm H₂O supine vs 109.6 ± 52.4 mL/cm H₂O prone), contributing to better oxygenation 3
• The PaO₂/FiO₂ ratio increases by a weighted mean difference of 25 mmHg with prone positioning 4
• Alveolar dead space remains stable during prolonged prone positioning (>3 hours), with no significant changes in the alveolar dead space to tidal volume ratio 5

Mechanical Changes

• Total respiratory system compliance remains largely unchanged because increased lung compliance is offset by decreased chest wall compliance (199.5 ± 58.7 vs 160.5 ± 45.4 mL/cm H₂O in obese patients) 3
• Respiratory resistance increases slightly (4.8 ± 2.5 vs 5.4 ± 2.7 cm H₂O·L⁻¹·s), primarily due to increased chest wall resistance (1.3 ± 0.6 vs 1.9 ± 0.8 cm H₂O·L⁻¹·s) 2
• Dynamic compliance may be reduced due to increased plateau pressure, particularly during prolonged procedures 5

Airway Management Considerations

Positioning and Intubation

• The patient's head should be positioned at the level of the anesthetist's xiphoid process or nipple when lying on an operating table for optimal ergonomics during intubation 6
• Induction of anesthesia and laryngeal mask airway (LMA) insertion can be performed in the prone position for minor ambulatory surgery, though this requires experience and appropriate patient selection 7
• When using LMA in prone position, additional propofol may be required for laryngospasm or inadequate depth (occurred in 10 of 73 patients in one series) 7

Airway Complications

• Endotracheal tube obstruction risk increases (RR 1.76,95% CI 1.24-2.50) with prone positioning 4
• LMA malpositioning occurs in approximately 5% of cases when inserted prone, though usually easily corrected 7
• Soft tissue trauma causing epistaxis can occur (observed in 2 of 73 patients) 7
• Manual ventilation may be required for arterial oxygen desaturation and hypoventilation 7

Hemodynamic Effects

Cardiovascular Stability

• Prone positioning is generally hemodynamically well tolerated and may positively affect right ventricular load 6
• Volume status should be optimized prior to positioning, though ongoing vasopressor therapy is not a contraindication 6
• Bradycardia can occur (observed in approximately 7% of patients) and should be anticipated 7

Blood Pressure Management

• Mean arterial pressure (MAP) may decrease with prone positioning, particularly when combined with increased PEEP in patients with normal lung compliance 6
• In some studies, MAP improved in prone position (78 ± 8 to 88 ± 8 mmHg), possibly due to better venous return 6

Special Populations and Considerations

Obese Patients

• Prone positioning provides particular benefit in obese patients (BMI >30 kg/m²), with marked improvements in FRC (0.894 ± 0.327 L supine vs 1.980 ± 0.856 L prone) and oxygenation (130 ± 31 vs 181 ± 28 mmHg) 3
• Intra-abdominal pressure increases from 12 ± 4 mmHg to 14 ± 5 mmHg in prone position 6
• Despite increased rates of hypoxic hepatitis and renal failure in some studies, obese patients do not experience more complications overall and may have greater oxygenation improvement 6

Patients with Intracranial Pathology

• Intracranial pressure (ICP) increases significantly with prone positioning (from 9.3 ± 5.2 to 14.8 ± 6.7 mmHg in subarachnoid hemorrhage patients, or 11 to 24 mmHg in other studies) 6
• Cerebral perfusion pressure (CPP) may decrease (73.0 ± 10.5 to 67.7 ± 10.7 mmHg) or remain stable depending on MAP response 6
• Brain tissue oxygen tension (PbtO₂) improves despite ICP increases (26.8 ± 10.9 to 31.6 ± 12.2 Torr) 6
• Individual risk-benefit assessment is required for patients with acute cerebral lesions, considering both potential ICP elevation and oxygenation benefits 6

Practical Anesthetic Management

Patient Positioning Ergonomics

• The anesthetic machine should be positioned so all controls are within a 45° arc from the midline, allowing the anesthetist to maintain proper shoulder and elbow positioning 6
• The patient should be at a height allowing the anesthetist to hold a facemask without bending knees or back, approximately at the level of the umbilicus or T10 dermatome 6
• The monitor screen should be at eye level and directly in front of the anesthetist 6

Ventilator Settings

• Maintain tidal volumes of 8-10 mL/kg (or 4-8 mL/kg predicted body weight in ARDS) 4, 5
• PEEP and prone positioning have additive effects on improving oxygenation 6, 4
• FiO₂ of 0.4 is typically adequate given the improved oxygenation with prone positioning 2, 5

Duration Considerations

• For elective surgery, measurements stabilize after 15-30 minutes in prone position 3
• Respiratory mechanics and gas exchange remain stable during procedures lasting >3 hours 5
• For ARDS patients requiring prone positioning, minimum duration of 12 hours is necessary for mortality benefit 6, 4

Complications to Monitor

Position-Related Injuries

• Pressure sores occur more frequently (RR 1.22,95% CI 1.06-1.41) and require careful examination of pressure points before and after positioning 4
• Postoperative hoarseness (1 of 73 patients) and sore throat (6 of 73 patients) can occur with LMA use in prone position 7

Common Pitfalls to Avoid

• Inadequate depth of anesthesia before prone positioning can lead to laryngospasm or patient movement 7
• Failure to secure the endotracheal tube adequately increases risk of obstruction or displacement 4
• Neglecting to optimize volume status before positioning may exacerbate hemodynamic changes 6
• In patients with intracranial pathology, increasing sedation may be necessary if ICP rises acutely with positioning 6