Is proning necessary for a patient with severe Acute Respiratory Distress Syndrome (ARDS)?

Prone Positioning in Severe ARDS

Yes, prone positioning is necessary for patients with severe ARDS (PaO₂/FiO₂ <150 mmHg) and should be implemented early as a first-line therapy, as it significantly reduces mortality when applied for prolonged durations (≥12-16 hours daily). 1, 2

Patient Selection Criteria

Prone positioning should be implemented when ALL of the following criteria are met:

• Severe ARDS with PaO₂/FiO₂ ratio <150 mmHg despite optimization 1, 2
• FiO₂ ≥0.6 and PEEP ≥5 cmH₂O 2
• Tidal volume ~6 ml/kg predicted body weight 2
• Within 48 hours of starting mechanical ventilation 1

The mortality benefit is most pronounced in this severe hypoxemia subgroup, with risk ratios of 0.74-0.77 for mortality reduction. 2 Patients with moderate ARDS (PaO₂/FiO₂ 100-300 mmHg) do not demonstrate the same survival advantage. 2

Duration and Timing Requirements

The critical factor for mortality benefit is prolonged prone positioning of at least 12-16 hours per day. 1, 2 Meta-analyses demonstrate that prone positioning reduced mortality only in trials with prone duration greater than 12 hours per day (RR 0.74; 95% CI 0.56-0.99), while shorter durations (7-8 hours) showed equivocal results. 1, 3

• Recommended duration: ≥16 hours daily 2
• Minimum effective duration: 12 hours per day 1, 3
• Early application within 48 hours of mechanical ventilation initiation 1

The time course of alveolar recruitment during prone positioning varies markedly between patients, with some requiring more than 8 hours to reach complete alveolar recruitment. 4

Ventilator Management During Proning

Lung-protective ventilation must be maintained throughout prone positioning, as tidal volume limitation is essential for the mortality benefit. 1, 2

Required ventilator settings:

• Tidal volume: 4-8 ml/kg predicted body weight (target ≤6 ml/kg) 2
• Plateau pressure: <30 cmH₂O 2
• PEEP: maintained or increased as tolerated 2

Prone positioning and PEEP have an additive effect on improving oxygenation. 1 The mortality benefit is particularly evident in patients receiving PEEP ≥10 cmH₂O, with reductions in both 60-day mortality (RR 0.82) and 90-day mortality (RR 0.57). 3

Monitoring Response and Discontinuation

Assess oxygenation response 8-12 hours after the first prone session. 2, 5 Early oxygenation improvement (increase in PaO₂/FiO₂ ratio ≥20 mmHg within 8-12 hours) is associated with improved 28-day outcomes and indicates the need to maintain prolonged prone positioning. 1, 5

Continue prone positioning daily until improvement persists in supine position, defined as: 1, 2

• PaO₂/FiO₂ ≥150 mmHg
• PEEP ≤10 cmH₂O
• FiO₂ ≤0.6
• Assessed 4 hours after returning to supine position

Terminate prone positioning therapy after 2 unsuccessful attempts (no oxygenation improvement). 1, 2

Safety Profile and Complications

Prone positioning is hemodynamically well-tolerated and may positively affect right ventricular function. 1 Volume status should be optimized prior to positioning, though ongoing vasopressor therapy is not a contraindication. 1

Expected complications with increased frequency include: 2

• Endotracheal tube obstruction (RR 1.76; 95% CI 1.24-2.50) 1
• Pressure sores (RR 1.22; 95% CI 1.06-1.41) 1
• Chest tube dislodgement 2

Notably, cardiac arrests are actually reduced with prone positioning compared to supine. 2 Intra-abdominal pressure increases from 12±4 mmHg to 14±5 mmHg in prone position. 1

Relative Contraindications

Proceed with caution but do not automatically exclude patients with: 2

• Recent abdominal surgery
• Increased intracranial pressure (ICP increases significantly with prone positioning) 1
• Spinal instability
• Hemodynamically significant arrhythmias or shock

Obesity is NOT a contraindication. 2 Obese patients do not experience more complications overall and may have greater oxygenation improvement. 1

For patients with acute cerebral lesions, individual risk-benefit assessment is required, considering both potential ICP elevation and oxygenation benefits. 1 Special considerations are needed for patients on concurrent ECMO therapy, which requires experienced centers. 1, 6

Physiological Rationale

The mortality benefit stems from multiple mechanisms beyond simple oxygenation improvement:

• More homogeneous distribution of ventilation 1, 2
• Improved ventilation-perfusion matching and reduced alveolar shunt 1
• Recruitment of well-perfused dorsal lung regions 1
• More even distribution of gravitational gradient in pleural pressure 1
• Decreased ventilator-induced lung injury through more uniform distribution of tidal volume 1, 7
• Reduced compression of lung segments by the heart 1

A critical caveat: While prone positioning consistently improves oxygenation in 70-80% of patients, improvement in oxygenation alone does not guarantee survival benefit. 8, 7 The mortality reduction requires the combination of prolonged prone duration (≥12 hours), severe ARDS criteria, and lung-protective ventilation. 4, 1, 2