Is prone positioning valid for non-ARDS (Acute Respiratory Distress Syndrome) patients?

Prone Positioning in Non-ARDS Patients

Prone positioning is not recommended for non-ARDS patients as its mortality benefit has only been demonstrated in patients with severe ARDS (PaO2/FiO2 <150 mmHg). 1, 2

Evidence for Prone Positioning in ARDS vs. Non-ARDS

• Prone positioning has strong evidence supporting its use specifically in severe ARDS patients, with the American Thoracic Society recommending it for more than 12 hours per day in this population 1, 2
• The mortality benefit is most pronounced in patients with moderate to severe ARDS, with a relative risk reduction of 26% in trials with prone duration greater than 12 hours per day 1
• Meta-analyses show that prone positioning is most effective when applied to patients with severe ARDS (PaO2/FiO2 <150 mmHg), not in milder forms of respiratory failure 1
• The landmark PROSEVA trial, which demonstrated mortality benefit, specifically included only patients with severe ARDS (mean baseline PaO2/FiO2 of 100 ± 30 in the prone group) 1

Physiological Mechanisms

• Prone positioning works through specific mechanisms that address the pathophysiology of ARDS:
  • Improved ventilation-perfusion matching 2
  • More even distribution of gravitational gradient in pleural pressure 1, 2
  • Reduced alveolar shunt through recruitment of well-perfused dorsal regions 2
  • Decreased ventilator-induced lung injury through more uniform distribution of tidal volume 1, 2
  • Reduced compression of lung segments by the heart 1

Limited Evidence in Non-ARDS Patients

• During the COVID-19 pandemic, prone positioning was used in non-intubated patients with acute hypoxemic respiratory failure, but the evidence remains limited and controversial 3, 4
• A meta-analysis of prone positioning in non-intubated patients showed improved oxygenation (PaO2/FiO2 increased by a mean difference of 47.89), but did not demonstrate clear mortality benefits 3
• The aggregated intubation rate in non-intubated patients using prone positioning was still 33%, suggesting that many patients ultimately required invasive ventilation despite prone positioning 3

Potential Risks and Complications

• Prone positioning is associated with specific complications:
  • Higher rates of endotracheal tube obstruction (RR 1.76) 1
  • Increased risk of pressure sores (RR 1.22) 1, 2
  • Potential increases in intra-abdominal pressure from 12±4 mmHg to 14±5 mmHg 2
  • Logistical challenges in patient care and monitoring 5

Clinical Decision Making

• For patients with respiratory distress who are not intubated and do not have ARDS, consider alternative positioning strategies:
  • The tripod position may be more appropriate for conscious patients with respiratory distress, as it allows use of accessory muscles and improves diaphragmatic function 6
• For patients with severe ARDS:
  • Apply prone positioning early (within 48 hours of mechanical ventilation) 2
  • Maintain prone positioning for more than 12 hours per day 1, 2
  • Use in conjunction with lung-protective ventilation strategies (tidal volumes 4-8 ml/kg PBW) 1, 2

Conclusion

While prone positioning has become a standard of care for patients with severe ARDS, the evidence does not support its routine use in non-ARDS patients. The physiological benefits and mortality reduction have been demonstrated specifically in the context of severe ARDS, where the pathophysiological abnormalities respond to the mechanisms of action of prone positioning.