What are the criteria for defining early responders to prone positioning in Acute Respiratory Distress Syndrome (ARDS)?

Criteria for Defining Early Responders to Prone Positioning in ARDS

Early responders to prone positioning in ARDS are defined by an improvement in oxygenation with PaO₂/FiO₂ ratio increasing by ≥20 mmHg within 8-12 hours after initial prone positioning. 1

Primary Response Criteria

• An early oxygenation response is defined as an increase in PaO₂/FiO₂ ratio of ≥20 mmHg within 8-12 hours after first placement in prone position 1
• More recent evidence suggests that a percentage increase in PaO₂/FiO₂ ratio of ≥53.5% between baseline and 8-12 hours after the first prone positioning is a stronger predictor of survival (sensitivity 91.5%, specificity 73.3%) 2
• Early responders to prone positioning show improved 28-day outcomes compared to non-responders 1, 3

Timing of Assessment

• Assessment of response should be performed within the first 8-12 hours after initiating prone positioning 1, 2
• The maximal oxygenation improvement typically occurs within the first 30 minutes of prone positioning and is maintained throughout the prone period 4
• Early response assessment is critical as it may guide decisions about continuing prone positioning for prolonged periods 3

Duration Recommendations Based on Response

• For responders, prone positioning should be maintained for at least 12 hours, preferably 16 hours per day 5, 6
• Prone positioning should be continued until there is persistent improvement in supine oxygenation (PaO₂/FiO₂ ≥150 with PEEP ≤10 cmH₂O and FiO₂ ≤0.6) measured 4 hours after returning to supine position 5
• If no improvement in oxygenation is observed after two positioning attempts, prone positioning therapy should be discontinued 5

Physiological Basis for Response

• Responders typically show:
  • Decreased calculated venous admixture (Qva/Qt) 4
  • Improved ventilation-perfusion matching 3
  • More homogeneous distribution of ventilation 5
  • Reduced risk of ventilator-induced lung injury 3

Clinical Implications of Early Response

• Early oxygenation improvement is associated with reduced mortality and may serve as an indicator to maintain prolonged prone positioning 1, 2
• In responders, the beneficial effects on oxygenation often persist even after returning to the supine position 4
• Patients who respond to prone positioning in the first session are more likely to respond to subsequent prone positioning sessions (71% response rate in subsequent sessions) 4

Important Considerations

• Even in non-responders, prone positioning does not typically cause worsening of gas exchange or hemodynamic parameters 4
• The decision to continue prone positioning should be based on oxygenation response rather than CO₂ clearance, as PaCO₂ response has not been shown to correlate with improved outcomes 1
• Complete (180°) prone positioning is recommended over incomplete prone positioning as it has a stronger effect on oxygenation 5

Pitfalls and Caveats

• Response to prone positioning does not eliminate the need for lung-protective ventilation strategies with limited tidal volumes (4-8 ml/kg PBW) 5, 3
• Prone positioning is associated with risks including endotracheal tube obstruction and pressure sores, which must be monitored closely 3
• Special consideration is needed for patients with increased intracranial pressure, obesity, recent abdominal surgery, or concurrent ECMO therapy 5, 3
• Patients should be hemodynamically stabilized with optimized volume status prior to prone positioning 5