What is the pathophysiology behind prone positioning in Acute Respiratory Distress Syndrome (ARDS)?

Pathophysiology of Prone Positioning in ARDS

Prone positioning improves oxygenation and reduces mortality in severe ARDS through more homogeneous distribution of ventilation, improved ventilation-perfusion matching, reduced alveolar shunt, and decreased ventilator-induced lung injury risk. 1

Key Physiological Mechanisms

• Redistribution of lung densities: Prone positioning causes redistribution of lung densities with recruitment of well-perfused dorsal regions that are often collapsed in supine position 1

• More even gravitational gradient: Prone position creates a more even distribution of the gravitational gradient in pleural pressure, leading to better distribution of ventilation to the dorsal areas of the lungs 1

• Improved ventilation-perfusion matching: While ventilation becomes more evenly distributed in prone position, pulmonary perfusion remains preferentially distributed to the dorsal lung regions, thus improving overall ventilation/perfusion relationships 2, 3

• Reduced alveolar shunt: The improved recruitment of dorsal lung regions and better ventilation-perfusion matching leads to a significant reduction in alveolar shunt 1

• More homogeneous distribution of ventilation: Prone positioning results in more uniform distribution of tidal volume throughout the lungs, reducing regional overdistension 3

• Decreased ventilator-induced lung injury (VILI) risk: The larger tissue mass suspended from a wider dorsal chest wall creates a more homogeneous distribution of pleural pressures, reducing abnormal strain and stress development that can lead to VILI 3

Effects on Lung Mechanics and Gas Exchange

• Improved lung recruitment: Although prone positioning increases chest wall elastance, this change is usually accompanied by improved lung recruitment 1

• Enhanced oxygenation: Prone positioning consistently improves oxygenation (PaO₂/FiO₂ ratio) in approximately 70% of ARDS patients, with an average increase of 23.5 mmHg by day 4 3, 4

• Improved CO₂ clearance: The more homogeneous ventilation distribution also enhances alveolar ventilation and CO₂ elimination 1

• Increased end-expiratory lung volume: Prone positioning improves resting lung volume in the dorsocaudal regions by reducing the superimposed pressure of both the heart and the abdomen 3

Clinical Implications

• Mortality benefit: Prone positioning significantly reduces mortality in severe ARDS (PaO₂/FiO₂ <150 mmHg) when applied early and for prolonged periods (≥16 hours per day) 1, 5

• Timing and duration: Early application of prone positioning (within 48 hours of mechanical ventilation) for sessions lasting at least 12-16 hours shows the greatest benefit 2, 6

• Synergy with lung-protective ventilation: The mortality benefit of prone positioning is enhanced when combined with lung-protective ventilation strategies (tidal volumes ≤8 mL/kg predicted body weight) 2, 4

• Additive effect with other strategies: Combining prone positioning with high PEEP, recruitment maneuvers, and inhaled vasodilators has an additive effect in improving oxygenation 3

Potential Complications

• Endotracheal tube complications: Higher rates of endotracheal tube obstruction (RR 1.76) can occur during prone positioning 2, 4

• Pressure sores: Increased risk of pressure sores (RR 1.22) with prolonged prone positioning 2, 4

• Increased intra-abdominal pressure: Prone positioning can increase intra-abdominal pressure from 12±4 mmHg to 14±5 mmHg 2

The pathophysiological benefits of prone positioning make it a crucial intervention in severe ARDS management, with clear mortality benefits when applied appropriately in the right patient population.