From the Guidelines
Proning improves outcomes in Acute Respiratory Distress Syndrome (ARDS) by enhancing oxygenation through the redistribution of lung perfusion and ventilation, reducing ventilation-perfusion mismatch, and improving secretion clearance, as recommended by the latest guideline on positioning and early mobilisation in the critically ill 1. The mechanism by which proning achieves this improvement involves several physiological changes. When patients with ARDS are placed in the prone position, gravity redistributes lung perfusion and ventilation more evenly, reducing ventilation-perfusion mismatch. This position allows for better expansion of the dorsal (back) regions of the lungs, which contain more alveoli and are typically compressed when patients lie supine. Key benefits of proning include:
- Improved oxygenation due to more even distribution of gravitational gradient in pleural pressure 1
- Better distribution of ventilation to the dorsal areas of the lungs 1
- Reduced compression of the lungs by the heart and abdominal contents
- Redistribution of edema fluid away from dependent regions, helping to open previously collapsed alveoli For implementation, patients should be proned for at least 12, preferably 16 hours daily, as recommended by the guideline 1, with careful attention to preventing pressure injuries by using proper positioning techniques and padding pressure points. A team approach is essential for safely turning patients, particularly those who are intubated. Proning has been shown to reduce mortality in moderate to severe ARDS when implemented early, especially in patients with PaO2/FiO2 ratios below 150 mmHg, as supported by the guideline 1 and other studies 1. The physiological benefits occur because the lungs are anatomically heterogeneous, with more lung tissue in the dorsal regions that can be recruited for gas exchange when gravity works in their favor during proning.
From the Research
Mechanism of Prone Positioning in ARDS
The mechanism by which proning improves outcomes in Acute Respiratory Distress Syndrome (ARDS) can be explained by the following factors:
- Redistribution of lung inflation: In the prone position, computed tomography scan densities redistribute from dorsal to ventral, allowing for more homogeneous lung inflation 2.
- Improvement in ventilation-perfusion distribution: Proning optimizes the ventilation-perfusion distribution, enabling lung protective ventilation and alleviating hypoxaemia 3.
- Reduction in lung strain: Prone positioning reduces lung strain by redistributing transpulmonary pressure, stress, and strain throughout the lung 4.
- Recruitment of alveoli: Proning recruits alveoli, particularly in the dorsal regions, leading to improved oxygenation 5.
- Decrease in intrapulmonary shunting: Prone positioning decreases intrapulmonary shunting by reducing compression atelectasis in the dorsal lung regions 3.
Physiological Effects of Prone Positioning
The physiological effects of prone positioning in ARDS patients include:
- Improved gas exchange: Proning improves oxygenation by increasing the ventilation-perfusion ratio 2, 3.
- Reduced lung injury: Prone positioning reduces lung injury by decreasing lung strain and promoting more homogeneous distribution of ventilation 4, 6.
- Improved hemodynamics: Proning unloads the right ventricle, leading to improved hemodynamics 4.
- Increased lung volumes: Prone positioning increases lung volumes, particularly in the dorsal regions, leading to improved oxygenation 5.
Clinical Benefits of Prone Positioning
The clinical benefits of prone positioning in ARDS patients include:
- Improved survival: Prone positioning has been shown to improve survival in patients with severe ARDS 2, 4.
- Reduced mortality: The use of prone positioning in conjunction with lung-protective ventilation has been shown to reduce mortality in ARDS patients 2, 4.
- Improved oxygenation: Proning improves oxygenation, reducing the need for high levels of supplemental oxygen 3, 5.