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What is Prone Ventilation?
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Prone ventilation is a technique of turning a patient with severe hypoxic respiratory failure from the supine to the prone position to improve oxygenation. This maneuver has been successful in the management of patients with acute lung injury or acute respiratory distress syndrome (ARDS). Proning is most commonly used for mechanically ventilated patients; however, prone positioning has been implemented successfully in non-intubated patients with COVID-19, improving oxygenation and reducing rates of intubation (see Prone Positioning for Non-intubated patients with COVID-19 ARDS).
Physiology Review
The severity of ARDS is defined by the degree of hypoxemia and is calculated as the ratio of partial pressure of arterial oxygen tension (PaO
2) to fraction of inspired oxygen (FiO
2). ARDS is subdivided into three main categories (ARDS Definition Task Force et al., 2012):
- Mild ARDS: PaO2/FiO2 ratio = 200-300
- Moderate ARDS: PaO2/FiO2 ratio = 100-199
- Severe ARDS: PaO2/FiO2 ratio less than 100
In the supine position, the lungs are compressed by gravity, the heart and the diaphragm. This position can cause hyperinflation of alveoli in the ventral lung while causing alveolar collapse (atelectasis) in the dorsal part of the lung (lying closest the bed). Gravity pushes blood downward toward the poorly oxygenated alveoli in the posterior lung, creating a ventilation/perfusion mismatch. This mismatch is thought to drive rapid deterioration of patients with ARDS.
Prone ventilation reduces ventilator-induced lung injury by promoting even distribution of lung volumes and pressures throughout the lung. Studies have shown prone positioning improves oxygenation between 60 to 80% compared to patients who were not proned (Hudack, 2013). Early initiation of prone ventilation is most effective and should be reserved for the following patients (Malholtra, 2021):
- Severe ARDS (PaO2/FiO2 less than 150 mmHg) with FiO2 greater than or equal to 0.6 and PEEP greater than or equal to 5 cm H2O OR
- Refractory hypoxemia due to ARDS defined as PaO2 less than or equal to 100 mmHg with a PaO2 less than or equal to 60 mmHg despite optimization of the ventilator settings on FiO2 of 1.0 (or 100%)
GAS EXCHANGE IN THE LUNGS (Gordon et al., 2019; Lucchini et al., 2020) |
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NORMAL VENTILATION |
ARDS |
PRONE |
Definition |
- Process of air movement into the lungs.
- Oxygen and carbon dioxide gas exchange occurs within the alveoli.
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- An acute, diffuse, inflammatory lung injury
- Symptoms include dyspnea, increasing need for supplemental oxygen, and alveolar infiltrates on chest X-ray.
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- A non-invasive maneuver of positioning a patient on their abdomen.
- Used to improve oxygenation.
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Ventilation (V) |
- Adequate tidal volume or inspired air is necessary to reach the alveoli.
- Influenced by lung compliance, chest wall compliance, and weight of cardiac and abdominal organs.
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- Pulmonary edema causes stiff and non-compliant lungs.
- Weight of fluid-filled lungs constricts dorsal lung fields, compressing alveoli, decreasing ventilation.
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- Reduces lung compression from weight of cardiac and abdominal organs.
- Improves lung compliance.
- Enhances the V/Q ratio.
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Perfusion (P) |
- Sufficient blood flow is required to perfuse the alveoli.
- Dependent lung fields (dorsal and basal lobes) typically receive greater perfusion in the supine position.
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- Atelectasis (collapsed lung tissue) and interstitial inflammation impair ventilation/perfusion (V/Q) ratio and contribute to hypoxemia.
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- Improves alveolar recruitment, particularly in the dependent and anterior lung fields.
- Increases functional residual capacity.
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Gas exchange at the alveolar level |
- Ample alveolar surface area is necessary for gas exchange to occur.
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- Decreased surfactant leads to collapsed alveoli.
- Fluid in alveoli inhibits gas exchange and oxygenation.
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- Increased drainage of pulmonary secretions improves alveolar surface area for gas exchange.
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