In This Article
- Prone positioning is well accepted as beneficial for patients with severe acute respiratory distress syndrome (ARDS) who are being mechanically ventilated
- In cases where intubation is not yet indicated, not feasible or is discordant with a patient's goals and preferences, there is a physiologic rationale for prone positioning without mechanical ventilation
- Changes in oxygenation should be assessed after prone positioning, and the patient should be as closely monitored as an intubated patient would be
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Prone positioning is well accepted as beneficial for patients with severe acute respiratory disease syndrome (ARDS) who are being mechanically ventilated. During the COVID-19 pandemic, physicians at Massachusetts General Hospital and other medical centers are considering prone positioning for non-intubated patients with ARDS to improve oxygenation.
Management of ARDS in COVID-19 should prioritize lung-protective ventilation. Prone positioning of a non-intubated patient is appropriate if intubation isn't feasible, is discordant with a patient's goals and values or is otherwise not indicated.
Recruitment (opening of alveoli) from relief of compressive weight: The dorsal lung (posterior lung) has a greater mass of tissue than the ventral lung (anterior lung), so in the supine position there's a greater amount of dependent lung. This dependent lung supports the weight of the lung and mediastinum above, worsening collapse in the setting of ARDS. Prone positioning results in dorsal recruitment, in part, simply from the relief of the weight of the mediastinum, which then rests on the sternum while prone. This repositioning results in a net increase in recruited lung and a decrease in overdistended lung.
In addition, derecruitment is exacerbated in the supine position by the weight of the mediastinum on the lung. In the prone position, the mediastinum is supported by the sternum.
Distribution of transpulmonary pressure (PTP, the pressure distending the lung): In the supine position, PTP is greater in the ventral lung. In ARDS, this difference is intensified because the weight of edematous tissue causes dorsal alveolar collapse. Simultaneously, the ventral lung receives greater airflow and is at risk of injury from overdistension. In the prone position, the PTP is more favorably distributed, resulting in less risk of overdistension of ventral lung along with the recruitment of dorsal lung.
Ventilation–perfusion matching: There are more alveoli and vessels in the dorsal lung (larger anatomic mass) and consequently more blood flow there regardless of patient position. Studies of perfusion in the supine and prone position confirm that perfusion is greatest to the dorsal lung in either position. Therefore, recruiting that region results in a net decrease in shunt (perfusion without ventilation) as more ventilation reaches the highly perfused dorsal lung.
Right ventricular function: If prone positioning accomplishes recruitment, the increases in aeration and oxygen tension should mitigate hypoxemic vasoconstriction. When these changes occur in the dorsal lung, which receives the majority of non–gravity-dependent blood flow, pulmonary vascular resistance and thus right ventricular afterload decrease.
There are no clinical trial data on prone positioning in non-intubated patients. Two reports from China describe COVID-19 patients who were prone for multiple sessions per day while receiving high-flow nasal oxygen or noninvasive positive-pressure ventilation. It appears safe and well-tolerated though data are limited.
Low tidal volumes cannot be guaranteed in spontaneously breathing patients, which may lead to injurious lung inflation patterns. Therefore, prone positioning should not serve as a "rescue" maneuver to prevent an otherwise indicated intubation.
If using prone positioning in a non-intubated patient, changes in oxygenation should be assessed after prone positioning, and the patient should be as closely monitored as an intubated patient would be. If the patient has progressive respiratory failure and continued increased work of breathing, we should still prioritize intubation and lung-protective ventilation.
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