Background
Current research suggests that proning improves oxygenation in patients with severe COVID-19 who are often hypoxemic and undergo endotracheal intubation.[1][2][4] The goal of awake self-proning is to enhance oxygenation, improve respiratory function by equalizing the pleural pressure gradient and distribution of ventilation, and prevent intubation.[6] Patients who practice proning by lying on their stomachs increase aeration of dense posterior lung segments. As they lie in the prone position, gravity pushes secretions from the dense posterior lung segments toward the less dense anterior lung tissue, which results in increased perfusion and oxygenation to the posterior lungs.[7] The improved mobilization and drainage of secretions from the posterior lungs increases oxygen saturation and reduces the risk of intubation and ventilator-associated pneumonia (VAP).[7]
Anatomical and physiological changes occur in the lungs of the COVID-19 patient when they are in the prone position. Published reports from 1988 to 1991 showed computerized tomography (CT) scans of patients with acute respiratory distress syndrome (ARDS) in the prone position, which revealed redistribution of pathological posterobasal lung densities to the new dependent lung positions.[7] The historical evidence led to the development of a pathophysiological ”sponge lung” model, which simply means that when a sponge is soaked in water, removed from the water, and placed in a horizontal or vertical position, the water drainage slows to a stop, and the sponge becomes wetter on the bottom with more empty pores on top.[7] The lungs have a sponge-like consistency similar to the sponge model. The model also reflects the mechanism through which the positive end-expiratory pressure (PEEP) opposes compressing forces by changing lung volume and intrathoracic pressure; when PEEP exceeds overlying pressure, it enables the dependent regions of the lung to remain open.[7]
Reports from clinical trials concluded that patients with severe ARDS who lie in the prone position have a significant survival advantage. Lung changes occur in the ARDS patient due to increased weight from fluids (edema) that squeeze out gases from the posterior part of the lungs or the most gravity-dependent regions.[8]Forces that result in compression atelectasis or alveolar collapse in the dependent regions appear as densities on CT scans.[9] The shape of the lungs and thoracic cavity, lung and cardiac mass, and displacement of the abdomen contribute to changes in transpulmonary pressure and the distribution of densities in the lungs due to gravitational forces.[6][9] While prone, the weight of the heart and abdominal contents are removed from the lungs, the production of cytokines associated with inflammation is decreased, pleural pressure and dorsal lung atelectasis decreases, and alveoli are opened with improved oxygenation.[5][7][9] PEEP and lying in the prone position contributes to anterior lung de-recruitment, dorsal lung recruitment with increased ventilation, and diminished ventilator-induced injury (VILI) in ARDS patients due to improved homogenous distribution of inflation and reduced lung strain.[7][9][10]