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]