The gravitational gradient of pleural pressure, regional end-expiratory and end-inspiratory lung volumes, regional ventilation and ventilation-perfusion ratios are all more uniform in the prone compared with the supine position. The primary reason is improved shape matching between the chest wall and the lung. As shown, the inflation of the pulmonary units is far more homogeneous in prone compared to supine, meaning that the forces applied to distend the lungs (the trans-pulmonary pressure, i.e., the lung stress) are more homogeneously distributed. 1, we represent the gas tissue ratio in prone and in supine position, both in normal and ARDS patients. The CT scan allows a precise quantification of the extent of the inflation as a ratio between gas and tissue. We believe it is extremely important to differentiate the concepts of inflation (a morphologic concept) and ventilation (a physiologic concept, consequence of inflating the lungs). Therefore, the simple observation of plateau pressure (or tidal volume) after a change from supine to prone may give an indication of the extent of lung recruitability. If these expected changes are not observed, it suggests improved lung compliance offsets the positional decrease in chest wall flexibility. With these considerations in mind, the expected response to prone position and decreased overall compliance would be an increase in plateau pressure (in volume control ventilation) or a reduction of tidal volume (in pressure control ventilation). In the prone position such a favorable shift may result from promoting the homogeneous distribution of total stress and strain. It follows that any change in lung compliance is primarily due to the opening of new pulmonary units and/or to improved mechanical characteristics of already opened units that reach a more favorable position on the volume–pressure curve. Of note, the specific lung compliance is similar in ARDS patients and in normal individuals, suggesting that surfactant alterations or early fibrosis do not predominate in altering the intrinsic mechanical characteristics of the lung. In ARDS patients, lung compliance is primarily determined by the lung open to ventilation (i.e., by the number of open pulmonary units). The effects of this intervention on outcomes are still uncertain. Recently, the use of prone position has been extended to non-intubated spontaneously breathing patients affected with COVID-19 ARDS. The most frequent adverse events are pressure sores and facial edema. The maneuver to change from supine to prone and vice versa requires a skilled team of 4–5 caregivers. The only absolute contraindication for implementing prone position is an unstable spinal fracture. The main reason explaining a decreased mortality is less overdistension in non-dependent lung regions and less cyclical opening and closing in dependent lung regions. Improvement in oxygenation and reduction in mortality are the main reasons to implement prone position in patients with ARDS. The change to prone position is generally accompanied by a marked improvement in arterial blood gases, which is mainly due to a better overall ventilation/perfusion matching. In ARDS patients, the change from supine to prone position generates a more even distribution of the gas–tissue ratios along the dependent–nondependent axis and a more homogeneous distribution of lung stress and strain.
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