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). Consequently, the natural response to prone position is a decrease in overall chest wall compliance. Conversely, in the prone position, the bed surface impedes expansion of the anterior structures while abdominal compliance remains relatively unmodified. For anatomical reasons, the posterior chest wall (including spine and the scapulae) isless compliant than the anterior component (sternum and ribs). In the supine position, variations of compliance are most strongly influenced by the abdominal and the anterior chest wall, while in prone position, the posterior chest and abdomen are the key determinants. Total chest wall compliance is influenced by the stiffness or flexibility of its three anatomic boundaries: anterior, posterior and abdominal. These possible changes and their causes may best be understood by considering chest wall and lung separately. Therefore, in transitioning to the prone position, the compliance of the integrated respiratory system may stay unmodified, deteriorate or improve. Regional compliance of the lung and chest wall varies in response to differences in the anatomic shape of these structures, the local effects of gravity and the heterogeneous mechanical properties of the diseased lung. Simultaneously, their compliance properties add in parallel: C rs = . The lungs and chest wall, whose structures expand together and share identical volumes, have elastance properties that add in series: ( E rs = E l + E w). 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.

Prone position