Prone position in ARDS patients: why, when, how and for whom

Abstract

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. 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. Improvement in oxygenation and reduction in mortality are the main reasons to implement prone position in patients with ARDS. 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. The only absolute contraindication for implementing prone position is an unstable spinal fracture. The maneuver to change from supine to prone and vice versa requires a skilled team of 4-5 caregivers. The most frequent adverse events are pressure sores and facial edema. Recently, the use of prone position has been extended to non-intubated spontaneously breathing patients affected with COVID-19 ARDS. The effects of this intervention on outcomes are still uncertain.

Keywords: Acute respiratory distress syndrome; Gravity; Lung protective ventilation; Prone position; Ventilation/perfusion.

Fig. 1

The gas/tissue ratio (it may be thought as a volume of the pulmonary unit) as a function of the distance between the sternum and the vertebrae. As shown, in supine position, the gas/tissue ratio sharply decreases from the sternum to the vertebrae suggesting that both in normal and in ARDS patients the distending forces is about three times higher closer to the sternum than to the vertebrae. In prone position, the gas/tissue ratio is far more homogeneous, indicating a more even distribution of forces throughout the lung parenchyma

Fig. 2

Due to the anatomical design, in supine position, the open, non-dependent lung mass (at 50% of the sternum-vertebra distance) is about 40% of the total mass, while the dependent accounts for the 60%. As collapse is primarily a function of the superimposed hydrostatic pressure (including the shape and weight of the heart, which is mainly located in the left chest side), it follows that, while prone, more mass opens in the non-dependent zones than collapses in the dependent sternal regions

Fig. 3

Improvement in right ventricular (RV) function after a proning session of 18 h in a patient ventilated for a severe ARDS. Long-axis mid-esophageal view by transesophageal echocardiography shows major RV dilatation (dotted yellow line) before prone positioning (upper image) and normalization when supine positioning was performed after several hours of proning (lower image). Main risk factors for RV overload are reported before and after in the tables. Pplat plateau pressure, DrivingP driving pressure, LV left ventricle