( 14) extended these findings by showing that this result was present in patients with low or high potential of recruitment in the supine position, whatever they were receiving low or high PEEP. CT studies consistently found that with prone the amount of overinflated lung mass declined and that of non or poorly aerated lung mass increased, indicating lung recruitment ( 13, 14). found a reduction in the lung concentration of pro-inflammatory cytokines after 12 hours in prone as compared to supine position ( 12). In humans, several lines of evidence argued in favor of the preventive role of proning against VILI. Lung strain was reduced and homogeneously distributed in prone ( 11). ( 10), demonstrated that prone position, as compared to supine position, attenuated and homogenized the distribution of lung injury across the ventral-to-dorsal direction when very high tidal volume were delivered in normal dogs. Animal studies, like that of Broccard et al. With the recognition of ventilator-induced lung injury (VILI) it turned out that prone position was also able to modulate it. Therefore, better ventilation towards well perfused areas accounts for the common scenario to explain better oxygenation in prone ( 9). Therefore, this unexpected finding argued against the second mechanism to explain the reduction in intra-pulmonary shunt. Several experiments found that the dorsal lung regions when in the prone position still had the highest amount of blood flowing through them ( 2- 8). The latter mechanism was considered as true as intuitively it was thought that the change in gravity direction will affect the lung perfusion in the same way, i.e., less perfusion towards dorsal lung regions, now non-dependent, in the prone position. For the intra-pulmonary shunt to go down two possibilities do exist, either more ventilation in well perfused areas or less perfusion in poorly ventilated lung regions. This effect resulted from a reduction in intra-pulmonary shunt. Therefore from the early onset the clinicians used proning to improve oxygenation. This effect, sometimes dramatic ( 1), was observed in the large majority of patients. The early reason that prompted clinicians to turn ARDS patient to prone was oxygenation improvement. Prone position is a key component of lung protective mechanical ventilation and should be used as a first line therapy in association with low tidal volume and neuromuscular blocking agents in patients with severe ARDS. The pressure sores are more frequent in prone and require a special attention.

The rate of complication is declining with the increase in centers expertise. The effect of proning on survival cannot be predicted and seems unrelated with both severity of oxygenation impairment and oxygenation response to proning. At the end, significant improvement in survival has been demonstrated in the most severe ARDS patients, at a threshold of 100–150 mmHg PaO 2/FiO 2 ratio. Over the time, several trials have been done, which regularly improved and refined from each other. After having demonstrated its capability to significantly improve oxygenation in a large number of patients, sometimes dramatically, this procedure has been found to prevent ventilator-induced lung injury, the primary concern for the intensivists managing ARDS patients. Prone position has been used in acute respiratory distress syndrome (ARDS) patients for more than 40 years in ICU.

Prone position definition