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Paper text:
prone positioning is widely used to improve oxygenation of patients with acute respiratory distress syndrome ( ards ) . in the previous issue , robak and colleagues report the short - term impact of combining two positioning strategies , prone and upright positioning . although randomized controlled trials failed to demonstrate an overall mortality benefit with the prone position , a trend toward better survival was observed for patients with severe ards [ 2 - 5 ] .
the use of the prone position is supported by a strong physiological rationale and a significant improvement of the oxygenation status is observed in roughly 70% of ards patients .
several mechanisms have been advocated to explain this effect , but the most determinant probably involves better aeration and recruitment of the vertebra - diaphragmatic lung regions , as a consequence of the reduction in ventral chest wall ( cw ) compliance due to direct compression on the bed .
interestingly , either high baseline cw compliance or a greater reduction in its magnitude with the prone position were associated with oxygenation improvement .
upright positioning has also been associated with oxygenation improvement in ards patients and was proposed as an alternative to the prone position .
we previously demonstrated , in a group of 16 ards patients submitted to upright positioning , a significant increase in the pao2/fio2 ratio ( 94  33 versus 142  49 mmhg , p < 0.0032 ) in comparison to the baseline value obtained with the supine position . with respect to the responders , the significant increase in the measured end - expiratory lung volume , coupled with the change in chord compliance after switching to upright positioning , suggested a time - dependent lung recruitment phenomenon .
based on physiological studies , one can postulate that the oxygenation improvement is mainly due to a lung volume redistribution induced by reduced ventral cw compliance during prone positioning , while an increase of the lung volume ( recruitment ) appears more significant during upright positioning . because of their complementary mechanisms of action , combining prone and upright positioning can make sense in severe ards patients .
likewise , it has already been shown that the combination of prone positioning with an adjunctive technique to increase lung volume , the recruitment maneuvers , has additive positive effects on oxygenation . in the physiological study of roback and colleagues ,
the upright - prone position was achieved by raising the head of the bed and lowering the foot end to obtain an angle of at least 20 ( reverse trendelenburg ) .
this was associated with a significant improvement in the pao2/fio2 ratio , without any short - term adverse events .
the proportion of responders increased by combining the prone with the upright position , which is particularly relevant since the prone position is usually discontinued in the absence of oxygenation improvement .
this study was the first to specifically investigate the effect on gas exchange of combining prone and upright positioning .
unfortunately , the study design precluded conclusions about the precise physiological mechanisms involved . measurements of the lung volumes would have been useful to better document the alteration of respiratory mechanics related to both prone and upright positioning .
regarding the redistribution of aerated lung regions , directly assessing the regional changes could have added to our current understanding . since computed tomography ( ct ) scanning
can not be done during upright positioning , alternative techniques , such as electrical impedance tomography , may have permitted a better delineation of the importance of ventilation redistribution from non - dependent toward dependent parts of the lung with the upright - prone position . beyond the impact on oxygenation , other beneficial effects
although the level of evidence is weak , both prone and upright positions have been associated with a decrease of ventilator - associated pneumonia .
an additive effect of these two strategies is plausible , but the pathophysiology of ventilator - associated pneumonia is complex and one should be prudent before making such an assumption .
furthermore , head elevation of patients ventilated in the prone position was shown to improve the tolerance of enteral feeding .
in summary , the use of prone and upright positioning should not be considered as opposing and mutually exclusive strategies , but more as complementary ones .
even if the level of evidence is debatable , combining prone and upright positioning seems to enhance the response rate in terms of oxygenation . whenever the prone position is used , combining it with the upright position
this strategy is easy to implement at no additional cost and with a low risk of major complications .
specific policies should be implemented to encourage caregivers to avoid as much as possible a strictly supine position .
future studies should aim at determining to what extent the addition of the upright position during , and perhaps also between , prone sessions can promote the maintenance of beneficial effects .
positional strategies have been proposed for mechanically ventilated patients with acute respiratory distress syndrome . despite different physiological mechanisms involved , oxygenation improvement has been demonstrated with both prone and upright positions . in the previous issue of critical care , robak and 
 colleagues reported the first study evaluating the short - term effects of combining prone and upright positioning . 
 the combined positioning enhanced the response rate in terms of oxygenation . 
 other benefits , such as a reduction in ventilator - associated pneumonia and better enteral feeding tolerance , can potentially be expected .