Prone Ventilation

Prone position was reported to improve oxygenation in patients with ARDS as long ago as 1976.31 The mechanism of the improvement in oxygenation on turning prone, seen in about two thirds of patients with ARDS, is complex. The intuitive explanation that regional lung perfusion is primarily dependent on gravity leading to improved perfusion of non-consolidated lung on turning is not substantiated by research. In fact, perfusion to dorsal lung regions predominates whatever the patient's position,32 and gravity accounts for less than half the perfusion heterogeneity seen in either the supine or prone position.33 Changes in regional pleural pressure are more important. The gradient of pleural pressure from negative ventrally to positive dorsally in the supine position is not completely reversed on turning prone, so that the distribution of positive pressure ventilation is more homogenous in the prone position.34 Thus, recruitment of dorsal lung appears to be the predominant mechanism of improved oxygenation.

Potential problems associated with prone positioning are pressure-induced skin damage, increased venous pressure in the head (facial oedema), eye damage (corneal abrasions, retinal and optic nerve ischaemia), dislodgment of endotra-cheal tubes and intravascular catheters, and increased intra-abdominal pressure.

A multicentre prospective randomised study of the prone position for adult patients with acute respiratory failure was undertaken in Italy.35 Patients randomised to prone positioning were assessed daily for the first 10 days and turned prone for at least 6 hours if severity criteria were met. There were no differences in clinical outcome.

Prone positioning is a useful adjunct to ventilation and may help to improve oxygenation and pulmonary mechanics but, as yet, has not been shown to alter outcome in ARDS.

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