Patients hospitalized in intensive care units (ICUs) are generally positioned supine. This approach disregards the adverse effects of the supine position on the lung which have been recognized for over 2500 years. Hippocrates noted in 500 BC that:
"It is well when the patient is.. .reclining upon either his right or left side.. .and the whole body lying in a relaxed state, for thus the most of persons of health recline. But to lie on one's back, with the hands, neck and the legs extended is far less favorable" .
The first study providing a scientific base for this observation appeared in 1922 when Christie and Beams  noted that lying supine reduced the vital capacity from that measured in the upright posture. In 1933, Hurtado and Frey ) noted that functional residual capacity (FRC) was similarly diminished by lying supine.
Moreno and Lyons  observed that the FRC was higher prone versus supine in 1961. This observation was cited by Mellins in 1974 when he suggested that body position could be an important determinant of airspace closure . Finally, Bryan  seems to have been the first to propose that ventilating patients in the prone position might be the only way to expand areas of dorsal lung that are collapsed as a result of the adverse mechanical effects attributable to lying supine.
This chapter will review the literature investigating the effects of position on regional ventilation and perfusion in the setting of acute lung injury (ALI) and summarize the results ofclinical studies in ALI and/or the acute respiratory distress syndrome (ARDS).
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