When is a patient prone for prone?

M echanical ventilation has been the main treatment of pulmonary insufficiency, especially acute respiratory failure and acute respiratory distress syndrome CARDS), for the last 30 years. Because mechanical ventilation itself may cause or propagate lung injury, therapeutic strategies increasingly focus on the avoidance of these iatrogenic factors by including adjunct measures such as inhaled vasodilators, negative intravascular fluid balance, or prone positioning. It was almost 10 years after the clinical description of ARDS that Piehl and Brown (1) first described the benefit of positional changes to augment arterial oxygenation in five patients prone to develop respiratory failure. Their data were confirmed by a second, more extensive study by Douglas et al. (2), who also described a significant increase in Pao, in most, but not all, patients studied. Possible mechanisms to explain the improvements of gas exchange were discussed and included the redistribution of blood flow and/or ventilation, an increase in functional residual capacity, and changes in the ventrodorsal transpulmonary pressure gradient. Both papers were based on the theoretical work of Bryan (3), who advocated the prone position in mechanically ventilated patients in order to improve regional inflation of the dorsal portions of the lung. Douglas et al. (2) Piehl and Brown (1) supported the use of positional changes to improve pulmonary gas exchange, to decrease possibly toxic inspiratory concentrations of oxygen, a potential iatrogenic factor in the promotion of the lung injury. Despite the encouraging clinical results, prone position did not become an integral part of treatment of respiratory failure, possibly due to the technical problems and unpredictability of prone position. Clinical research focused instead on the development of new therapeutic strategies, e.g., ventilator modes and drugs to cope with acute respiratory failure and its high mortality. However, studies continued to investigate the interdependence of body position, gas exchange, regional ventilation, and blood flow.