Objective: In acute respiratory distress syndrome the body posture
effects on pressure-volume (PV) curves are still unclear. We examined
the effects of prone position on inflation PV curves and their potential
relationships with postural alterations in gas exchange. Design and
setting: Prospective study with patients serving as their own controls
in a university-affiliated 30-bed intensive care unit. Patients and
participants: Thirteen anesthetized, paralyzed, semirecumbent,
mechanically ventilated patients with early/severe/diffuse ARDS.
Interventions: Sequential body posture changes: preprone semirecumbent,
prone, and postprone semirecumbent. Measurements and results: In each
posture hemodynamics, gas exchange, and lung volumes were determined
before/during removal and after restoration of positive end-expiratory
pressure ( PEEP=10.2 +/- 0.6 cmH(2)O). At zero PEEP PV curves of
respiratory system, lung, and chest wall were constructed. Prone
position vs. preprone semirecumbent resulted in significantly reduced
pressure lower inflection point of lung PV curve (2.2 +/- 0.2 vs. 3.7
+/- 0.5 cmH(2)O) and increased volume at upper inflection point (0.87
+/- 0.03 vs. 0.69 +/- 0.051). Postural reduction in lower inflection
point pressure of lung PV curve was the sole independent predictor of
pronation-induced increases in PaO2/FIO2 (R-2 = 0.76). PaO2/FIO2
increases were also significantly related with increases in functional
residual capacity (R-2 = 0.60). Conclusions: In early/severe/diffuse
ARDS prone position reduces lower inflection point pressure and
increases upper inflection point UIP volume of the lung PV curve. Lower
inflection point pressure reductions explain oxygenation improvements,
which are also associated with a postural increase in functional
residual capacity
