Effects of positive end-expiratory pressure on the sigmoid equation in experimental acute lung injury

Objectives: To describe inflation and deflation volume-pressure (V-P) curves of the respiratory system by the sigmoidal equation at different levels of positive end-expiratory pressure (PEEP) in acute lung injury. Design: Experimental study. Setting: Physiological laboratory in a university setting. Subjects: Six pigs of 25 kg each. Interventions: Acute lung injury was induced by oleic acid. PEEP was applied from 0 to 15 cmH(2)O and from 15 to 0 cmH(2)O for 10 min in steps of 5 cmH(2)O. Measurements and results: Inflation and deflation V-P curves were constructed from an automated super-syringe that delivers a constant flow of 7 l/min in both inspiratory and expiratory directions. V-P curves were obtained at each level of PEEP without disconnecting the animal from the ventilator. The experimental data were fitted to the sigmoid equation which provided the true inflection point (c), the point of maximal compliance increase (Pmci) reflecting opening/closure and the point of maximal compliance decrease (Pmcd) reflecting end of recruitment/onset of de-recruitment. The sigmoid equation provided an excellent fit. The values of the coefficients of determination were greater than 0.970 (median 0.996, IQR 0.994-0.997 for the 84 determinations). Negative values of Pmci in the deflation limb of the V-P curve were recorded in five pigs, suggesting closure below the volume range studied. Conclusions: Inflation and deflation V-P curves at different PEEPs can be fitted by the sigmoid equation. However, further work is needed to investigate the meaning of negative values for Pmci.