A comprehensive equation for the pulmonary pressure-volume curve

Venegas, José G., R. Scott Harris, and Brett A. Simon.A comprehensive equation for the pulmonary pressure-volume curve.J. Appl. Physiol. 84(1): 389–395, 1998.—Quantification of pulmonary pressure-volume (P-V) curves is often limited to calculation of specific compliance at a given pressure or the recoil pressure (P) at a given volume (V). These parameters can be substantially different depending on the arbitrary pressure or volume used in the comparison and may lead to erroneous conclusions. We evaluated a sigmoidal equation of the form, V =a +b[1 +$e^{-(P-c)/d}$ ]⁻¹, for its ability to characterize lung and respiratory system P-V curves obtained under a variety of conditions including normal and hypocapnic pneumoconstricted dog lungs (n = 9), oleic acid-induced acute respiratory distress syndrome (n = 2), and mechanically ventilated patients with acute respiratory distress syndrome (n = 10). In this equation,a corresponds to the V of a lower asymptote, b to the V difference between upper and lower asymptotes, cto the P at the true inflection point of the curve, andd to a width parameter proportional to the P range within which most of the V change occurs. The equation fitted equally well inflation and deflation limbs of P-V curves with a mean goodness-of-fit coefficient (R²) of 0.997 ± 0.02 (SD). When the data from all analyzed P-V curves were normalized by the best-fit parameters and plotted as (V −a)/bvs. (P −c)/d, they collapsed into a single and tight relationship (R² = 0.997). These results demonstrate that this sigmoidal equation can fit with excellent precision inflation and deflation P-V curves of normal lungs and of lungs with alveolar derecruitment and/or a region of gas trapping while yielding robust and physiologically useful parameters.