Exponential Description of the Static Pressure-Volume Curve of Normal and Diseased Lungs1, 2

We examined the practical value of a simple mathematical description of the static pressure-volume (PV) curve of the lungs above functional residual capacity by fitting a monoexponential function of the form V = Vmax - Ae-KP (where Vmax is the volume extrapolated to infinite P, and A and K are constants) to PV curves obtained in 45 normal subjects and 39 patients with emphysema, intermediate α1-antitrypsin deficiency, fibrosing alveolitis, or sarcoidosis. Quality of fit was assessed by calculation of the standard errors of the parameters, by the reduction of original variance (R2), and by the application of sign and runs tests. The runs test proved particularly useful for detecting minor deviations from a monoexponential function; the value of R2 was less discriminating and largely depended on the magnitude of experimental error. The exponential function described the expiratory pressure-volume data well in 42 of the 45 normal subjects and in all of the patients with lung disease. The shape of the pressur...