Use of an exponential function for elastic recoil.

To improve the quantitative representation of elastic recoil, an exponential function (V = A - B exp (-KP), where V is the lung volume, P is the recoil pressure, and A, B, and K are constants), has been fitted to static pressure-volume (PV) data by a least-squares technique using a digital computer. The PV points below about 50% total lung capacity (TLC) usually deviate from an exponential expression. Increasing the lower volume limit to which the exponential was fitted from 40 to 75% of TLC in 20 subjects, increased K and B/A significantly. Residual variance was lowest (approx 1.0 +/- 0.5%) when the lower volume limit was between 50 and 60% TLC. The PV points in the lower volume range deviated significantly less from exponential in a group of older subjects than for young subjects. The coefficient of variation of K and B/A for duplicate studies in 10 subjects was 9 and 14%, respectively. For inflation PV data, K was significantly lower and A significantly higher than for deflation. For PV data obtained after a submaximal inspiration, K did not change significantly. From this exponential function, the PV curve above 50% TLC can be satisfactorily described by K, B/A, and recoil pressure at TLC; other conventional parameters may be derived.