Exponential analysis of the lung pressure-volume curve in newborn mammals.

The compliance of the lung (per unit of lung weight) is less in newborn mammals than in adults. This could result from a smaller volume of airspaces per unit weight and/or a lower lung distensibility. The isolated role of lung distensibility was evaluated by using a mathematical description of the pressure-volume (P-V) curve during lung deflation. Deflation limbs of static P-V curves in newborns of six species (four experimentally obtained and two taken from the literature) ranging from total lung capacity to the resting volume (Vr) were fitted by a monoexponential function of the type V = B - Ae-KP, where B equals Vmax at infinite P, A equals the difference between Vmax and V at P = O, and K is a constant representing lung distensibility. Unlike in adults, in newborns the monoexponential fitting provided an adequate description of the P-V curve for only a relatively small range of transpulmonary pressure (from P at Vr to 10-15 cm H2O). The K value of this portion of the curve was similar among species but higher than in adult mammals, averaging 0.240 cm H2O-1. This suggests a similar lung structure in the different species. Since lung distensibility in newborns is larger than in adults, the fact that a unit mass of lung in the newborn is less compliant should be due to the smaller volume of its airspaces.