Effects of Gas Density and Viscosity on the Maximal Expiratory Flow-volume Relationship

It has been shown that a functional relationship exists between transpulmonary pressure, respira-tory gas flow, and the degree of lung inflation (1, 2). This relationship may be expressed in the form of a family of isovolumetric pressure-flow (PF) curves, each obtained at different degrees of lung inflation. Figure la shows four expiratory isovolumetric PF curves from a normal subject. The shape of each curve is different. Expiratory flows on PF curves 1, 2, and 3 were measured at volumes low in the vital capacity. Flow in-creases to the maxima A, B, and C, and then diminishes with increasing pressure. On PF curves high in the vital capacity, such as curve 4, effort adequate to achieve flow maxima cannot be developed. If the maximal flows from curves 1 through 4 are plotted against the volume at which each was measured, a maximal expiratory flow-volume (FV) curve results (Figure lb). On the basis of the PF curves, this FV curve may be divided into two segments, the a-/3 segment and the,8-y segment. The a-/8 segment (the solid portion of the curve in Figure lb) extends over the lower portion of the vital capacity and corre-sponds to the PF-curve maxima. The 8--y seg-ment (the dashed portion of the curve in Figure ib) is the remaining part of the FV curve and extends over the upper portion of the vital ca-pacity. The /-3y segment corresponds to the ends of the isovolumetric PF curves not having maxima. In practice, it is not necessary to obtain the FV curve from PF curves as outlined above un-less careful identification of the /3 point is de-sired. The FV curve may be simply and directl