Bronchial mechanical properties and maximal expiratory flows

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Abstract
Flow limitation in a collapsible elastic tube is dependent on the area (A) vs. pressure (P) relationship (the “tube law”) for the tube. In this paper, a tube law in which A varies as (1-P)-n1 at negative pressures is assumed. It is shown that wave-speed limitation is possible at negative pressures only if n1 is greater than 0.5. Dissipative limitation is also investigated. Viscous limitation can occur if n1 is greater than 0.5, and turbulent limitation can occur if n1 is not less than 0.4. For values of n1 less than 0.4, flow cannot be limited at negative pressures. Model simulations are used to show that a combination of a value of n1 less than 0.3 together with an area minimum in the bronchial tree produce a minimum (a “hook”) in the flow-volume curve. In the vicinity of such hooks, density dependence exceeds the usually accepted theoretical maximum value. Simulations also show that, when n1 is sufficiently small, apparently supramaximal flows appear to be possible.