Fractional carbon monoxide uptake and "diffusing capacity" in models of pulmonary maldistribution

Two models of pulmonary non-uniformity have been studied to determine the effects of maldistribution on "diffusing capacity", measured by steady state (DSS) and single breath (DSB) methods, and on the fractional carbon monoxide uptake (U). The distribution of ventilation with respect to lung volume, perfusion and diffusing capacity, as well as breath-holding time, series inhomogeneity and sequential emptying, variously affect DSS and DSB. Of these, only the distribution of ventilation with respect to diffusing capacity (V/D) influences U. In addition, U is very sensitive to dead space ventilation and thus to respiratory frequency, and this is the chief defect of this very simple measurement. The models described indicate ways in which frequency dependence of U may be allowed for, leaving total diffusing capacity and V/D distribution as its sole determinants.