Effects of pulmonary blood flow on the fractal nature of flow heterogeneity in sheep lungs.

The spatial heterogeneity of pulmonary blood flow can be described by the relative dispersion (RD) of weight-flow histograms (RD = SD/mean). Glenny and Robertson (J. Appl. Physiol. 69: 532-545, 1990) showed that RD of flow in the lung is fractal in nature, characterized by the fractal dimension (D) and RD for the smallest realizable volume element (RDref). We studied the effects of increasing total pulmonary blood flow on D and RDref. In eight in situ perfused sheep lung preparations, 15-microns radio-labeled microspheres were injected into the pulmonary artery at five different blood flows ranging, in random order, from 1.5 to 5.0 l/m. The lungs were in zone 2 at the lower flows and in zone 3 at the higher flows. The lungs were removed, dried, cut into 2 x 2 x 2-cm3 pieces, weighed, and then counted for microsphere radioactivity. Fractal plots of log(weight) vs. log(RD) were constructed by iteratively combining neighboring pieces and then calculating RD with the increasingly larger portion size. D, which is one minus the slope of the fit through this plot, was 1.14 +/- 0.09 and did not change as blood flow increased. However, RDref decreased significantly (P < 0.01) as total flow increased. We conclude that the fractal nature of pulmonary blood flow distribution is not altered by changes in overall flow.