Physiological implications of the fractal distribution of ventilation and perfusion in the lung

Both regional ventilation and regional perfusion demonstrate progressive in creases in heterogeneity as the resolution of measurement is improved. Beca use the efficiency of pulmonary gas exchange is dependent on the match betw een local ventilation and local perfusion, the correlation between these tw o parameters was examined over a range of scale. We marked regional ventila tion and perfusion in three anesthetized pigs with aerosolized 1 mum fluore scent microspheres (FMS) and injected 15 mum FMS. The lungs were dried infl ated, cut into similar to2 cm(3) cubes, and regional ventilation and blood flow were calculated from measurements of the fluorescence signals extracte d from each piece. Adjacent pieces were clustered into successively larger aggregate volumes, and the averages of ventilation and of perfusion were ca lculated for each cluster size. While the coefficient of variation for both ventilation and perfusion increased predictably as the cluster size decrea sed, the correlation between ventilation and perfusion within clusters rema ined high, averaging between 0.82 and 0.92 among animals. Thus, while both ventilation and perfusion heterogeneity increase as the resolution of measu rement improves, the strong correlation between these two parameters in a n ormal prone lung is nearly sample size invariant. This finding explains the observed efficiency of normal gas exchange in the face of the substantial degree of ventilation and perfusion heterogeneity observed in the normal lu ng with high-resolution measurement. (C) 2000 Biomedical Engineering Societ y. [S0090-6964(00)01708-2].