Spatial and Temporal Heterogeneity of Regional Pulmonary Blood Flow in Piglets

Abstract

ABSTRACT:Regional pulmonary blood flow (PBF) in adult animals varies over space and time, following a fractal pattern. We hypothesized that PBF would follow a fractal pattern in young animals. Five, two-week old piglets were sedated and mechanically ventilated. After stabilization, fluorescent microspheres were injected via the right atrium at baseline and then again at 5, 20, 20.5, 40 and 60 min. The lungs were subsequently excised, dried, inflated, and cored into 0.12-cm 3 pieces (mean n ϭ 561 Ϯ 106 per animal) with the spatial coordinates recorded for each piece. Regional PBF was spatially and temporally heterogeneous with a spatial coefficient of variation of 43.3 Ϯ 7.9% and a temporal coefficient of variation of 14.3 Ϯ 0.4%. PBF followed a fractal pattern with a fractal dimension of 1.20 Ϯ 0.06 at 20 min, remaining stable throughout the experiment. PBF decreased with distance from the hilum but did not follow a lobar pattern. Temporal heterogeneity did not significantly increase with time but low flow regions demonstrated the greatest temporal variability throughout the study. Hence, PBF in young piglets was characterized both spatial and temporal heterogeneity. A considerable amount of information describing the structural and functional characteristics of the pulmonary circulation during postnatal lung development exists. Features such as vascular reactivity, pulmonary capillary recruitment, and metabolic capabilities of the microvasculature appear to differ between adult and immature animals (1-8). Recently, the fractal spatial distribution and isogravitational heterogeneity of pulmonary blood flow (PBF) have been delineated in adult animals using injectable microspheres that achieved greater resolution than earlier studies (9,10). The results of these studies diverge from the findings of homogeneous gravitational zones described by West et al. (11). Temporal heterogeneity also occurs in the pulmonary circulation, which may subserve the conservation of energetics rather than ventilation-perfusion relationships (12).Hitherto, these physiologic characteristics remain unstudied in newborn or very young animals. Speculation suggests that the fractal nature of PBF is based on the pulmonary anatomy, particularly of the precapillary bed (13). If true, the spatial distribution of PBF in young animals should be similar to that in adults. Furthermore, the presence of temporal heterogeneity of PBF in young animals has not been reported. Previous studies suggest a higher degree of pulmonary vascular reactivity in neonatal animals (2,3,6) compared with adults, and hence temporal heterogeneity should be greater than that previously described in adult animals.We measured spatial and temporal heterogeneity in piglets because morphometric studies of pulmonary development in the piglet suggest that the vascular and alveolar changes during the first 3 months of life are similar to those seen in humans (14,15). We hypothesized that under normoxic conditions the pulmonary blood flow would demonstrate greater spatial heteroge...

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