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1. With the advent of single-photon emission computerized tomography, controversy has arisen with regard to the significance of gravitational influences on regional pulmonary perfusion (Q̊r) in the supine versus prone postures. We investigated the dorsal—ventral distributions of Q̊r in prone (n = 5) and supine (n = 5) normal subjects, as assessed by single-photon emission computerized tomography after intravenous injection of technetium-99m-labelled macroaggregated albumin at end-tidal expiration. Reconstructed serial (one pixel thickness) coronal sections were traced on a computer screen, to yield the encompassed radioactive counts and number of pixels per each image. Coronal section data (expressed as mean radioactive counts/pixel) were expressed in a ‘profile’ and normalized to the maximum coronal section of each lung (%maximum). 2. Coefficients of variation and linear regression slopes for the prone versus supine profiles for left and right lungs were not statistically different (unpaired Student's t-test). The coronal section with maximum Q̊r was identified in the more dependent lung regions and, hence, affected by gravity. 3. We conclude that, in contrast to previous canine models, which have suggested postural differences in dorsal-ventral perfusion gradients, in normal man gravity primarily determines the non-dependent to dependent distribution of Q̊r. We speculate that interspecies differences in physiology may be teleological and related to the different perfusion demands of the quadruped compared with upright man.

1. Single-photon emission computerized tomography in both an intact canine model and man has demonstrated an aspect of pulmonary perfusion to be independent of gravitational forces. 2. Using technetium 99 m-labelled macroaggregated albumin single-photon emission computerized tomographic imaging, we investigated normal human subjects (n = 5), stable unilateral lung transplant recipients (n = 6) and transplant recipients with chronic allograft dysfunction related to obliterative bronchiolitis (n = 5). 3. In coronal isogravitational sections, a 1 × 1 × N pixel strip (medial to lateral) was constructed through the ‘core’ pixel of maximal radioactive counts. The counts were measured for the ‘core’ pixel and at two mid-points (medial and lateral) between the core pixel and the lung edges. Coefficients of variation were computed for each isogravitational strip and compared between groups. Fractional whole-lung perfusion was determined for left versus right lungs of normal subjects and allograft versus native lungs of transplant recipients. 4. Using these indices, ‘isogravitational heterogeneity’ (i.e. increased ‘core’ versus peripheral perfusion) was observed in allografts and native diseased lungs after unilateral transplantation. Despite significantly increased fractional whole-lung perfusion directed to the allografts (84.8% ± 3.0% and 75.8% + 12.1% for stable unilateral lung transplant recipients and patients with obliterative bronchiolitis respectively) compared with normal lungs (50.2% ± 1.2% and 49.8% ± 1.2% for left and right respectively), ‘isogravitational flow heterogeneity’ (i.e. increased ‘core’ versus peripheral perfusion) was preserved after transplantation. 5. These findings suggest that ‘isogravitational heterogeneity’ was maintained despite increased unilateral pulmonary perfusion and the presumed increase in pulmonary capillary recruitment and/or distension.