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.

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