No gravity-independent gradient of blood flow distribution in dog lung

The existence of a major gravity-independent gradient of blood flow in lungs has recently been described based on single photon emission computed tomography after intravenous injection of radioactively labeled macroaggregates. We wanted to test this hypothesis of a major gravity-independent gradient in lung blood flow in experiments with direct measurement of macroaggregate distribution in the dog lung. In six anesthetized (4 prone spontaneously breathing, 2 mechanically ventilated) dogs we injected 111In-labeled albumin macroaggregates intravenously. We killed the dogs, removed, inflated, and froze the lower lobes. We sliced the lobes 1 cm thick and made gamma camera images of the slices. We then cut three or four slices in each lobe into two or three concentric layers and measured the radioactivity per gram of tissue in a well-type gamma counter. In three of the dogs we also labeled the red cells (99mTc) so that blood volume in each sample could be determined. The gamma camera images were acquired on a 64 X 64 matrix with 4 X 4 mm pixels. On the numeric printouts from the individual slices we made two or three concentric layers and calculated activity per pixel in each layer. Neither by the well counting nor by the pixel analysis of the gamma scans did we detect any gravity-independent distribution of blood flow. With the well counting the distribution was the same whether macroaggregate activity was expressed per gram of tissue or per gram of blood-free tissue. We conclude that by direct measurements no major gravity-independent gradient of pulmonary blood flow can be detected in dog lungs.