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

Abstract

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.

Spatial and Temporal Heterogeneity of Regional Pulmonary Blood Flow in Piglets

“…The hilar-peripheral gradient of PBF has also been found previously in a number of studies of adult animals including humans (9), dogs (19), sheep (20) and ponies (21) with contradictory evidence in dogs (22). A radial gradient is consistent with the fractal branching pattern of the pulmonary circulation in which vascular resistances is determined by the size of branching vessels and the length of circuit pathways.…”

Gravity is not the only determinant for the distribution of pulmonary blood flow

“…This large central-peripheral gradient was not confirmed by other investigators, who found either much smaller (Glenny et al 1991;Jarvis et al 1992) or no gradients (Newhouse et al 1968;Hedenstierna, White & Wagner 1979;Beck & Rehder 1986;Nicolaysen et al 1987). It has been suggested that the large gradient observed by Hakim and associates may be caused by an artefact due to the complicated data analysis (Nicolaysen et al 1987;Beck, OConnor & Schroeder 1990). Nevertheless, some central-peripheral gradient may exist, which emphasize the point that factors other than gravity are important in the blood flow distribution (Glenny et al 1991).…”

Microsphere maps of regional blood flow and regional ventilation

“…Beck and Rehder (8) also studied microsphere-injected TLC-dried lungs, but they systematically sampled and mapped small cores from transverse slices of the dried specimen. Nicolaysen et al (45) used circumferentially cut sections from partially inflated and frozen RMS-labeled lungs to refute the existence of a significant radial gradient of flow in the lung. After intravenous injection of a flow label, Glenny and Robertson (25) employed in situ supine tissue fixation and acquired gamma camera images of transverse slices of the fixed lung.…”

“…As additional high-resolution studies accumulated over time (8,45), it became clear that some factor unrelated to the gravitational gradient was responsible for a major component of the spatial heterogeneity of regional blood flow. Inspired by a demonstration of scale-dependent heterogeneity of cardiac blood flow (5, 37), a high-resolution study of lung perfusion distribution confirmed that the spatial heterogeneity of regional blood flow increased in a similar predictable fashion as resolution was enhanced (25).…”