There is a gravitational influence on pulmonary perfusion, including in the most dependent lung, where perfusion is reduced, termed Zone 4. Studies using xenon-133 show Zone 4 behaviour, present in the dependent 4 cm at total lung capacity (TLC), affects the dependent 11 cm at functional residual capacity (FRC) and almost all the lung at residual volume (RV). These differences were ascribed to increased resistance in extra-alveolar vessels at low lung volumes although other mechanisms have been proposed. To further evaluate the behaviour of perfusion in dependent lung using a technique that directly measures pulmonary perfusion and corrects for tissue distribution by measuring regional proton density, seven healthy subjects (age = 38 ± 6 years, FEV1= 104 ± 7% predicted) underwent magnetic resonance imaging in supine posture. Data were acquired in the right lung during breath-holds at RV, FRC and TLC. Arterial spin labelling quantified regional pulmonary perfusion, which was normalized for regional proton density measured using a fast low-angle shot technique. The height of the onset of Zone 4 behaviour was not different between lung volumes (P= 0.23). There were no significant differences in perfusion (expressed as ml min-1 g-1) between lung volumes in the gravitationally intermediate (RV = 8.9 ± 3.1, FRC = 8.1 ± 2.9, TLC = 7.4 ± 3.6; P= 0.26) and dependent lung (RV = 6.6 ± 2.4, FRC = 6.1 ± 2.1, TLC = 6.4 ± 2.6; P= 0.51). However, at TLC perfusion was significantly lower in non-dependent lung than at FRC or RV (3.6 ± 3.3, 7.7 ± 1.5, 7.9 ± 2.0, respectively; P < 0.001). These data suggest that the mechanism of the reduction in perfusion in dependent lung is unlikely to be a result of lung volume related increases in resistance in extra-alveolar vessels. In supine posture, the gravitational influence on perfusion is remarkably similar over most of the lung, irrespective of lung volume.The lung is subject to multiple influences that affect its function. In particular the effect of gravity has attracted considerable interest, especially as it relates to the distribution of blood flow. In the most gravitationally dependent part of the lung, blood flow is reduced, and this region is known as Zone 4. Zone 4 has been suggested to result from increased resistance in some of the blood vessels, and this effect is thought to be the smallest when the lung is at a high volume as the elastic structures of the lung hold the vessels open minimizing resistance. Using novel magnetic resonance imaging techniques to measure blood flow, this study shows that the distribution of blood flow in the dependent lung is unchanged when the lung volume is changed and suggests that the intrinsic structure of the vessels is a more likely explanation for this behaviour.
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