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1 Department of Medicine, University of California, San Diego, La Jolla, California, United States
2 Department of Radiology, University of California, San Diego, La Jolla, California, United States
3 Department of Medicine, University of California, San Diego, La Jolla, California, United States; Department of Radiology, University of California, San Diego, La Jolla, California, United States
* To whom correspondence should be addressed. E-mail: achenderson{at}ucsd.edu.
Head-down tilt has been shown to increase lung water content in animals and alter the distribution of ventilation in humans; however its effects on the distribution of pulmonary blood flow in humans are unknown. We hypothesized that head-down tilt would increase the heterogeneity of pulmonary blood flow in humans, an effect analogous to the changes seen in the distribution of ventilation, by increasing capillary hydrostatic pressure and fluid efflux in the lung. To test this, we evaluated changes in the distribution of pulmonary blood flow in 7 normal subjects before and after 1 hour of 30-degree head-down tilt using the magnetic resonance imaging technique of arterial spin labeling. Data were acquired in triplicate before tilt and at 10 minute intervals for one hour following tilt. Pulmonary blood flow heterogeneity was quantified by the relative disperson (standard deviation / mean) of signal intensity for all voxels within the right lung. Relative dispersion was significantly increased by 29% following tilt and remained elevated during the one hour of measurements following tilt (0.84±0.06 pre-tilt, 1.09±0.09 calculated for all time points post-tilt, p<0.05). We speculate that the mechanism most likely responsible for our findings is that increased pulmonary capillary pressures and fluid efflux in the lung resulting from head-down tilt alters regional blood flow distribution.
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