Prone positioning improves distribution of pulmonary perfusion: noninvasive magnetic resonance imaging study in healthy humans

The purpose of this study was to evaluate the effects of prone positioning on pulmonary perfusion using flow-sensitive alternating inversion recovery (FAIR), a noninvasive magnetic resonance imaging technique that requires no contrast medium. Seven healthy volunteers were studied in the supine and prone positions under three respiratory conditions: normal breathing of room air, unassisted breathing of 45% O₂, and controlled mechanical ventilation (CMV) with positive end-expiratory pressure. Signal intensities (SIs) were obtained from ventral, middle, and dorsal regions on sagittal lung images and dependent/nondependent SI ratios were calculated to evaluate pulmonary perfusion distribution. In the supine position, SIs increased significantly from the ventral to dorsal region under all three respiratory conditions and prone positioning inverted the perfusion distribution under all conditions. Right lung SI ratios were 2.34 ± 0.29, 2.74 ± 0.66, and 2.42 ± 0.73 in the supine position and 1.68 ± 0.48, 1.78 ± 0.36, and 1.92 ± 0.21 in prone for room air, 45% O₂, and CMV, respectively. The difference between supine and prone positions was statistically significant. The left lung showed a similar pattern and the difference was significant only under CMV. No difference was observed between the different respiratory conditions in both lungs. This study demonstrated that the distribution of pulmonary perfusion was more uniform in prone than in the supine position.