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1 Medicine, University of California, San Diego, La Jolla, California, United States
2 Medicine, University of California, San Diego, La Jolla, California, United States; School of Medicine, UCSD, La Jolla, California, United States
3 Department of Medicine, University of California, San Diego, La Jolla, California, United States
4 Medicine, University of California, San Diego, La Jolla, California, United States; UCSD, United States
5 Department of Radiology, Mail Code 8756, UC-San Diego Medical Center, San Diego, California, United States
6 UCSD Medical Center, Radiology, 8756, San Diego, California, United States
* To whom correspondence should be addressed. E-mail: kprisk{at}ucsd.edu.
Prone posture increases cardiac output and improves pulmonary gas exchange. We hypothesized that in the supine posture, greater compression of dependent lung limits regional blood flow. To test this, MRI-based measures of regional lung density, MRI arterial spin labeling quantification of pulmonary perfusion, and density normalized perfusion were made in 6 healthy subjects. Measurements were made in both the prone and supine posture at FRC. Data were acquired in 3 non-overlapping 15mm sagittal slices covering most of the right lung: central, middle and lateral, which were further divided into vertical zones: anterior, intermediate and posterior. The density of the entire lung was not different between prone and supine, but the increase in lung density in the anterior lung with prone posture was less than the decrease in the posterior lung (change: +0.07 g/cm3 anterior, -0.11 posterior; P<0.0001) indicating greater compression of dependent lung in supine posture, principally in the central lung slice (P<0.0001). Overall density normalized perfusion was significantly greater in prone posture (7.9±3.6 ml/min/g prone, 5.1±1.8 supine, a 55% increase, P<0.05) and showed the largest increase in the posterior lung as it became non-dependent (change: +71% posterior, +58% intermediate, +31% anterior, P=0.08), most marked in the central lung slice (P<0.05). These data indicate that central posterior portions of the lung are more compressed in the supine posture, likely by the heart and adjacent structures, than are central anterior portions in the prone, and that this limits regional perfusion in the supine posture.
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