The effect of supine exercise on the distribution of regional pulmonary blood flow measured using proton MRI

Abstract

The Zone model of pulmonary perfusion predicts that exercise reduces perfusion heterogeneity because increased vascular pressure redistributes flow to gravitationally nondependent lung, and causes dilation and recruitment of blood vessels. However, during exercise in animals, perfusion heterogeneity as measured by the relative dispersion (RD, SD/mean) is not significantly decreased. We evaluated the effect of exercise on pulmonary perfusion in six healthy supine humans using magnetic resonance imaging (MRI). Data were acquired at rest, while exercising (∼27% of maximal oxygen consumption) using a MRI-compatible ergometer, and in recovery. Images were acquired in most of the right lung in the sagittal plane at functional residual capacity, using a 1.5-T MR scanner equipped with a torso coil. Perfusion was measured using arterial spin labeling (ASL-FAIRER) and regional proton density using a fast multiecho gradient-echo sequence. Perfusion images were corrected for coil-based signal heterogeneity, large conduit vessels removed and quantified (in ml·min−1·ml−1) (perfusion), and also normalized for density and quantified (in ml·min−1·g−1) (density-normalized perfusion, DNP) accounting for tissue redistribution. DNP increased during exercise (11.1 ± 3.5 rest, 18.8 ± 2.3 exercise, 13.2 ± 2.2 recovery, ml·min−1·g−1, P < 0.0001), and the increase was largest in nondependent lung (110 ± 61% increase in nondependent, 63 ± 35% in mid, 70 ± 33% in dependent, P < 0.005). The RD of perfusion decreased with exercise (0.93 ± 0.21 rest, 0.73 ± 0.13 exercise, 0.94 ± 0.18 recovery, P < 0.005). The RD of DNP showed a similar trend (0.82 ± 0.14 rest, 0.75 ± 0.09 exercise, 0.81 ± 0.10 recovery, P = 0.13). In conclusion, in contrast to animal studies, in supine humans, mild exercise decreased perfusion heterogeneity, consistent with Zone model predictions.