The gravitational distribution of ventilation-perfusion ratio is more uniform in prone than supine posture in the normal human lung

The gravitational gradient of intrapleural pressure is suggested to be less in prone posture than supine. Thus the gravitational distribution of ventilation is expected to be more uniform prone, potentially affecting regional ventilation-perfusion ((V) over dot(A)/(Q) over dot) ratio. Using a novel functional lung magnetic resonance imaging technique to measure regional (V) over dot(A)/(Q) over dot ratio, the gravitational gradients in proton density, ventilation, perfusion, and (V) over dot(A)/(Q) over dot ratio were measured in prone and supine posture. Data were acquired in seven healthy subjects in a single sagittal slice of the right lung at functional residual capacity. Regional specific ventilation images quantified using specific ventilation imaging and proton density images obtained using a fast gradient-echo sequence were registered and smoothed to calculate regional alveolar ventilation. Perfusion was measured using arterial spin labeling. Ventilation (ml.min(-1).ml(-1)) images were combined on a voxel-by-voxel basis with smoothed perfusion (ml.min(-1).ml(-1)) images to obtain regional (V) over dot(A)/(Q) over dot ratio. Data were averaged for voxels within 1-cm gravitational planes, starting from the most gravitationally dependent lung. The slope of the relationship between alveolar ventilation and vertical height was less prone than supine (-0.17 +/- 0.10 ml.min(-1).ml(-1).cm(-1) supine, -0.040 +/- 0.03 prone ml.min(-1).ml(-1).cm(-1), P = 0.02) as was the slope of the perfusion-height relationship (-0.14 +/- 0.05 ml.min(-1).ml(-1).cm(-1) supine, -0.08 +/- 0.09 prone ml.min(-1).ml(-1).cm(-1), P = 0.02). There was a significant gravitational gradient in (V) over dot(A)/(Q) over dot ratio in both postures (P < 0.05) that was less in prone (0.09 +/- 0.08 cm(-1) supine, 0.04 +/- 0.03 cm(-1) prone, P = 0.04). The gravitational gradients in ventilation, perfusion, and regional (V) over dot(A)/(Q) over dot ratio were greater supine than prone, suggesting an interplay between thoracic cavity configuration, airway and vascular tree anatomy, and the effects of gravity on (V) over dot(A)/(Q) over dot matching.