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

Abstract

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 (Va/Q) ratio. Using a novel functional lung magnetic resonance imaging technique to measure regional Va/Q ratio, the gravitational gradients in proton density, ventilation, perfusion, and Va/Q 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 Va/Q 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 Va/Q 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 Va/Q ratio were greater supine than prone, suggesting an interplay between thoracic cavity configuration, airway and vascular tree anatomy, and the effects of gravity on Va/Q matching.

Keywords: arterial spin labeling; gravity; magnetic resonance imaging; specific ventilation imaging; ventilation-perfusion ratio.

Fig. 1.

Example images of density (A), alveolar ventilation (B), perfusion (C), and ventilation-perfusion (V̇a/Q̇) ratio (D) in a sagittal slice of the right lung in a normal subject in the supine posture. Images are also shown for the prone posture (E–H, respectively). Voxels with larger conduit blood vessels are removed for the calculation of regional perfusion and V̇a/Q̇ ratio, since they do not represent perfusion and incorrectly map as regions of shunt.

Fig. 2.

The effect of gravity on lung density, ventilation, perfusion, and V̇a/Q̇ ratio (means and SDs) in prone (n = 6) and supine posture (n = 7). Voxel-by-voxel data are binned for each centimeter of vertical distance from the most dependent portion of the lung for a sagittal lung slice in each posture. Note that the gravitational distributions of ventilation, perfusion, and V̇a/Q̇ ratio are more uniform in prone posture compared with supine.

Fig. 3.

Local transpulmonary pressure calculated using the analysis of Glazier et al. (14) for all subjects in supine and prone postures with linear regression model fits to the aggregate data in each posture. Since this analysis considers each 1-cm plane as supporting all of the weight of the lung below the plane, unlike the other plots, data are plotted in 1-cm horizontal bins using the most gravitationally nondependent portion of the lung (posterior lung in prone posture, anterior lung in supine) as a reference point. The mean calculated gradient in transpulmonary pressure is significantly less in prone posture compared with supine (P < 0.005).