Topographical distribution of pulmonary perfusion and ventilation, assessed by PET in supine and prone humans

Using positron emission tomography (PET) and intravenously injected ¹³N₂, we assessed the topographical distribution of pulmonary perfusion (Q̇) and ventilation (V̇) in six healthy, spontaneously breathing subjects in the supine and prone position. In this technique, the intrapulmonary distribution of ¹³N₂, measured during a short apnea, is proportional to regional Q̇. After resumption of breathing, regional specific alveolar V̇ (sV̇A, ventilation per unit of alveolar gas volume) can be calculated from the tracer washout rate. The PET scanner imaged 15 contiguous, 6-mm-thick, slices of lung. Vertical gradients of Q̇ and sV̇A were computed by linear regression, and spatial heterogeneity was assessed from the squared coefficient of variation (CV²). Both CV_Q̇² and CV_sV̇A² were corrected for the estimated contribution of random imaging noise. We found that 1) both Q̇ and V̇ had vertical gradients favoring dependent lung regions, 2) vertical gradients were similar in the supine and prone position and explained, on average, 24% of Q̇ heterogeneity and 8% of V̇ heterogeneity, 3) CV_Q̇² was similar in the supine and prone position, and 4) CV_sV̇A² was lower in the SVA prone position. We conclude that, in recumbent, spontaneously breathing humans, 1) vertical gradients favoring dependent lung regions explain a significant fraction of heterogeneity, especially of Q̇, and 2) although Q̇ does not seem to be systematically more homogeneous in the prone position, differences in individual behaviors may make the prone position advantageous, in terms of V̇-to-Q̇ matching, in selected subjects.