Distributions of V/Q heterogeneity during bronchoconstriction measured with PET: contribution of sub-resolution heterogeneity to gas exchange impairment

Positron emission tomography (PET) imaging of intravenously injected ¹³N-labeled saline was used to quantify V/Q heterogeneity and estimate arterial blood gases in methacholine bronchoconstricted sheep. Imaging data were analyzed assuming a uniform V/Q distribution within the resolution elements, and assuming a double compartment model of sub-resolution heterogeneity. Results indicate that V/Q heterogeneity is present within volumes smaller than a spatial resolution of 2.2 cm³ and that such heterogeneity is responsible for approximately 30% of the total alveolar-arterial O₂ and CO₂ gradients. Tracer kinetics analysis of PET images provides a quantitative assessment of global V/Q heterogeneity including that corresponding to length scales smaller than the spatial resolution of the imaging method.