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Published ahead of print on January 7, 2005, doi:10.1164/rccm.200409-1296OC
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American Journal of Respiratory and Critical Care Medicine Vol 171. pp. 714-721, (2005)
© 2005 American Thoracic Society
doi: 10.1164/rccm.200409-1296OC

Original Article

Topographic Basis of Bimodal Ventilation–Perfusion Distributions during Bronchoconstriction in Sheep

Marcos F. Vidal Melo, R. Scott Harris, J. Dominick H. Layfield and Jose G. Venegas

Department of Anesthesia and Critical Care and Department of Medicine (Pulmonary and Critical Care Unit), Massachusetts General Hospital; Harvard Medical School, Boston, Massachusetts

Correspondence and requests for reprints should be addressed to Marcos F. Vidal Melo, M.D., Ph.D., Cardiac Anesthesia Group, Department of Anesthesia and Critical Care, Massachusetts General Hospital, 55 Fruit Street, Boston, MA 02114. E-mail: mvidalmelo{at}partners.org

The distribution of ventilation–perfusion (A/) ratios during bronchoconstriction measured with the multiple inert gases elimination technique is frequently bimodal. However, the topographic basis and the cause of that bimodality remain unknown. In this article, regional A/ is quantified by three-dimensional positron emission tomography (PET) imaging of methacholine-induced bronchoconstriction in sheep. Regional A/ ratios were calculated from the imaged kinetics of intravenously injected 13NN-saline bolus, assembled into global A/ distributions, and used to estimate gas exchange. During bronchoconstriction, large regions with impaired tracer washout were observed adjacent to regions of normal ventilation. PET-derived A/ distributions during bronchoconstriction were consistently bimodal, with areas of low A/ receiving a large fraction of . The standard deviation of the A/ distribution was 38% lower if small-scale (subresolution) heterogeneity (< 2.2 cm3) was ignored. Arterial blood gases predicted from PET data correlated well with measured values for PaO2 (r2 = 0.91, p < 0.01) and PaCO2 (r2 = 0.90, p < 0.01). We conclude that the bimodality of A/ distributions in bronchoconstriction reflects the involvement of large contiguous regions of hypoventilation with substantial subresolution intraregional A/ heterogeneity. Assessment of the subresolution A/ heterogeneity is therefore essential to accurately quantify global gas exchange impairment during bronchoconstriction.

Key Words: lung imaging • positron emission tomography • pulmonary gas exchange • theoretical models




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