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Regional Pulmonary Perfusion, Inflation, and Ventilation Defects in Bronchoconstricted Patients with Asthma

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

Correspondence and requests for reprints should be addressed to R. Scott Harris, M.D., Pulmonary and Critical Care Unit, Bulfinch 148, Massachusetts General Hospital, 55 Fruit Street, Boston, MA 02114. E-mail: rharris{at}partners.org

Rationale: Bronchoconstriction in asthma leads to heterogeneous ventilation and the formation of large and contiguous ventilation defects in the lungs. However, the regional adaptations of pulmonary perfusion () to such ventilation defects have not been well studied.

Methods: We used positron emission tomography to assess the intrapulmonary kinetics of intravenously infused tracer nitrogen-13 (¹³NN), and measured the regional distributions of ventilation and perfusion in 11 patients with mild asthma. For each subject, the regional washout kinetics of ¹³NN before and during methacholine-induced bronchoconstriction were analyzed. Two regions of interest (ROIs) were defined: one over a spatially contiguous area of high tracer retention (TR) during bronchoconstriction and a second one covering an area of similar size, showing minimal tracer retention (NR).

Results: Both ROIs demonstrated heterogeneous washout kinetics, which could be described by a two-compartment model with fast and slow washout rates. We found a systematic reduction in regional to the TR ROI during bronchoconstriction and a variable and nonsignificant change in relative for NR regions. The reduction in regional was associated with an increase in regional gas content of the TR ROI, but its magnitude was greater than that anticipated solely by the change in regional lung inflation.

Conclusion: During methacholine-induced bronchoconstriction, perfusion to ventilation defects are systematically reduced by a relative increase in regional pulmonary vascular resistance.

Key Words: emission computed tomography • pulmonary gas exchange • vascular resistance • vasoconstriction • ventilation–perfusion ratio

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