Self-Organized Patchiness in Asthma as a Prelude to Catastrophic Shifts

Purpose of the Study. To reveal self-organized small airway constriction contributing to large ventilation defects in asthmatics. Study Population. Mild and moderate asthmatics. Methods. Ventilation defects in asthmatics were studied during methacholine bronchoprovocation by using serial dynamic positron emission tomography with a positron-emitter nitrogen-13 tracer and a single terminal-airway model. Results. Heterogeneity of ventilation defects in asthmatics was demonstrated. In this model, constriction of terminal bronchioles was the main feature of bronchoconstriction, contributing to nonuniform ventilation defects. Consequently, on the basis of the mechanical interdependence in expansion between airways and surrounding parenchyma, clusters of constricted terminal bronchioles fed by a common tree branch developed and led to large ventilation defects. Conclusion. Clustered groups of self-organized terminal bronchiolar constriction, not large airways obstruction, contribute to large ventilation defects in acute asthma. Reviewer Comments. The nature of functional changes of both small and large airways affecting ventilation during acute asthma attacks has been unclear. Previously, MRIs of asthmatic lungs during bronchoprovocation suggested large-airway obstruction as a major cause of large ventilation defects (eg, J Allergy Clin Immunol. 2003;111:1205–1211). In this study, Venegas et al demonstrated the role of clustered terminal bronchiolar constriction resulting in ventilation defects in acute asthma. On the basis of this model, inhaled bronchodilators could be ineffective because the inhaled form might reach only well-ventilated regions and could further impede lung expansion of problematic regions and exacerbate regional ventilation defects. The concept of catastrophic shifts might account for sudden, unexplained, and severe asthma attacks in some patients. Systemic bronchodilators may be needed in some asthmatics to bypass this problem. This line of investigation is further elucidated elsewhere (J Appl Physiol. 2005;99:2388–2397).