Relationship between airway narrowing, patchy ventilation and lung mechanics in asthmatics

¹ Anesthesia and Critical Care, Massachusetts General Hospital, Boston, MA; and Dept of Biomedical Engineering, Boston University, Boston, MA
² Anesthesia and Critical Care, Massachusetts General Hospital, Boston, MA
³ Dept of Medicine (Pulmonary and Critical Care Unit and Clinical Epidemiology Unit), Massachusetts General Hospital and Harvard Medical School
⁴ Dept of Biomedical Engineering, Boston University, Boston, MA

^* To whom correspondence should be addressed. E-mail: noratgav{at}yahoo.com.

	Abstract

Bronchoconstriction in asthma results in patchy ventilation forming ventilation defects (VDef's). Patchy ventilation is clinically important because it affects obstructive symptoms, impairs gas exchange and the distribution of inhaled medications. This report combined functional imaging, oscillatory mechanics and theoretical modelling to test whether the degrees of constriction of airways feeding units outside VDef's were related to the extent of VDef's in bronchoconstricted asthmatic subjects.

Positron emission tomography was used to quantify the regional distribution of ventilation and oscillatory mechanics were measured in asthmatic subjects before and after bronchoconstriction. For each subject, the ventilation data was mapped into an anatomically based lung model that was used to evaluate whether airway constriction patterns, consistent with the imaging data, were capable of matching the measured changes in airflow obstruction.

The degree and heterogeneity of constriction of airways feeding alveolar units outside VDef's was similar amongst the subjects studied despite large inter-subject variability in airflow obstruction and extent of the ventilation defects. Analysis of the data amongst the subjects showed an inverse relationship between the reduction in mean airway conductance, measured in the breathing frequency range during bronchoconstriction, and the fraction of lung involved in ventilation defects.

The data supports the concept that patchy ventilation is an expression of the integrated system and not just the sum of independent responses of individual airways.

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