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INNOVATIVE METHODOLOGY

A new and more accurate technique to characterize airway nitric oxide using different breath-hold times

Departments of ¹Biomedical Engineering and ²Chemical Engineering and Materials Science, University of California, Irvine, California

Submitted 13 September 2004 ; accepted in final form 20 December 2004

Exhaled nitric oxide (NO) arises from both airway and alveolar regions of the lungs, which provides an opportunity to characterize region-specific inflammation. Current methodologies rely on vital capacity breathing maneuvers and controlled exhalation flow rates, which can be difficult to perform, especially for young children and individuals with compromised lung function. In addition, recent theoretical and experimental studies demonstrate that gas-phase axial diffusion of NO has a significant impact on the exhaled NO signal. We have developed a new technique to characterize airway NO, which requires a series of progressively increasing breath-hold times followed by exhalation of only the airway compartment. Using our new technique, we determined values (means ± SE) in healthy adults (20–38 yr, n = 8) for the airway diffusing capacity [4.5 ± 1.6 pl·s^–1·parts per billion (ppb)^–1], the airway wall concentration (1,340 ± 213 ppb), and the maximum airway wall flux (4,350 ± 811 pl/s). The new technique is simple to perform, and application of this data to simpler models with cylindrical airways and no axial diffusion yields parameters consistent with previous methods. Inclusion of axial diffusion as well as an anatomically correct trumpet-shaped airway geometry results in significant loss of NO from the airways to the alveolar region, profoundly impacting airway NO characterization. In particular, the airway wall concentration is more than an order of magnitude larger than previous estimates in healthy adults and may approach concentrations (5 nM) that can influence physiological processes such as smooth muscle tone in disease states such as asthma.

gas exchange; axial diffusion; model

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