Probing the impact of axial diffusion on nitric oxide exchange dynamics with heliox

Hye Won Shin, Peter Condorelli, Christine M. Rose-Gottron, Dan M. Cooper, Steven George

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Abstract

Exhaled nitric oxide (NO) is a potential noninvasive index of lung inflammation and is thought to arise from the alveolar and airway regions of the lungs. A two-compartment model has been used to describe NO exchange; however, the model neglects axial diffusion of NO in the gas phase, and recent theoretical studies suggest that this may introduce significant error. We used heliox (80% helium, 20% oxygen) as the insufflating gas to probe the impact of axial diffusion (molecular diffusivity of NO is increased 2.3-fold relative to air) in healthy adults (21-38 yr old, n = 9). Heliox decreased the plateau concentration of exhaled NO by 45% (exhalation flow rate of 50 ml/s). In addition, the total mass of NO exhaled in phase I and II after a 20-s breath hold was reduced by 36%. A single-path trumpet model that considers axial diffusion predicts a 50% increase in the maximum airway flux of NO and a near-zero alveolar concentration (CANO) and source. Furthermore, when NO elimination is plotted vs. constant exhalation flow rate (range 50-500 ml/s), the slope has been previously interpreted as a nonzero CANO (range 1-5 ppb); however, the trumpet model predicts a positive slope of 0.4-2.1 ppb despite a zero CANO because of a diminishing impact of axial diffusion as flow rate increases. We conclude that axial diffusion leads to a significant backdiffusion of NO from the airways to the alveolar region that significantly impacts the partitioning of airway and alveolar contributions to exhaled NO.

Original languageEnglish (US)
Pages (from-to)874-882
Number of pages9
JournalJournal of Applied Physiology
Volume97
Issue number3
DOIs
StatePublished - Sep 1 2004

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Keywords

  • Exhaled breath
  • Gas exchange
  • Model

ASJC Scopus subject areas

  • Physiology
  • Physiology (medical)

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