title: Probing the impact of axial diffusion on nitric oxide exchange dynamics with heliox.
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 (Ca(NO)) 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 Ca(NO) (range 1-5 ppb); however, the trumpet model predicts a positive slope of 0.4-2.1 ppb despite a zero Ca(NO) 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.
authors.affiliation: Department of Biomedical Engineering, University of California, Irvine, California 92697-2575, USA.
authors.forename: Hye-Won
authors.initials: HW
authors.lastname: Shin
authors.forename: Peter
authors.initials: P
authors.lastname: Condorelli
authors.forename: Christine M
authors.initials: CM
authors.lastname: Rose-Gottron
authors.forename: Dan M
authors.initials: DM
authors.lastname: Cooper
authors.forename: Steven C
authors.initials: SC
authors.lastname: George
mesh_terms: D000328:Adult; D003198:Computer Simulation; D004058:Diffusion; D005260:Female; D006371:Helium / Q000008:administration & dosage* / Q000378:metabolism*; D006801:Humans; D008168:Lung / Q000502:physiology*; D008297:Male; D008954:Models, Biological; D009569:Nitric Oxide / Q000378:metabolism*; D010100:Oxygen / Q000008:administration & dosage* / Q000378:metabolism*; D011650:Pulmonary Alveoli / Q000502:physiology; D011659:Pulmonary Gas Exchange / Q000502:physiology*; D012119:Respiration; D012129:Respiratory Function Tests / Q000379:methods*
chemical_list: D006371:Helium; D009569:Nitric Oxide; C038949:heliox; D010100:Oxygen