Quantifying proximal and distal sources of NO in asthma using a multicompartment model

Accurate real-time FENO expirograms using complementary optical sensors

The fraction of exhaled nitric oxide (F_ENO) is an important biomarker for the diagnosis and management of asthma and other pulmonary diseases associated with airway inflammation. In this study we report on a novel method for accurate, highly time-resolved, real time detection of F_ENO at the mouth. The experimental arrangement is based on a combination of optical sensors for the determination of the temporal profile of exhaled NO and CO₂ concentrations. Breath CO₂ and exhalation flow are measured at the mouth using diode laser absorption spectroscopy (at 2 μm) and differential pressure sensing, respectively. NO is determined in a sidestream configuration using a quantum cascade laser based, cavity-enhanced absorption cell (at 5.2 μm) which simultaneously measures sidestream CO₂. The at-mouth and sidestream CO₂ measurements are used to enable the deconvolution of the sidestream NO measurement back to the at-mouth location. All measurements have a time resolution of 0.1 s, limited by the requirement of a reasonable limit of detection for the NO measurement, which on this timescale is 4.7 ppb (2 σ). Using this methodology, NO expirograms (F_ENOgrams) were measured and compared for eight healthy volunteers. The F_ENOgrams appear to differ qualitatively between individuals and the hope is that the dynamic information encoded in these F_ENOgrams will provide valuable additional insight into the location of the inflammation in the airways and potentially predict a response to therapy. A validation of the measurements at low-time resolution is provided by checking that results from previous studies that used a two-compartment model of NO production can be reproduced using our technology.

Bayesian estimation of physiological parameters governing a dynamic two-compartment model of exhaled nitric oxide

The fractional concentration of nitric oxide in exhaled breath (fe_NO) is a biomarker of airway inflammation with applications in clinical asthma management and environmental epidemiology. fe_NO concentration depends on the expiratory flow rate. Standard fe_NO is assessed at 50 mL/sec, but “extended NO analysis” uses fe_NO measured at multiple different flow rates to estimate parameters quantifying proximal and distal sources of NO in the lower respiratory tract. Most approaches to modeling multiple flow fe_NO assume the concentration of NO throughout the airway has achieved a “steady‐state.” In practice, this assumption demands that subjects maintain sustained flow rate exhalations, during which both fe_NO and expiratory flow rate must remain constant, and the fe_NO maneuver is summarized by the average fe_NO concentration and average flow during a small interval. In this work, we drop the steady‐state assumption in the classic two‐compartment model. Instead, we have developed a new parameter estimation approach based on measuring and adjusting for a continuously varying flow rate over the entire fe_NO maneuver. We have developed a Bayesian inference framework for the parameters of the partial differential equation underlying this model. Based on multiple flow fe_NO data from the Southern California Children's Health Study, we use observed and simulated NO concentrations to demonstrate that our approach has reasonable computation time and is consistent with existing steady‐state approaches, while our inferences consistently offer greater precision than current methods.

A method for mapping regional oxygen and CO2 transfer in the lung

This paper presents a novel approach to visualizing regional lung function, through quantitative three-dimensional maps of O2 and CO2 transfer rates. These maps describe the contribution of anatomical regions to overall gas exchange and demonstrate how transfer rates of the two gas species' differ regionally. An algorithm for generating such maps is presented, and for illustration, regional gas transfer maps were generated using values of ventilation and perfusion imaged by PET/CT for a healthy subject and an asthmatic patient after bronchoprovocation. In a sensitivity analysis, compartment values of gas transfer showed minor sensitivity to imaging noise in the ventilation and perfusion data, and moderate sensitivity to estimation errors in global lung input values, chiefly global alveolar ventilation, followed by cardiac output and arterial-venous O2 content difference. Gas transfer maps offer an intuitive display of physiologically relevant lung function at a regional level, the potential for an improved understanding of pulmonary gas exchange in health and disease, and potentially a presurgical evaluation tool.

Axial distribution of nitric oxide airway production in asthma patients

In healthy subjects, axial distribution of nitric oxide (NO) airway production is likely heterogeneous: notably a distal peak of production in terminal bronchioles and a quasi-nil NO production in the most of the conducting airways. In asthma, few information exists about the contributions of the proximal and distal airways to NO overproduction. In 18 asthma patients, sites of constriction after methacholine and adenosine 5'-monophosphate (AMP) challenges were assessed by ventilation distribution tests with He and SF(6). The resulting decreases in fractional exhaled NO (FENO) were measured. Changes in He and SF(6) slopes indicated a pre-acinar bronchoconstriction due to AMP and a more proximal action for methacholine. FENO decreased by 38.7% and 20.2% (p<0.001) after AMP and methacholine challenges, respectively. Significant FENO decreases after AMP and methacholine implies substantial pre-acinar but also, contrary to healthy subjects, more proximal airway production. In conclusion, nitric oxide overproduction in asthma patients appears to involve the most part of the conducting airways.

Nitric oxide in adaptation to altitude

This review summarizes published information on the levels of nitric oxide gas (NO) in the lungs and NO-derived liquid-phase molecules in the acclimatization of visitors newly arrived at altitudes of 2500 m or more and adaptation of populations whose ancestors arrived thousands of years ago. Studies of acutely exposed visitors to high altitude focus on the first 24-48 h with just a few extending to days or weeks. Among healthy visitors, NO levels in the lung, plasma, and/or red blood cells fell within 2h, but then returned toward baseline or slightly higher by 48 h and increased above baseline by 5 days. Among visitors ill with high-altitude pulmonary edema at the time of the study or in the past, NO levels were lower than those of their healthy counterparts. As for highland populations, Tibetans had NO levels in the lung, plasma, and red blood cells that were at least double and in some cases orders of magnitude greater than other populations regardless of altitude. Red blood cell-associated nitrogen oxides were more than 200 times higher. Other highland populations had generally higher levels although not to the degree shown by Tibetans. Overall, responses of those acclimatized and those presumed to be adapted are in the same direction, although the Tibetans have much larger responses. Missing are long-term data on lowlanders at altitude showing how similar they become to the Tibetan phenotype. Also missing are data on Tibetans at low altitude to see the extent to which their phenotype is a response to the immediate environment or expressed constitutively. The mechanisms causing the visitors' and the Tibetans' high levels of NO and NO-derived molecules at altitude remain unknown. Limited data suggest processes including hypoxic upregulation of NO synthase gene expression, hemoglobin-NO reactions, and genetic variation. Gains in understanding will require integrating appropriate methods and measurement techniques with indicators of adaptive function under hypoxic stress.

Clinical study of multiple breath biomarkers of asthma and COPD (NO, CO(2), CO and N(2)O) by infrared laser spectroscopy

Breath analysis is a powerful non-invasive technique for the diagnosis and monitoring of respiratory diseases, including asthma and chronic obstructive pulmonary disease (COPD). Exhaled nitric oxide (NO) and carbon monoxide (CO) are markers of airway inflammation and can indicate the extent of respiratory diseases. We have developed a compact fast response quantum cascade laser system for analysis of multiple gases by tunable infrared absorption spectroscopy. The ARI breath analysis instrument has been deployed in a study of exhaled breath from patients with asthma or COPD. A total of 173 subjects participated, including both adult and pediatric patients. Patients in asthma or COPD exacerbations were evaluated twice-during the exacerbation and at a follow-up visit-to compare variations in breath biomarkers during these events. The change in exhaled NO levels between exacerbation and 'well' visits is consistent with spirometry data collected. Respiratory models are important for understanding the exchange dynamics of nitric oxide and other species in the lungs and airways. At each patient's visit, tests were conducted at four expiratory flow rates. We have applied a trumpet model with axial diffusion to the multi-flow exhaled nitric oxide data, obtaining NO alveolar concentrations and airway fluxes. We found higher airway fluxes for those with more severe asthma and during exacerbation events. The alveolar concentrations from the model were higher in adults with asthma and COPD, but this trend was less clear among the pediatric subjects.

Small airway function, exhaled NO and airway hyper-responsiveness in paediatric asthma

BACKGROUND

Asthma is a chronic inflammatory airway disorder known to involve the peripheral airways. Current guidelines state that diagnosis and treatment should be guided by symptoms and spirometry. FEV(1) is, however, a poor marker of small airway function and correlates poorly with asthma control and airway inflammation.

AIMS

To assess the contribution of small airway dysfunction and inflammation in paediatric asthma. A secondary aim was to study the associations between small airway dysfunction, airway inflammation and airway hyper-responsiveness (AHR).

METHODS

Small airway function was measured as LCI, S(cond) and S(acin), evaluated with the SF(6) multiple breath inert gas washout (MBW) technique, in 47 asthmatic children and 74 healthy controls. Exhaled NO at multiple exhalation flow rates including 50 mL/s (FENO(50)) was assessed to calculate alveolar NO and bronchial NO flux (a surrogate for airway inflammation). AHR was evaluated with isocapnoic dry air hyperventilation challenge in the asthmatic children.

RESULTS

S(cond) was elevated in 31 (66%) and S(acin) in 18 (38%) of the asthmatic subjects. LCI was increased and FEV(1) decreased in 7 (15%) of the subjects with asthma. Individuals with AHR had higher S(cond) (p = 0.001) and FENO(50) (p < 0.0001) than those without AHR. The levels of FENO(50) and bronchial NO flux were elevated in asthmatic subjects compared with healthy controls. In asthma, S(cond) correlated with FENO(50) (r(s) = 0.42, p = 0.003) and alveolar NO (r(s) = 0.40, p = 0.011), and S(acin) correlated with alveolar NO (r(s) = 0.40, p = 0.015).

CONCLUSION

Dysfunction of small conducting airways is associated with airway inflammation and hyper-responsiveness in paediatric asthma.