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

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.

Modeling pulmonary nitric oxide exchange

Nitric oxide (NO) was first detected in the exhaled breath more than a decade ago and has since been investigated as a noninvasive means of assessing lung inflammation. Exhaled NO arises from the airway and alveolar compartments, and new analytical methods have been developed to characterize these sources. A simple two-compartment model can adequately represent many of the observed experimental observations of exhaled concentration, including the marked dependence on exhalation flow rate. The model characterizes NO exchange by using three flow-independent exchange parameters. Two of the parameters describe the airway compartment (airway NO diffusing capacity and either the maximum airway wall NO flux or the airway wall NO concentration), and the third parameter describes the alveolar region (steady-state alveolar NO concentration). A potential advantage of the two-compartment model is the ability to partition exhaled NO into an airway and alveolar source and thus improve the specificity of detecting altered NO exchange dynamics that differentially impact these regions of the lungs. Several analytical techniques have been developed to estimate the flow-independent parameters in both health and disease. Future studies will focus on improving our fundamental understanding of NO exchange dynamics, the analytical techniques used to characterize NO exchange dynamics, as well as the physiological interpretation and the clinical relevance of the flow-independent parameters.

Characterizing airway and alveolar nitric oxide exchange during tidal breathing using a three-compartment model

Exhaled nitric oxide (NO) may be a useful marker of lung inflammation, but the concentration is highly dependent on exhalation flow rate due to a significant airway source. Current methods for partitioning pulmonary NO gas exchange into airway and alveolar regions utilize multiple exhalation flow rates or a single-breath maneuver with a preexpiratory breath hold, which is cumbersome for children and individuals with compromised lung function. Analysis of tidal breathing data has the potential to overcome these limitations, while still identifying region-specific parameters. In six healthy adults, we utilized a three-compartment model (two airway compartments and one alveolar compartment) to identify two potential flow-independent parameters that represent the average volumetric airway flux (pl/s) and the time-averaged alveolar concentration (parts/billion). Significant background noise and distortion of the signal from the sampling system were compensated for by using a Gaussian wavelet filter and a series of convolution integrals. Mean values for average volumetric airway flux and time-averaged alveolar concentration were 2,500 +/- 2,700 pl/s and 3.2 +/- 3.4 parts/billion, respectively, and were strongly correlated with analogous parameters determined from vital capacity breathing maneuvers. Analysis of multiple tidal breaths significantly reduced the standard error of the parameter estimates relative to the single-breath technique. Our initial assessment demonstrates the potential of utilizing tidal breathing for noninvasive characterization of pulmonary NO exchange dynamics.

Airway diffusing capacity of nitric oxide and steroid therapy in asthma

Exhaled nitric oxide (NO) concentration is a noninvasive index for monitoring lung inflammation in diseases such as asthma. The plateau concentration at constant flow is highly dependent on the exhalation flow rate and the use of corticosteroids and cannot distinguish airway and alveolar sources. In subjects with steroid-naive asthma (n = 8) or steroid-treated asthma (n = 12) and in healthy controls (n = 24), we measured flow-independent NO exchange parameters that partition exhaled NO into airway and alveolar regions and correlated these with symptoms and lung function. The mean (+/-SD) maximum airway flux (pl/s) and airway tissue concentration [parts/billion (ppb)] of NO were lower in steroid-treated asthmatic subjects compared with steroid-naive asthmatic subjects (1,195 +/- 836 pl/s and 143 +/- 66 ppb compared with 2,693 +/- 1,687 pl/s and 438 +/- 312 ppb, respectively). In contrast, the airway diffusing capacity for NO (pl.s-1.ppb-1) was elevated in both asthmatic groups compared with healthy controls, independent of steroid therapy (11.8 +/- 11.7, 8.71 +/- 5.74, and 3.13 +/- 1.57 pl.s-1.ppb-1 for steroid treated, steroid naive, and healthy controls, respectively). In addition, the airway diffusing capacity was inversely correlated with both forced expired volume in 1 s and forced vital capacity (%predicted), whereas the airway tissue concentration was positively correlated with forced vital capacity. Consistent with previously reported results from Silkoff et al. (Silkoff PE, Sylvester JT, Zamel N, and Permutt S, Am J Respir Crit Med 161: 1218-1228, 2000) that used an alternate technique, we conclude that the airway diffusing capacity for NO is elevated in asthma independent of steroid therapy and may reflect clinically relevant changes in airways.

Impact of high-intensity exercise on nitric oxide exchange in healthy adults

PURPOSE

After exercise, exhaled NO concentration has been reported to decrease, remain unchanged, or increase. A more mechanistic understanding of NO exchange dynamics after exercise is needed to understand the relationship between exercise and NO exchange.

METHODS

We measured several flow-independent NO exchange parameters characteristic of airway and alveolar regions using a single breath maneuver and a two-compartment model (maximum flux of NO from the airways, J'(awNO), pL x s-1; diffusing capacity of NO in the airways, D(awNO), pL x s-1 x ppb-1; steady state alveolar concentration, C(alv,ss), ppb; mean airway tissue NO concentration, C(awNO), ppb), as well as serum IL-6 at baseline, 3, 30, and 120 min after a high-intensity exercise challenge in 10 healthy adults (21-37 yr old).

RESULTS

D(awNO) (mean +/- SD) increased (37.1 +/- 44.4%), whereas J'(awNO) and C(awNO) decreased (-7.27 +/- 11.1%, -26.1 +/- 24.6%, respectively) 3 min postexercise. IL-6 increased steadily after exercise to 481% +/- 562% above baseline 120 min postexercise.

CONCLUSION

High-intensity exercise acutely enhances the ability of NO to diffuse between the airway tissue and the gas phase, and exhaled NO might be used to probe both the metabolic and physical properties of the airways.

Expression of matrix proteins in an in vitro model of airway remodeling in asthma

Acute asthma is characterized by a decrease in the pH of the exhaled breath condensate and bronchoconstriction. These perturbations may injure the epithelium in a chronic, intermittent pattern, leading to subepithelial fibrosis. We used an in vitro three-dimensional model of the bronchial mucosa to elucidate the response to a repeated chemical or physical insult to the epithelium in the postcontraction phase. We used enzyme-linked immunosorbent assay and reverse transcriptase--polymerase chain reaction to assess the production of the following proteins: matrix metalloproteinase (MMP) 3, MMP-9, tissue inhibitor of MMP-1, transforming growth factor beta 1, thrombospondin 1, tenascin, and fibronectin. The presence of the epithelium enhanced the degree of tissue contraction (50.1 +/- 4.4% of original area versus 75.4 +/- 2.3%). In the absence of injury, tenascin, fibronectin, MMP-3, and tissue inhibitor of MMP-1 are actively expressed. However, the chronic chemical wound markedly inhibited the expression of all proteins. We conclude that the epithelium, wound type, and age of the tissue (contracting versus postcontraction) impact the expression of key proteins in an in vitro model of subepithelial fibrosis in asthma.

Impact of axial diffusion on nitric oxide exchange in the lungs

Nitric oxide (NO) appears in the exhaled breath and is a potentially important clinical marker. The accepted model of NO gas exchange includes two compartments, representing the airway and alveolar region of the lungs, but neglects axial diffusion. We incorporated axial diffusion into a one-dimensional trumpet model of the lungs to assess the impact on NO exchange dynamics, particularly the impact on the estimation of flow-independent NO exchange parameters such as the airway diffusing capacity and the maximum flux of NO in the airways. Axial diffusion reduces exhaled NO concentrations because of diffusion of NO from the airways to the alveolar region of the lungs. The magnitude is inversely related to exhalation flow rate. To simulate experimental data from two different breathing maneuvers, NO airway diffusing capacity and maximum flux of NO in the airways needed to be increased approximately fourfold. These results depend strongly on the assumption of a significant production of NO in the small airways. We conclude that axial diffusion may decrease exhaled NO levels; however, more advanced knowledge of the longitudinal distribution of NO production and diffusion is needed to develop a complete understanding of the impact of axial diffusion.

Free nitric oxide diffusion in the bronchial microcirculation

Theoretical mass transfer rates and concentration distributions were determined for transient diffusion of free nitric oxide (NO) generated in vivo from vascular endothelial cells. Our analytical framework is typical of the bronchial circulation in the human pulmonary system but is applicable to the microvascular circulation in general. We characterized mass transfer rates in terms of the fractional mass flux across a boundary relative to the total endothelial NO production rate. NO concentration in the tissue surrounding blood vessels was expressed in terms of fractional soluble guanylate cyclase (sGC) activity. Our results suggest that endothelium-derived free NO is capable of vascular smooth muscle dilation despite its rapid consumption by hemoglobin in blood. An optimal blood vessel radius of 20 microm was estimated for NO signaling. We hypothesize intermittent generation of endothelial NO as a possible mechanism for sGC activation in vascular smooth muscle. This mechanism enhances the efficacy of NO-modulated vascular smooth muscle dilation while minimizing NO losses to blood and surrounding tissue.

Flow-independent nitric oxide exchange parameters in cystic fibrosis

Exhaled nitric oxide (NO) remains a promising noninvasive index for monitoring inflammatory lung diseases; however, the plateau concentration (C(NO,plat)) is nonspecific and requires a constant exhalation flow rate. We utilized a new technique that employs a variable flow rate to estimate key flow-independent parameters characteristic of NO exchange in a group (n = 9) of 10 to 14 yr-old healthy children and children with cystic fibrosis (CF): maximum flux of NO from the airways (J(NO,max'), pl s(-1)), diffusing capacity of NO in the airways (D(NO,air'), pl s(-1) ppb(-1)), steady-state alveolar concentration (C(alv,ss'), ppb), and mean tissue concentration of NO in the airways (C(tiss,air'), ppb). We determined the following mean (+/- SD) values in the healthy children and patients with CF for J(NO,max'), D(NO,air'), C(alv,ss'), and C(tiss,air'), respectively: 784 +/- 465 and 607 +/- 648 pl s(-1); 4.82 +/- 3.07 and 17.6 +/- 12.1 pl s(-1) ppb(-1); 4.63 +/- 3.59 and 1.96 +/- 1.18 ppb; and 198 +/- 131 and 38 +/- 25 ppb. D(NO,air) is elevated (p = 0.007), and both C(alv,ss) and C(tiss,air) are reduced (p = 0.05 and 0.002, respectively) in CF. In contrast, C(NO,plat) for healthy control subjects and patients with CF are not statistically different at both exhalation flow rates of 50 ml/s (17.5 +/- 11.5 and 11.5 +/- 8.97) and at 250 ml/s (7.11 +/- 5.36 and 4.28 +/- 3.43). We conclude that D(NO,air'), C(tiss,air'), and C(alv,ss) may be useful noninvasive markers of CF.

Mechanisms of synergistic cytokine-induced nitric oxide production in human alveolar epithelial cells

Nitric oxide (NO) derived from inducible NO synthase (iNOS) at sites of inflammation is closely related to host defense against infection and airway inflammation. Cytokines are known to stimulate NO production in human alveolar epithelial cells in a synergistic (nonlinear or nonadditive) manner. The mechanism of this synergy is not known. We measured the activation of the transcription factor NF-kappaB, the iNOS protein, and NO production in A549 monolayers (human alveolar epithelial cell line) in response to different combinations of IL-1beta, INF-gamma, and TNF-alpha (100 ng/ml), and the cofactors FMN, FAD, and BH4. We found that both IL-1beta and TNF-alpha could independently activate cytosolic NF-kappaB, direct its translocation into the nucleus, and induce iNOS monomer synthesis. In addition, different combinations of cytokines produced synergistic amounts of iNOS monomers. Exogenous BH4 (0.1 microM) had no impact on NO production induced by cytokine combinations that included IL-1beta, but significantly enhanced NO production in the presence of INF-gamma and TNF-alpha, and allowed TNF-alpha independently to produce NO. We conclude that there are at least three mechanisms of synergistic cytokine-induced NO production: (1) the biosynthesis of iNOS monomer due to nonlinear interactions by transcription factors, (2) synergistic cytosolic activation of NF-kappaB, and (3) parallel biosynthesis of BH4 in the presence of cytokine combinations that include IL-1beta.

Flow-independent nitric oxide exchange parameters in healthy adults

Currently accepted techniques utilize the plateau concentration of nitric oxide (NO) at a constant exhalation flow rate to characterize NO exchange, which cannot sufficiently distinguish airway and alveolar sources. Using nonlinear least squares regression and a two-compartment model, we recently described a new technique (Tsoukias et al. J Appl Physiol 91: 477-487, 2001), which utilizes a preexpiratory breath hold followed by a decreasing flow rate maneuver, to estimate three flow-independent NO parameters: maximum flux of NO from the airways (J(NO,max), pl/s), diffusing capacity of NO in the airways (D(NO,air), pl x s(-1) x ppb(-1)), and steady-state alveolar concentration (C(alv,ss), ppb). In healthy adults (n = 10), the optimal breath-hold time was 20 s, and the mean (95% intramaneuver, intrasubject, and intrapopulation confidence interval) J(NO,max), D(NO,air), and C(alv,ss) are 640 (26, 20, and 15%) pl/s, 4.2 (168, 87, and 37%) pl x s(-1) x ppb(-1), and 2.5 (81, 59, and 21%) ppb, respectively. J(NO,max) can be estimated with the greatest certainty, and the variability of all the parameters within the population of healthy adults is significant. There is no correlation between the flow-independent NO parameters and forced vital capacity or the ratio of forced expiratory volume in 1 s to forced vital capacity. With the use of these parameters, the two-compartment model can accurately predict experimentally measured plateau NO concentrations at a constant flow rate. We conclude that this new technique is simple to perform and can simultaneously characterize airway and alveolar NO exchange in healthy adults with the use of a single breathing maneuver.

Impact of volume-dependent alveolar diffusing capacity on exhaled nitric oxide concentration

Exhaled endogenous nitric oxide (NO) holds promise as a potential biomarker of pulmonary inflammation. Previous experimental and theoretical work has concluded that the alveolar concentration approaches a constant steady state value at end exhalation due to both a constant maximum flux or release of NO (J(max,alv)) and a constant diffusing capacity (D(NO,alv)) in the alveolar region. We have recently demonstrated that D(NO,alv) is not constant, but increases with alveolar volume (VA) given by the following average relationship: D(NO,alv) =48*VA(2/3) ml/min/mmHg (where VA is expressed in liters, STPD). We investigated the potential impact of a variable D(NO,alv) on exhaled concentration by incorporating the volume dependence into the currently accepted two-compartment model for NO exchange dynamics. Our results suggest that the mechanism underlying the plateau in exhaled concentration is a constant ratio J(max,alv)/D(NO,alv) This constant ratio requires a volume dependence of J(max,alv) similar to D(NO,alv), and is likely due to a decreasing alveolar surface area during exhalation.

A single-breath technique with variable flow rate to characterize nitric oxide exchange dynamics in the lungs

Current techniques to estimate nitric oxide (NO) production and elimination in the lungs are inherently nonspecific or are cumbersome to perform (multiple-breathing maneuvers). We present a new technique capable of estimating key flow-independent parameters characteristic of NO exchange in the lungs: 1) the steady-state alveolar concentration (C(alv,ss)), 2) the maximum flux of NO from the airways (J(NO,max)), and 3) the diffusing capacity of NO in the airways (D(NO,air)). Importantly, the parameters were estimated from a single experimental single-exhalation maneuver that consisted of a preexpiratory breath hold, followed by an exhalation in which the flow rate progressively decreased. The mean values for J(NO,max), D(NO,air), and C(alv,ss) do not depend on breath-hold time and range from 280-600 pl/s, 3.7-7.1 pl. s(-1). parts per billion (ppb)(-1), and 0.73-2.2 ppb, respectively, in two healthy human subjects. A priori estimates of the parameter confidence intervals demonstrate that a breath hold no longer than 20 s may be adequate and that J(NO,max) can be estimated with the smallest uncertainty and D(NO,air) with the largest, which is consistent with theoretical predictions. We conclude that our new technique can be used to characterize flow-independent NO exchange parameters from a single experimental single-exhalation breathing maneuver.

In vivo control of soluble guanylate cyclase activation by nitric oxide: a kinetic analysis

Free nitric oxide (NO) activates soluble guanylate cyclase (sGC), an enzyme, within both pulmonary and vascular smooth muscle. sGC catalyzes the cyclization of guanosine 5'-triphosphate to guanosine 3',5'-cyclic monophosphate (cGMP). Binding rates of NO to the ferrous heme(s) of sGC have been measured in vitro. However, a missing link in our understanding of the control mechanism of sGC by NO is a comprehensive in vivo kinetic analysis. Available literature data suggests that NO dissociation from the heme center of sGC is accelerated by its interaction with one or more cofactors in vivo. We present a working model for sGC activation and NO consumption in vivo. Our model predicts that NO influences the cGMP formation rate over a concentration range of approximately 5-100 nM (apparent Michaelis constant approximately 23 nM), with Hill coefficients between 1.1 and 1.5. The apparent reaction order for NO consumption by sGC is dependent on NO concentration, and varies between 0 and 1.5. Finally, the activation of sGC (half-life approximately 1-2 s) is much more rapid than deactivation (approximately 50 s). We conclude that control of sGC in vivo is most likely ultra-sensitive, and that activation in vivo occurs at lower NO concentrations than previously reported.

Two-photon laser scanning microscopy of epithelial cell-modulated collagen density in engineered human lung tissue

Tissue remodeling is a complex process that can occur in response to a wound or injury. In lung tissue, abnormal remodeling can lead to permanent structural changes that are characteristic of important lung diseases such as interstitial pulmonary fibrosis and bronchial asthma. Fibroblast-mediated contraction of three-dimensional collagen gels is considered an in vitro model of tissue contraction and remodeling, and the epithelium is one factor thought to modulate this process. We studied the effects of epithelium on collagen density and contraction using two-photon laser scanning microscopy (TPLSM). TPLSM was used to image autofluorescence of collagen fibers in an engineered tissue model of the human respiratory mucosa -- a three-dimensional co-culture of human lung fibroblasts (CCD-18 lu), denatured type I collagen, and a monolayer of human alveolar epithelial cell line (A549) or human bronchial epithelial cell line (16HBE14o(-)). Tissues were imaged at days 1, 8, and 15 at 10 depths within the tissue. Gel contraction was measured concurrently with TPLSM imaging. Image analysis shows that gels without an epithelium had the fastest rate of decay of fluorescent signal, corresponding to highest collagen density. Results of the gel contraction assay show that gels without an epithelium also had the highest degree of contraction (19.8% +/- 4.0%). We conclude that epithelial cells modulate collagen density and contraction of engineered human lung tissue, and TPLSM is an effective tool to investigate this phenomenon.

Microscopic modeling of NO and S-nitrosoglutathione kinetics and transport in human airways

Nitric oxide (NO) appears in the exhaled breath and is elevated in inflammatory diseases. We developed a steady-state mathematical model of the bronchial mucosa for normal small and large airways to understand NO and S-nitrosoglutathione (GSNO) kinetics and transport using data from the existing literature. Our model predicts that mean steady-state NO and GSNO concentrations for large airways (generation 1) are 2.68 nM and 113 pM, respectively, in the epithelial cells and 0.11 nM (approximately 66 ppb) and 507 nM in the mucus. For small airways (generation 15), the mean concentrations of NO and GSNO, respectively, are 0.26 nM and 21 pM in the epithelial cells and 0.02 nM (approximately 12 ppb) and 132 nM in the mucus. The concentrations in the mucus compare favorably to experimentally measured values. We conclude that 1) the majority of free NO in the mucus, and thus exhaled NO, is due to diffusion of free NO from the epithelial cell and 2) the heterogeneous airway contribution to exhaled NO is due to heterogeneous airway geometries, such as epithelium and mucus thickness.

Effect of alveolar volume and sequential filling on the diffusing capacity of the lungs: II. Experiment

The diffusing capacity of the lung, DL, is a critical physiological parameter, yet the currently accepted clinical model (Jones-Meade) assumes a well-mixed alveolar region, and a constant DL independent of alveolar volume, VA, despite experimental evidence to the contrary. We have formulated a new mathematical model [Tsoukias, N.M, Wilson, A.F., George, S.C., 2000. Respir. Physiol. 120, 231-249] that considers variable alveolar mixing through a single parameter, k (0<k<1), and a DL that is a positive function of VA (DL=a+bVA or DL=alphaVA(beta)). The goal of this study is to determine the suitability of this model to determine the unknown parameters a, b, alpha, beta, and k from experimental data in normal subjects. The model predicts that the normal lung fills, in part, sequentially (k=0.51+/-0.35). The following average values in all seven subjects were obtained: DLNO=48.VA(2/3) ml/min/mmHg and DLCO=20+0.7.VA ml/min/mmHg (STPD) where VA is expressed in L (STPD). We conclude that the mathematical model is suitable for identifying the unknown parameters and thus can be used to characterize the degree of alveolar mixing (or sequential filling) as well as the volume dependence of DL.

Effect of alveolar volume and sequential filling on the diffusing capacity of the lungs: I. theory

The diffusing capacity, DL, is a critical physiological parameter of the lung used to assess gas exchange clinically. Most models developed to analyze experimental data from a single breath maneuver have assumed a well-mixed or uniform alveolar region, including the clinically accepted Jones-Meade method. In addition, all previous models have assumed a constant DL, which is independent of alveolar volume, VA. In contrast, experimental data provide evidence for a non-uniform alveolar region coupled with sequential filling of the lung. In addition, although the DL for carbon monoxide is a weak function of VA, the DL of nitric oxide depends strongly on VA. We have developed a new mathematical model of the single breath maneuver that considers both a variable degree of sequential filling and a variable DL. Our model predicts that the Jones-Meade method overestimates DL when the exhaled gas sample is collected late in the exhalation, but underestimates DL if the exhaled gas sample is collected early in the exhalation phase due to the effect of sequential filling. Utilizing a prolonged constant exhalation method, or a three-equation method, will also produce erroneous predictions of DL. We conclude that current methods may introduce significant error in the estimation of DL by ignoring the sequential filling of the lung, and the dependence of DL on VA.

Synergistic cytokine-induced nitric oxide production in human alveolar epithelial cells

Nitric oxide (NO) is an important mediator molecule in regulating normal airway function, as well as in the pathophysiology of inflammatory airway diseases. In addition, cytokines are potent messenger molecules at sites of inflammation. The specific relationship among IL-1beta, TNF-alpha, and IFN-gamma on iNOS induction and NO synthesis in human alveolar epithelial cells has not been determined. In addition, rigorous methods to determine potential synergistic action between the cytokines have not been employed. We exposed monolayer cultures of A549 cells to a factorial combination of three cytokines (IL-1beta, TNF-alpha, and IFN-gamma) and three concentrations (0, 5, and 100 ng/mL). TNF-alpha alone does not induce NO production directly; however, it does have a stimulatory effect on IL-1beta-induced NO production. IL-1beta and INF-gamma both induce NO production alone, yet at different concentration thresholds, and act synergistically when present together. In the presence of all three cytokines, the net effect of NO production exceeds the predicted additive effect of each individual cytokine and the two-way interactions. Several plausible mechanisms of synergy among IL-1beta, TNF-alpha, and IFN-gamma in NO production from human alveolar epithelial cells (A549) are proposed. In order to verify the proposed mechanisms of synergy, future experimental and theoretical studies must address several molecular steps through which the iNOS gene is expressed and regulated, as well as the expression and regulation of enzyme cofactors and substrates.

Theoretical gas phase mass transfer coefficients for endogenous gases in the lungs

Gas phase mass transfer coefficients for nitric oxide (NO), ethanol (EtOH), and water vapor (H2O) were determined for typical conducting airway geometry and tracheal flows (5 x 10(-5)and 5 x 10(-4) m3 s(-1)), by solving the steady-state two-dimensional diffusion equation. A constant absolute production rate with first order consumption reactions in pulmonary tissue was assumed for NO. For EtOH and H2O, constant concentrations were assumed in the blood and tissue, respectively. Results, expressed in terms of the average Sherwood number (Sh), were correlated with the Peclet (Pe(r)) number, and the length-to-diameter (L/D) ratio for each airway branch in terms of a lumped variable, Pe(r)(L/D)n. (Sh) increases as the solubility of the gas in tissue and blood increases. In addition, Sh passes through a minimum value at Pe(r)(D/L)n equal to approximately one when axial convection and diffusion have equal but opposite magnitudes. We conclude that Sh is not a monotonic function of Pe(r)(L/D)n within the entire airway tree and that it depends on the physical properties of the gas in the tissue. This conclusion contrasts with previous experimental and theoretical correlations.

A two-compartment model of pulmonary nitric oxide exchange dynamics

The relatively recent detection of nitric oxide (NO) in the exhaled breath has prompted a great deal of experimentation in an effort to understand the pulmonary exchange dynamics. There has been very little progress in theoretical studies to assist in the interpretation of the experimental results. We have developed a two-compartment model of the lungs in an effort to explain several fundamental experimental observations. The model consists of a nonexpansile compartment representing the conducting airways and an expansile compartment representing the alveolar region of the lungs. Each compartment is surrounded by a layer of tissue that is capable of producing and consuming NO. Beyond the tissue barrier in each compartment is a layer of blood representing the bronchial circulation or the pulmonary circulation, which are both considered an infinite sink for NO. All parameters were estimated from data in the literature, including the production rates of NO in the tissue layers, which were estimated from experimental plots of the elimination rate of NO at end exhalation (ENO) vs. the exhalation flow rate (VE). The model is able to simulate the shape of the NO exhalation profile and to successfully simulate the following experimental features of endogenous NO exchange: 1) an inverse relationship between exhaled NO concentration and VE, 2) the dynamic relationship between the phase III slope and VE, and 3) the positive relationship between ENO and VE. The model predicts that these relationships can be explained by significant contributions of NO in the exhaled breath from the nonexpansile airways and the expansile alveoli. In addition, the model predicts that the relationship between ENO and VE can be used as an index of the relative contributions of the airways and the alveoli to exhaled NO.

Single-exhalation profiles of NO and CO2 in humans: effect of dynamically changing flow rate

Endogenous production of nitric oxide (NO) in the human lungs has many important pathophysiological roles and can be detected in the exhaled breath. An understanding of the factors that dictate the shape of the NO exhalation profile is fundamental to our understanding of normal and diseased lung function. We collected single-exhalation profiles of NO and CO2 from normal human subjects after inhalation of ambient air (approximately 15 parts/billion) and examined the effect of a 15-s breath hold and exhalation flow rate (VE) on the following features of the NO profile: 1) series dead space, 2) average concentration in phase III with respect to time and volume, 3) normalized slope of phase III with respect to time and volume, and 4) elimination rate at end exhalation. The dead space is approximately 50% smaller for NO than for CO2 and is substantially reduced after a breath hold. The concentration of exhaled NO is inversely related to VE, but the average NO concentration with respect to time has a stronger inverse relationship than that with respect to volume. The normalized slope of phase III NO with respect to time and that with respect to volume are negative at a constant VE but can be made to change signs if the flow rate continuously decreases during the exhalation. In addition, NO elimination at end exhalation vs. VE produces a nonzero intercept and slope that are subject dependent and can be used to quantitate the relative contribution of the airways and the alveoli to exhaled NO. We conclude that exhaled NO has an airway and an alveolar source.

Modeling bronchial circulation with application to soluble gas exchange: description and sensitivity analysis

The steady-state exchange of inert gases across an in situ canine trachea has recently been shown to be limited equally by diffusion and perfusion over a wide range (0.01-350) of blood solubilities (betablood; ml . ml-1 . atm-1). Hence, we hypothesize that the exchange of ethanol (betablood = 1,756 at 37 degrees C) in the airways depends on the blood flow rate from the bronchial circulation. To test this hypothesis, the dynamics of the bronchial circulation were incorporated into an existing model that describes the simultaneous exchange of heat, water, and a soluble gas in the airways. A detailed sensitivity analysis of key model parameters was performed by using the method of Latin hypercube sampling. The model accurately predicted a previously reported experimental exhalation profile of ethanol (R2 = 0.991) as well as the end-exhalation airstream temperature (34.6 degrees C). The model predicts that 27, 29, and 44% of exhaled ethanol in a single exhalation are derived from the tissues of the mucosa and submucosa, the bronchial circulation, and the tissue exterior to the submucosa (which would include the pulmonary circulation), respectively. Although the concentration of ethanol in the bronchial capillary decreased during inspiration, the three key model outputs (end-exhaled ethanol concentration, the slope of phase III, and end-exhaled temperature) were all statistically insensitive (P > 0.05) to the parameters describing the bronchial circulation. In contrast, the model outputs were all sensitive (P < 0.05) to the thickness of tissue separating the core body conditions from the bronchial smooth muscle. We conclude that both the bronchial circulation and the pulmonary circulation impact soluble gas exchange when the entire conducting airway tree is considered.

Gas exchange in the airways

The primary function of the lungs is to exchange the respiratory gases, O2 and CO2, between the atmosphere and the blood. Our overall understanding of the lungs as a gas-exchanging organ has improved considerably over the past four decades. We now know that the dynamics of gas exchange depend on the blood solubility (beta b, ml gas ml blood-1 atm-1) of the gas. While the major focus of research has rightly been on the respiratory gases, the lungs exchange a wide spectrum of gases ranging from very low solubility gases such as SF6 or helium (beta b = 0.01) to water vapor (beta b = 20,000). O2 (beta b = 0.7) and CO2 (beta b = 3.0) exchange primarily in the alveolar region of the lung and their exchange is limited by the rate of ventilation and perfusion. In contrast, highly soluble gases (beta b > 100) are likely to exchange primarily in the airways of the lung. We have used exhaled ethanol (beta b = 1756) profiles for humans, steady-state exchange of six inert gases (0.01 < beta b < 300) in an in situ dog trachea, and a mathematical model to analyze the dynamics of airway gas exchange. We make the following conclusion: (1) ethanol exchanges entirely within the airways, and (2) the magnitude of perfusion- and diffusion-related resistance to airway gas exchange is the same.

Diffusion of nonelectrolytes in the canine trachea: effect of tight junction

We recently demonstrated through theoretical modeling that the exhaled ethanol (EtOH) profile from humans is consistent with a molecular diffusion coefficient (cm2/s) in the bronchial mucosa (Dti) that is only 8% of the diffusion coefficient in water (Dw; J. Appl. Physiol. 75: 2439-2449, 1993). Because of the small oil-water partition coefficient (lambda o:w) of EtOH (lambda o:w = 0.074), the reduced diffusion coefficient may be due, in part, to the epithelial tight junction in the paracellular pathway. We hypothesized that opening the tight junction would open an aqueous pathway and increase the diffusion coefficient of small (mol wt < 100) hydrophilic compounds. We mounted the mucosa from the membranous canine trachea in an Ussing-type diffusion cell and measured the diffusion coefficient of 2-ethoxyethanol (2-Ethx; lambda o:w = 0.042), EtOH, and methyl ethyl ketone (MEK; lambda o:w = 1.04) in the presence and absence of the epithelial tight junction. The tight junction was opened using a phosphate-buffered saline free of Ca2+ and Mg2+ with 0.5 mM ethylene glycol-bis (beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid, and its integrity was assessed by measuring the transepithelial electrical resistance. Dti/Dw in the presence of Ca2+ and Mg2+ was 0.39, 0.34, and 0.39 for 2-Ethx, EtOH, and MEK, respectively, and increased 24.6, 11.7, and 1.11% in the absence of Ca2+ and Mg2+. We conclude that the effect of the tight junction on Dti increases with increasing water solubility but can account for only a small portion of the reduced Dti of EtOH as predicted by exhaled profiles.