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J Appl Physiol 96: 65-75, 2004. First published September 5, 2003; doi:10.1152/japplphysiol.00575.2003
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Airway diffusing capacity of nitric oxide and steroid therapy in asthma

Hye-Won Shin,1,2 Christine M. Rose-Gottron,3 Dan M. Cooper,3,4 Robert L. Newcomb,3,5 and Steven C. George1,2

Departments of 1Biomedical Engineering, 2Chemical Engineering and Materials Science, and 4Pediatrics, 3General Clinical Research Center, and 5Center for Statistical Consulting, University of California, Irvine, California 92697-2575

Submitted 2 June 2003 ; accepted in final form 28 August 2003


   ABSTRACT
TOP
 ABSTRACT
METHODS
 RESULTS
 DISCUSSION
 APPENDIX A: ASTHMA CONTROL...
 APPENDIX B: MATHEMATICAL...
 ACKNOWLEDGMENTS
 REFERENCES
 
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.

model; airways; alveoli; inflammation

NITRIC OXIDE (NO) WAS FIRST detected in the exhaled breath of humans more than a decade ago (19) and remains a promising noninvasive index of lung pathophysiology. Substantial evidence suggests that both the airway and alveolar regions are significant sources of exhaled NO [fraction of exhaled NO (FENO)] (8, 20, 37, 42, 44-46, 48, 52, 53). Thus, in contrast to a respiratory gas like CO2 that is evolved predominantly in the alveolar compartment and whose presence in the exhaled breath primarily reflects alveolar gas exchange, FENO measurements might lead to specific insights about pathophysiology throughout the respiratory tract. Guidelines for characterizing FENO by the American Thoracic Society (ATS) and the European Respiratory Society (ERS) include only the plateau concentration in phase III (CNO,plat) at a constant exhalation flow rate (E) (2, 29). However, a single measurement of CNO,plat cannot distinguish airway and alveolar contributions and thus may not be the optimal parameter to describe pulmonary NO exchange.

The potential for greater clinical insight is accompanied by the need for new and robust analytic approaches to characterize NO in the exhaled breath. Because NO is produced throughout the respiratory tract, factors like expiratory flow rate substantially influence the NO concentration in the exhaled breath (Cexh) (21, 47, 54). To account for this and other determinants of NO concentration, we and others have described NO exchange using a biologically relevant two-compartment model (airway and alveolar compartments) and a series of flow-independent NO exchange parameters (20, 42, 48, 52). The flow-independent parameters potentially provide clinically relevant information about NO exchange. For example, the alveolar NO concentration is elevated in allergic alveolitis (alveolar inflammation), whereas airway wall NO flux is elevated in asthma (bronchial inflammation) (37).

Inflammation is characteristic of asthma and induces the expression of several steroid-sensitive enzymes, such as NO synthase (NOS) and glutaminase, which impact NO metabolism (3, 23, 43). Consequently, corticosteroids, which attenuate the inflammatory process, also reduce the concentration of NO in the exhaled breath (31, 41). This feature of corticosteroid therapy may be useful in monitoring the inflammatory status of the airways, but, by reducing the concentration of NO in the exhaled breath to near normal, may mask steroid-independent alterations in airway NO physiology that are of potential clinical significance.

The airway diffusing capacity of NO (Daw,NO) is the conductance for the transfer of NO between the airway wall and the gas stream (48, 52, 53). It depends on both the physical features of the airway wall (e.g., airway surface area or tissue thickness) and the rate of chemical consumption (4, 53), both of which may be altered in asthma. Recently, Silkoff et al. (48) demonstrated that Daw,NO was elevated in asthma independent of steroid therapy by measuring multiple CNO,plat at small flow rates (<50 ml/s). However, values for the flow-independent NO exchange parameters may depend on the breathing maneuver and analytic technique utilized. Thus the goal of the present study was to apply our alternate breathing and analytic technique (20-s preexpiratory breath hold followed by a decreasing flow-rate maneuver) in asthma to confirm the results of Silkoff et. al. and potentially provide additional insight into the pathophysiology that marks chronic asthma.


   METHODS
TOP
 ABSTRACT
 METHODS
RESULTS
 DISCUSSION
 APPENDIX A: ASTHMA CONTROL...
 APPENDIX B: MATHEMATICAL...
 ACKNOWLEDGMENTS
 REFERENCES
 
Subjects. Twenty-four healthy adults and 20 subjects with a clinical history of asthma (8 steroid naive and 12 steroid treated) participated in this study. Inclusion criteria for the healthy subjects was a forced expiratory volume in 1 s (FEV1)-to-forced vital capacity (FVC) ratio (FEV1/FVC) >0.80; exclusion criteria were a history of smoking at any time, heart disease, or lung disease. Inclusion criteria for the asthma group were a clinical history of reversible bronchoconstriction and a current FEV1/FVC <0.75, regardless of the use of corticosteroids; exclusion criteria were a history of smoking at any time, heart disease, and lung disease other than asthma. We then subdivided the adults with a clinical history of asthma into two groups: 1) steroid naive and 2) steroid treated. In addition, each of the adult subjects with asthma also completed a previously validated asthma control questionnaire (see APPENDIX A) to assess clinical symptoms of asthma over the past 7 days (27, 28). Subject characteristics are presented in Tables 1 and 2, including details of their clinical history. The Institutional Review Board at the University of California, Irvine approved the protocol, and written, informed consent was obtained from all subjects.


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  		Table 1. Physical characteristics of subjects

 

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  		Table 2. Clinical history of adults with asthma

 

Experimental protocol. Each subject performed two types of exhalation maneuvers: one necessary to estimate the flow-independent NO exchange parameters, and the other according to the ATS guidelines (2). The first maneuver was five repetitions of a 20-s preexpiratory breath hold followed by a decreasing flow rate (from ~6 to ~1% of vital capacity per second) maneuver (53) to estimate several flow-independent NO exchange parameters. A positive pressure of >5 cmH2O was maintained to prevent nasal contamination during the breath hold (2), and a Starling resistor (Hans Rudolph, Kansas City, MO) with a variable resistance was used to progressively decrease the flow rate during the exhalation. After breath hold, the exhalation valve was opened, allowing the patient to expire. A schematic of the experimental apparatus has been previously presented (53). The second maneuver was a vital capacity maneuver performed in triplicate to collect plateau NO concentration based on the ATS guidelines (2). We also included an exhalation flow rate of 250 ml/s (ATS guideline is 50 ml/s) consistent with the guidelines of the ERS (29). After measuring the indexes of NO exchange dynamics, general spirometry, such as FVC, and FEV1 normalized by FVC (FEV1/FVC) were measured in all subjects (Vmax229; Sensormedics, Yorba Linda, CA) by using the best performance (see Table 1) from three consecutive maneuvers.

Airstream analysis. A chemiluminesence NO analyzer (NOA280, Sievers, Boulder, CO) was used to measure the Cexh. The instrument was calibrated on a daily basis by using a certified NO gas (45 parts/million in N2; Sievers, Boulder, CO). The zero-point calibration was performed with a NO filter (Sievers) immediately before the collection of a profile. The flow rate and pressure signals were measured by using a pneumotachometer (RSS100, Hans Rudolph, Kansas City, MO). The pneumotachometer was calibrated daily and was set to provide the flow in units of STPD.

Data analysis and parameter estimation. Experimental single-exhalation profiles with the 20-s preexpiratory breath hold were characterized by the peak concentration in phases I and II (CNO,peak); the peak width (W50) in phases I and II, defined as the exhaled volume in which the NO concentration was >50% of CNO,peak; and the total volume of phases I and II (VI,II), defined as the point of zero slope (dCexh/dV = 0, where V is volume) in the exhalation profile (53) (Fig. 1). The constant-flow-rate single exhalations were characterized by the CNO,plat, as previously described by the ATS and the ERS (2, 29).



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  		Fig. 1. Definition of CNO,peak, W50, and VI,II are presented by a schematic of a representative exhalation nitric oxide (NO) profile using the single-breath technique with a preexpiratory breath hold and a decreasing exhalation flow rate. CNO,peak is the maximum concentration of NO in phases I and II; W50 is the width of the phase I and II peak calculated by taking the volume (V) at which the exhaled concentration (Cexh) is >50% of CNO,peak; and VI,II is the volume of phases I and II. The distinction between phases I and II and phase III is the point of zero slope in the exhalation profile, as previously described (53). ppb, Parts per billion.

 

A previously described two-compartment model was used to estimate four flow-independent NO exchange parameters: 1) maximum flux of NO from the airways (; pl/s); 2)Daw,NO [pl·s-1·parts per billion (ppb)-1]; 3) steady-state alveolar concentration (Calv,ss; ppb); and 4) mean airway tissue NO concentration (Caw,NO; ppb; equal to the ratio of to Daw,NO). A simple schematic of the two-compartment model and flow-independent parameters is presented in Fig. 2, and a detailed description of the mathematical estimation of the parameters has been previously described (53).



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  		Fig. 2. Schematic of 2-compartment model used to describe NO exchange dynamics. Cexh is the sum of two contributions, the alveolar region and the airway region, which depends on 3 flow-independent parameters: maximum total volumetric flux of NO from the airway wall (; pl/s), diffusing capacity of NO in the airways (Daw,NO; pl·s-1·ppb-1), and steady-state alveolar concentration (Calv,ss; ppb). Jaw,NO is the total flux (pl/s) of NO between the tissue and gas phase in the airway and is an inverse function of the exhalation flow rate (E) and is the sum of two terms: - Daw,NO * Cair (airway gas phase concentration). If Daw,NO increases while is held constant (note that this necessitates a decrease in the wall concentration, Caw,NO, as is the product of Daw,NO * Caw,NO), then Jaw,NO decreases (see text for details). If exhalation flow rate is held constant (i.e., 50 ml/s as suggested by the American Thoracic Society), then Cexh approaches a constant value in phase III of the exhalation profile and is equivalent to CNO,plat (NO plateau concentration in phase III). t, Time.

 

The source of NO from the airways can be described by the instantaneous flux of NO from the airways (Jaw,NO; pl/s). Jaw,NO depends on the flow-independent parameters and is expressed as a linear function of the airway gas-phase concentration (Cair) by the following

(1)
or

(2)
is equal to the product Daw,NO * Caw,NO (Eq. 2). Conceptually, approaches Jaw,NO as the product Daw,NO * Cair approaches zero. Daw,NO is the conductance for mass transfer (transfer factor or airway diffusing capacity) of NO between the airway tissue and the gas phase. The alveolar region is characterized by Calv,ss, which is equivalent to the alveolar tissue concentration (25, 52). Fig. 3 illustrates the independent (i.e., all other parameters are held constant) impact of Daw,NO, , and Calv,ss on the single-exhalation profile with a 20-s preexpiratory breath hold and a decreasing exhalation flow rate.



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  		Fig. 3. The 2-compartment model prediction of the exhaled NO profile is shown for the single-exhalation maneuver with a 20-s preexpiratory breath hold. Representative values for lung volumes of a healthy adult have been used, and the "control" values for the flow-independent parameters are as follows: Daw,NO = 5 pl·s-1·ppb-1 (A); = 750 pl·s-1 (B); Calv,ss = 3 ppb (C). In each panel, the control profile (solid line) is shown together with the exhaled profile when one of the flow-independent parameters is doubled (dashed line). A: the decreasing E is also shown on the y-axis. This informal sensitivity analysis demonstrates graphically which part of the profile is impacted by each parameter. It can be seen that each parameter uniquely impacts the exhaled profile and can thus be uniquely determined. Note that Daw,NO primarily impacts phases I and II, Calv,ss impacts primarily phase III, whereas impacts all 3 phases. In addition, note that an increase in Daw,NO (while holding and Calv,ss) decreases the NO concentration in phases I and II if (Eq. 2) and Calv,ss are held constant, but would increase the concentration in phases I and II if Caw,NO (Eq. 2) and Calv,ss were held constant (Eq. 2). In the former case, Caw,NO must be decreased to hold constant (product of Daw,NO * Caw,NO), whereas, in the later case, would increase as Caw,NO is constant.

 

Once the flow-independent parameters are known, the two-compartment model can be used to predict CNO,plat at any constant exhalation flow, and thus there is no loss of information in characterizing NO exchange with the flow-independent NO parameters (53)

(3)
where is the plateau concentration of NO predicted by the model using the flow-independent parameters. Our laboratory (44, 53) has previously demonstrated that is not different than the experimentally measured CNO,plat in healthy adults, with the advantage that intersubject and interpopulation variations in flow rate can be accounted for by calculating CNO,plat at a precise desired flow rate (e.g., 50 ml/s).

Statistics. To detect differences among the three groups of subjects, data were analyzed by using ANOVA and post hoc paired comparisons of treatment means. In those instances in which Levene's test rejected homogeneity of variance, tests for group differences relied on Welch's ANOVA or Satterthwaite's method to adjust the test to account for this problem. To detect significant relationships between the parameters that characterize NO exchange and either asthma symptoms or standard indexes of lung function (e.g., FEV1), we utilized first- and second-order partial correlation coefficients, respectively. For example, to determine the relationship between NO parameters and lung function for all subjects, the second-order partial correlation coefficient factors out the effect of having asthma or being treated with steroids by subtracting the group mean from each individual score. As to the question of normality, in addition to screening variables for excessive skewness, all tests of group differences were rerun by using a log transformation of the dependent variables. Because the log transformation of each variable did not impact the results, all statistical tests were reported by using the untransformed data. Finally, a P value <0.05 was considered statistically significant, and all results were produced by using the GLM procedure of SAS.


   RESULTS
TOP
 ABSTRACT
 METHODS
 RESULTS
DISCUSSION
 APPENDIX A: ASTHMA CONTROL...
 APPENDIX B: MATHEMATICAL...
 ACKNOWLEDGMENTS
 REFERENCES
 
FVC, FEV1, FEV1/FVC, and the clinical history of the subjects with asthma are presented in Tables 1 and 2, respectively. FEV1/FVC was more reproducible than FEV1 alone. The mean maximum variability (defined as the difference between the maximum and minimum value normalized by the mean of the three repeated maneuvers) for FEV1/FVC was 5.8% (range 1.5-10.2%) and 2.9% (range 0-10.2%) for steroid-naive and steroid-treated asthma subjects, respectively. For FEV1 alone, the mean maximum variability was slightly higher for each group: 8.9% (range 0.7-20.6%) and 5.2% (range 1.5-17.9%) for steroid-naive and steroid-treated asthma subjects, respectively. FEV1/FVC was significantly lower in both groups of subjects with asthma compared with healthy adults. However, there was no difference in FEV1/FVC or clinical symptoms (as assessed by the composite score on the asthma control questionnaire) between the two groups of subjects with asthma.

Of the 20 subjects with asthma, three of the steroid treated (subjects 2, 10, and 12) were not able to complete the 20-s breath hold, and thus we utilized a 10-s breath hold, which may increase the confidence interval of Daw,NO (44, 53). To highlight differences among groups in exhaled concentrations, a composite exhalation profile for each group was attained (Fig. 4) by taking the mean exhaled concentration at equivalent exhaled volume intervals for each of the three groups. The three asthmatic subjects who were not able to complete the 20-s breath hold were excluded from the composite exhalation profile. Steroid-naive subjects with asthma had an increased concentration of NO in all phases of the exhalation profile compared with both steroid-treated subjects with asthma and healthy controls. Although the NO exhalation profile for steroid-treated subjects with asthma and healthy controls is similar (Fig. 4B), there are important differences that reflect alterations in the flow-independent NO parameters. Steroid-treated subjects with asthma have elevated NO in phase III that is reflected in a steeper phase III slope. This steeper slope reflects a greater airway wall flux () as opposed to an elevated Calv,ss, which would cause a uniform increase in NO concentration over phase III (52, 53). The elevated would result in a much larger CNO,peak than actually observed, and this results in an elevated Daw,NO as described below.



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  		Fig. 4. A: composite experimental NO exhalation profiles are presented for the 20-s breath hold followed by a decreasing flow rate maneuver for steroid-naive (SN) asthma subjects (thick line) and healthy adults (HA) (thin line). B: composite experimental NO exhalation profiles are presented for the 20-s breath hold followed by a decreasing flow rate maneuver for steroid-treated (ST) asthma subjects (thick line) and HA (thin line). Values are means ± SD.

 

Mean (±SD) CNO,peak for steroid-naive, steroid-treated, and healthy subjects was 192 ± 127, 82 ± 42, and 67 ± 29 ppb, respectively. CNO,peak for steroid-naive subjects with asthma was statistically larger than that for the other two groups. W50 for steroid-naive, steroid-treated, and healthy subjects was 189 ± 60, 171 ± 49, and 190 ± 51 ml, respectively, and was not different among groups. VI,II for steroid-naive, steroid-treated, and healthy subjects was 657 ± 98, 604 ± 127, and 668 ± 142 ml, respectively, and was also not different among the groups.

As shown in Fig. 5, and Daw,NO are elevated in steroid-naive subjects with asthma relative to healthy controls. The use of corticosteroids does not impact Daw,NO, but is associated with a significantly lower and Caw,NO that are equivalent to those in healthy adults. Calv,ss is not different among the three groups.



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  		Fig. 5. Individual and population mean (solid bar) values of 4 flow-independent parameters (A: ; B: Daw,NO; C: Calv,ss; D: Caw,NO) for SN ({bullet}) and ST ({circ}) asthma subjects and HA ({diamond}). The mean ± SD , Daw,NO, Calv,ss, and Caw,NO for HA, SN, and ST are as follows: HA: 530 ± 234 pl/s, 3.13 ± 1.57 pl·s-1·ppb-1, 3.08 ± 2.39 ppb, and 220 ± 177 ppb; SN: 2,693 ± 1,687 pl/s, 8.71 ± 5.74 pl·s-1·ppb-1, 5.68 ± 3.22 ppb, and 438 ± 312 ppb; ST: 1,196 ± 837 pl/s, 11.8 ± 11.7 pl·s-1·ppb-1, 3.30 ± 2.74 ppb, and 143 ± 66 ppb, respectively. Statistically different from *HA and #SN subjects with asthma: P < 0.05.

 

The experimental values of CNO,plat at the target flow rates of 50 and 250 ml/s, respectively, are presented in Table 3 (for healthy adults and subjects with asthma) along with the model-predicted CNO,plat (Eq. 3, ) at exhalation flow rates of exactly 50 and 250 ml/s. and CNO,plat were not statistically different from each other, with the exception of the steroid-naive group of asthmatic subjects at 250 ml/s (see Table 3). Statistical differences between the groups did not depend on the choice of CNO,plat or . Thus, to control for small variations in the exhalation flow rate between groups (e.g., mean exhalation flow rate at the target of 50 ml/s was 62 ml/s and 55 ml/s for steroid-naive and steroid-treated groups, respectively), statistical differences between groups are presented by using (Fig. 6). was 13.0 ± 5.97 and 5.17 ± 2.97 ppb for healthy adults, 53.9 ± 33.0 and 16.1 ± 9.46 ppb for steroid-naive adults with asthma, and 23.2 ± 14.3 and 7.76 ± 5.34 ppb for steroid-treated adults with asthma at flow rates of 50 and 250 ml/s, respectively. at 50 ml/s is significantly higher for both groups of subjects with asthma compared with healthy controls (Fig. 6), whereas only the steroid-naive subjects with asthma have a higher at 250 ml/s.


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  		Table 3. Model-predicted and experimental CNO,plat of subjects

 


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  		Fig. 6. Individual and population mean (solid bar) values of the plateau exhaled concentration for nitric oxide as predicted by the model (; Eq. 3) using the flow-independent parameters for each subject. A: exhalation flow rate of exactly 50 ml/s; B: exhalation flow rate of exactly 250 ml/s. {bullet}, SN; {circ}, ST; {diamond}, HA. Statistically different from *HA and #SN subjects with asthma: P < 0.05.

 

Daw,NO was inversely correlated with both FEV1 (%predicted) and FVC (%predicted) (Fig. 7, A and B). In contrast, Caw,NO was positively correlated with FVC (%predicted). and Calv,ss were not correlated with any lung function indexes. at either E was not correlated with indexes of lung function, but CNO,plat was inversely correlated with FEV1/FVC (%predicted) (Fig. 8). The asthma control questionnaire composite score was not correlated with any of the NO exchange parameters.



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  		Fig. 7. Second-order partial correlation analysis demonstrates a significant inverse relationship between Daw,NO and forced expiratory volume in 1 s (FEV1; %predicted) (A), Daw,NO and forced vital capacity (FVC; %predicted) (B), and a positive relationship between Caw,NO and FVC (%predicted) (C) in a total of 44 subjects. {Delta}, Difference between the individual score of each subject and the group mean value to which each subject belongs. +, HA (n = 24); {circ}, ST (n = 12); {bullet}, SN (n = 8).

 


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  		Fig. 8. Second-order partial correlation analysis demonstrates a significant inverse relationship between CNO,plat and ratio of FEV1 to FVC (FEV1/FVC; %predicted) at an exhalation flow rate of 50 ml/s (A) and 250 ml/s (B) in a total of 44 subjects. +, HA (n = 24); {circ}, ST (n = 12); {bullet}, SN (n = 8).

 


   DISCUSSION
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
APPENDIX A: ASTHMA CONTROL...
 APPENDIX B: MATHEMATICAL...
 ACKNOWLEDGMENTS
 REFERENCES
 
In the present study, we estimate flow-independent NO exchange parameters with a single exhalation breathing technique and plateau Cexh by following ATS and ERS guidelines in a group of subjects with a low FEV1 (FEV1/FVC < 0.75) and a clinical history of asthma. We found that the use of corticosteroids was associated with a decrease in CNO,plat at flow rates of 50 and 250 ml/s, as well as a decrease in the flow-independent parameters that reflect airway tissue concentration ( and Caw,NO, respectively). In contrast, Daw,NO was elevated in both groups of asthmatic subjects and was independent of the use of corticosteroids. These findings are in good agreement with previously published data by Silkoff et al. (48), despite using a different breathing maneuver and analytic technique to estimate the flow-independent NO exchange parameters. In addition, we found that Daw,NO is inversely correlated with both FEV1 and FVC (%predicted), independent of the presence of asthma and steroid use. Thus we confirm that Daw,NO may reflect physiological changes in the lungs that impact lung function independent of the use of corticosteroids.

Because the initial reports that FENO in asthma was elevated (1, 30), subsequent studies have focused on exploring the correlation between Cexh and other inflammatory markers (i.e., eosinophils), clinical interventions such as corticosteroids, and standard indexes of lung function (i.e., FEV1/FVC). Corticosteroid treatment significantly decreases CNO,plat in subjects with asthma (31, 40, 41), and the dose of steroid is inversely related to CNO,plat (32). In addition, an increase in CNO,plat has recently been shown to be equally effective as sputum eosinophils and airway hyperresponsiveness to hypertonic saline as a predictor for loss of asthma control (26). However, the present study, as well as that of Silkoff et al. (48), demonstrates the presence of steroid-independent factors (i.e., Daw,NO) that can also contribute to the elevated levels of NO in the exhaled breath of asthmatic subjects.

We are also now aware of disease states in which exhaled concentration of NO is in the normal range only because abnormalities in the flow-independent determinants of NO concentration balance each other. For example, in scleroderma, the alveolar concentration of NO is elevated, whereas the airway wall flux of NO is reduced (15). In cystic fibrosis, the Daw,NO (transfer factor) is elevated, but the airway wall concentration is reduced, leading to an Cexh that is similar to that of healthy controls (45).

Silkoff et al. (48) first reported that Daw,NO is fourfold higher in subjects with asthma and that this increase is independent of steroid treatment, whereas decreases. Lehtimaki et al. (38) then demonstrated that steroid treatment reduces in newly diagnosed asthma subjects (previously steroid naive) by utilizing multiple constant-flow rate maneuvers (52, 54). Most recently, Hogman et al. (22) also recently demonstrated that Daw,NO is increased 1.5-fold in a group of atopic asthmatic subjects. Although we utilized a different breathing maneuver and technique to estimate the flow-independent NO parameters, our results are consistent with previously reported trends (22, 38, 48) and also demonstrates that Daw,NO is inversely correlated with FEV1 and FVC (%predicted) and Caw,NO is positively correlated with FVC. The positive correlation of Caw,NO with FVC is likely due to the fact that it is inversely related to Daw,NO (i.e., Caw,NO = /Daw,NO). Of note is the fact that Silkoff et al. (48) reported that values of , Daw,NO, and Caw,NO after steroid use in asthmatic subjects were all positively correlated with FEV1/FVC (%predicted). These important differences may be due to differences in study design and the technique used to estimate the flow-independent NO parameters. Nonetheless, future studies will need to continue to investigate the relationship between NO flow-independent parameters and lung function.

Cexh necessarily reflects both the chemical and physical properties of the airway wall and alveoli, as well as the endogenous production rate from NOS isoforms in the airway and alveoli. Our ability to estimate the flow-independent NO parameters, which depend on these properties from the exhaled concentration signal, can be illustrated by using the composite exhalation profile (Fig. 4A). Our laboratory (53) has previously demonstrated that only phases I and II are sensitive to changes in Daw,NO (if Daw,NO increases, less NO is exhaled in phases I and II), only phase III is sensitive to Calv,ss (if Calv,ss increases, there is a uniform increase across exhaled volume in phase III), and all three phases are sensitive to (if increases, there is more NO exhaled in all phases, and the impact on phase III is a steeper slope) (see Fig. 3). Thus the observed changes in the composite profile of each group are consistent with our reported values of the flow-independent parameters. For example, steroid-treated subjects with asthma have a steeper slope in phase III and a higher concentration (necessitating a larger ), yet a similar amount of NO in phases I and II (necessitating a larger Daw,NO to balance the increased ). Of note is the fact that, among the parameters characterizing phases I and II of the exhalation profile, only CNO,peak differs among the groups (W50 and VI,II are not different among the three groups). This is consistent with altered NO production and transport in the airway wall during the breath hold, but also suggests that the volume accumulating NO during the breath hold and subsequently eliminated during exhalation is similar among the three groups.

Our laboratory has previously reported analytic expressions for the flow-independent parameters that approximate the functional dependence on the surface area emitting NO [Ai, where i is either airways (aw) or alveoli (alv); cm2], solubility [partition coefficient ({lambda}t:air)], molecular diffusion [molecular diffusivity (Dt,NO); cm2/s], chemical consumption (lumped first-order rate reaction constant k; s-1), thickness of the tissue layer (Lt,i; cm), and chemical production [airway (Saw,NO) and alveolar (Salv,NO) production rate per unit volume; ml NO·s-1·cm-3] (45, 52). The analytic expressions are summarized in APPENDIX B and provide a level of quantitative insight into the mechanism of the observed changes in the flow-independent parameters.

Daw,NO is independent of Saw,NO; is a positive function of total surface area of airway space (Aaw), {lambda}t:air, Dt,NO, and k; and is an inverse function of the thickness of the airway tissue layer (Lt,aw) (Eq. B2 in APPENDIX B). Thus the increase in Daw,NO may be due to alterations in any of these parameters. The airway wall in asthma is generally considered to be thicker than in healthy controls due to remodeling processes, such as subepithelial fibrosis and increased mucous production (56). The thicker airway wall would tend to increase the diffusion distance for NO, and the mucus tends to be more viscous, which would decrease the "ease" at which NO can diffuse (i.e., decrease Dt,NO) (4). Both of these observations would decrease Daw,NO and contrast with our experimental observation, as well as that of Silkoff et al. (48), of an elevated Daw,NO. Enhanced chemical consumption, primarily with superoxide (9), can increase Daw,NO due to an increase in the radial concentration gradient (4, 45). However, Daw,NO remains elevated after steroid treatment, which has been reported to suppress super-oxide release (10).

An increase in Aaw is a plausible mechanism for the increase in Daw,NO. Silkoff et al. (48) postulated that extension of the NO-producing nonadrenergic noncholinergic nerves from the large airways into the small airways may increase the Ai, which is supported both directly and indirectly by several studies (6, 7, 16, 17, 39, 55). Expression of inducible NOS (iNOS) in the airways of subjects with asthma has been demonstrated (12, 36, 50), which could potentially increase Aaw; however, this possible mechanism would likely be sensitive to corticosteroid therapy, which is not the observation.

has a similar functional dependence on the physical and chemical parameters of the airways (Aaw, Dt,NO, and Lt,aw) (see APPENDIX B) as Daw,NO. However, in contrast to Daw,NO, is inversely related to k and is a positive function of an additional parameter, Saw,NO. An increase in Saw,NO by an increase in neuronal NOS expression from nonadrenergic noncholinergic nerves (6, 7, 16, 17, 39, 55) or prokaryotic denitrification (13) may increase the exhaled concentration of NO and thus contribute to the observed increase in for both steroid-naive and steroid-treated subjects with asthma. Other enzymatic and nonenzymatic chemical events in the airways, such as increased iNOS expression in the epithelium (18), nitrite reduction to NO at lower pH (23, 24, 35), and S-nitrosoglutathione catabolism (5, 11, 14, 49), could also increase Saw,NO and contribute to the increase in for steroid-naive subjects with asthma.

Steroid treatment dramatically decreases the Cexh (see Fig. 5), which corresponds to observed decreases in and Caw,NO as well as CNO,plat at both 50 and 250 ml/s flow rates. As previously discussed, steroid therapy decreases superoxide production, which would correspond to a reduced consumption rate and an increase in , which is not observed. The decrease in in steroid-treated subjects with asthma may be related to 1) the reduced iNOS activity in the epithelial and inflammatory cells in the airways (12, 34, 36, 50, 57); 2) reduced nitrite to NO reduction due to normalized airway pH (23, 24, 35); 3) decreased prokaryotic colonization (13); and 4) inhibition of arginase upregulation (33). The decrease in Caw,NO (a ratio of over Daw,NO) for steroid-treated subjects with asthma is due to the decrease in , whereas Daw,NO is not changed.

In summary, we have estimated both flow-independent NO exchange parameters and plateau Cexh following ATS guidelines, in subjects with low FEV1/FVC and a clinical history of asthma. Daw,NO is elevated independent of corticosteroid use, whereas , Caw,NO, and CNO,plat (at both 50 and 250 ml/s) are all reduced by the use of steroids. In addition, Daw,NO is inversely correlated with pulmonary function, independent of the presence of asthma and steroid use. In agreement with Silkoff et al. (48), we conclude that Daw,NO may reflect changes in the lungs that impact function and that are not impacted by steroid therapy and thus may provide clinical information not available from Cexh alone.


   APPENDIX A: ASTHMA CONTROL QUESTIONNAIRE
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
 APPENDIX A: ASTHMA CONTROL...
APPENDIX B: MATHEMATICAL...
 ACKNOWLEDGMENTS
 REFERENCES
 
The following six questions are from a previously published and validated asthma control questionnaire (27, 28).

  1. ) On average, during the past week, how often were you woken by your asthma during the night?
  2. ) On average, during the past week, how bad were your asthma symptoms when you woke up in the morning?
  3. ) In general, during the past week, how limited were you in your activities because of your asthma?
  4. ) In general, during the past week, how much shortness of breath did you experience because of your asthma?
  5. ) In general, during the past week, how much of the time did you wheeze?
  6. ) On average, during the past week, how many puffs of short-acting bronchodilator (e.g., Ventolin) have you used each day?

Each question is answered by the subject on a scale of 0-6, representing the absence of symptoms (score of 0) to severe symptoms (score of 6). The composite score is then the mean of the six scores. Thus a higher composite score reflects more asthmatic symptoms. The questionnaire has been shown to have improved discriminative and evaluative measurement properties than an asthma control diary (27).


   APPENDIX B: MATHEMATICAL DESCRIPTION OF FLOW-INDEPENDENT NO PARAMETERS
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
 APPENDIX A: ASTHMA CONTROL...
 APPENDIX B: MATHEMATICAL...
ACKNOWLEDGMENTS
 REFERENCES
 
The following analytic expressions for the steady-state values of , Daw,NO, and Calv,ss have been previously derived (52) and presented in a slightly different form (45, 51)

(B1)

(B2)

(B3)
where , where k (s-1) is the first-order rate constant that characterizes the rate of chemical consumption by substrates such as superoxide. The {xi}i represents the ratio of the rate of chemical consumption (k; s-1) to the rate of molecular diffusion (Dt,NO/L2; s-1) for NO. The hyperbolic tangent (tanh) is bounded between -1 and 1 and is a monotonically increasing function of its argument. Eq. B1 provides units of milliliters per second for Daw,NO that are equivalent in magnitude to picoliters per second per ppb.


   ACKNOWLEDGMENTS
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
 APPENDIX A: ASTHMA CONTROL...
 APPENDIX B: MATHEMATICAL...
 ACKNOWLEDGMENTS
REFERENCES
 
We thank Dr. Philip Silkoff at the National Jewish Medical and Research Center for insightful comments during the preparation of this manuscript, and the General Clinical Research Center at the University of California, Irvine, and Dr. Arthur Gelb for assistance in recruiting subjects with asthma.

GRANTS

This work was supported by National Institutes of Health Grants HL-60636, HD-23969, and RR-00827.


   FOOTNOTES
 

Address for reprint requests and other correspondence: S. C. George, Dept. of Chemical Engineering and Materials Science, 916 Engineering Tower, Univ. of California, Irvine, CA 92697-2575 (E-mail: scgeorge{at}uci.edu).

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.


   REFERENCES
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
 APPENDIX A: ASTHMA CONTROL...
 APPENDIX B: MATHEMATICAL...
 ACKNOWLEDGMENTS
 REFERENCES
 

  1. Alving K, Weitzberg E, and Lundberg JM. Increased amount of nitric oxide in exhaled air of asthmatics. Eur Respir J 6: 1368-1370, 1993.[Abstract]
  2. American Thoracic Society. Recommendations for standardized procedures for the online and offline measurement of exhaled lower respiratory nitric oxide and nasal nitric oxide in adults and children-1999. Am J Respir Crit Care Med 160: 2104-2117, 1999.[Free Full Text]
  3. Ardawi MS. Glutamine metabolism in the lungs of glucocorticoid-treated rats. Clin Sci (Lond) 81: 37-42, 1991.[Medline]
  4. Bird RB, Stewart WE, and Lightfoot EN. Transport Phenomena. New York: Wiley, 1960.
  5. Corradi M, Montuschi P, Donnelly LE, Pesci A, Kharitonov SA, and Barnes PJ. Increased nitrosothiols in exhaled breath condensate in inflammatory airway diseases. Am J Respir Crit Care Med 163: 854-858, 2001.[Abstract/Free Full Text]
  6. De Sanctis GT, MacLean JA, Hamada K, Mehta S, Scott JA, Jiao A, Yandava CN, Kobzik L, Wolyniec WW, Fabian AJ, Venugopal CS, Grasemann H, Huang PL, and Drazen JM. Contribution of nitric oxide synthases 1, 2, and 3 to airway hyperresponsiveness and inflammation in a murine model of asthma. J Exp Med 189: 1621-1630, 1999.[Abstract/Free Full Text]
  7. De Sanctis GT, Mehta S, Kobzik L, Yandava C, Jiao A, Huang PL, and Drazen JM. Contribution of type I NOS to expired gas NO and bronchial responsiveness in mice. Am J Physiol Lung Cell Mol Physiol 273: L883-L888, 1997.[Abstract/Free Full Text]
  8. DuBois AB, Kelley PM, Douglas JS, and Mohsenin V. Nitric oxide production and absorption in trachea, bronchi, bronchioles, and respiratory bronchioles of humans. J Appl Physiol 86: 159-167, 1999.[Abstract/Free Full Text]
  9. Dweik RA, Comhair SA, Gaston B, Thunnissen FB, Farver C, Thomassen MJ, Kavuru M, Hammel J, Abu-Soud HM, and Erzurum SC. NO chemical events in the human airway during the immediate and late antigen-induced asthmatic response. Proc Natl Acad Sci USA 98: 2622-2627, 2001.[Abstract/Free Full Text]
  10. Dweik RA, Lewis M, Kavuru M, Buhrow L, Erzurum SC, and Thomassen MJ. Inhaled corticosteroids and beta-agonists inhibit oxidant production by bronchoalveolar lavage cells from normal volunteers in vivo. Immunopharmacology 37: 163-166, 1997.[CrossRef][ISI][Medline]
  11. Fang K, Johns R, Macdonald T, Kinter M, and Gaston B. S-nitrosoglutathione breakdown prevents airway smooth muscle relaxation in the guinea pig. Am J Physiol Lung Cell Mol Physiol 279: L716-L721, 2000.[Abstract/Free Full Text]
  12. Gaston B, Drazen JM, Loscalzo J, and Stamler JS. The biology of nitrogen oxides in the airways. Am J Respir Crit Care Med 149: 538-551, 1994.[Abstract]
  13. Gaston B, Ratjen F, Vaughan JW, Malhotra NR, Canady RG, Snyder AH, Hunt JF, Gaertig S, and Goldberg JB. Nitrogen redox balance in the cystic fibrosis airway: effects of antipseudomonal therapy. Am J Respir Crit Care Med 165: 387-390, 2002.[Abstract/Free Full Text]
  14. Gaston B, Sears S, Woods J, Hunt J, Ponaman M, McMahon T, and Stamler JS. Bronchodilator S-nitrosothiol deficiency in asthmatic respiratory failure. Lancet 351: 1317-1319, 1998.[CrossRef][ISI][Medline]
  15. Girgis RE, Gugnani MK, Abrams J, and Mayes MD. Partitioning of alveolar and conducting airway nitric oxide in scleroderma lung disease. Am J Respir Crit Care Med 165: 1587-1591, 2002.[Abstract/Free Full Text]
  16. Grasemann H, Yandava CN, and Drazen JM. Neuronal NO synthase (NOS1) is a major candidate gene for asthma. Clin Exp Allergy 29, Suppl 4: 39-41, 1999.[CrossRef]
  17. Grasemann H, Yandava CN, Storm van's Gravesande K, Deykin A, Pillari A, Ma J, Sonna LA, Lilly C, Stampfer MJ, Israel E, Silverman EK, and Drazen JM. A neuronal NO synthase (NOS1) gene polymorphism is associated with asthma. Biochem Biophys Res Commun 272: 391-394, 2000.[CrossRef][ISI][Medline]
  18. Guo FH, Comhair SA, Zheng S, Dweik RA, Eissa NT, Thomassen MJ, Calhoun W, and Erzurum SC. Molecular mechanisms of increased nitric oxide (NO) in asthma: evidence for transcriptional and post-translational regulation of NO synthesis. J Immunol 164: 5970-5980, 2000.[Abstract/Free Full Text]
  19. Gustafsson LE, Leone AM, Persson MG, Wiklund NP, and Moncada S. Endogenous nitric oxide is present in the exhaled air of rabbits, guinea pigs and humans. Biochem Biophys Res Commun 181: 852-857, 1991.[CrossRef][ISI][Medline]
  20. Hogman M, Drca N, Ehrstedt C, and Merilainen P. Exhaled nitric oxide partitioned into alveolar, lower airways and nasal contributions. Respir Med 94: 985-991, 2000.[CrossRef][ISI][Medline]
  21. Hogman M, Hjoberg J, Mork AC, Roomans GM, and Anderson SD. Dry gas hypernea changes airway reactivity and ion content of rabbit tracheal wall. Respir Physiol 109: 65-72, 1997.[CrossRef][ISI][Medline]
  22. Hogman M, Holmkvist T, Wegener T, Emtner M, Andersson M, Hedenstrom H, and Merilainen P. Extended NO analysis applied to patients with COPD, allergic asthma and allergic rhinitis. Respir Med 96: 24-30, 2002.[CrossRef][ISI][Medline]
  23. Hunt JF, Erwin E, Palmer L, Vaughan J, Malhotra N, Platts-Mills TA, and Gaston B. Expression and activity of pH-regulatory glutaminase in the human airway epithelium. Am J Respir Crit Care Med 165: 101-107, 2002.[Abstract/Free Full Text]
  24. Hunt JF, Fang K, Malik R, Snyder A, Malhotra N, Platts-Mills TA, and Gaston B. Endogenous airway acidification. Implications for asthma pathophysiology. Am J Respir Crit Care Med 161: 694-699, 2000.[Abstract/Free Full Text]
  25. Hyde RW, Geigel EJ, Olszowka AJ, Krasney JA, Forster RE 2nd, Utell MJ, and Frampton MW. Determination of production of nitric oxide by lower airways of humans—theory. J Appl Physiol 82: 1290-1296, 1997.[Abstract/Free Full Text]
  26. Jones SL, Kittelson J, Cowan JO, Flannery EM, Hancox RJ, McLachlan CR, and Taylor DR. The predictive value of exhaled nitric oxide measurements in assessing changes in asthma control. Am J Respir Crit Care Med 164: 738-743, 2001.[Abstract/Free Full Text]
  27. Juniper EF, O'Byrne PM, Ferrie PJ, King DR, and Roberts JN. Measuring asthma control. Clinic questionnaire or daily diary? Am J Respir Crit Care Med 162: 1330-1334, 2000.[Abstract/Free Full Text]
  28. Juniper EF, O'Byrne PM, Guyatt GH, Ferrie PJ, and King DR. Development and validation of a questionnaire to measure asthma control. Eur Respir J 14: 902-907, 1999.[Abstract/Free Full Text]
  29. Kharitonov S, Alving K, and Barnes PJ. Exhaled and nasal nitric oxide measurements: recommendations. The European Respiratory Society Task Force. Eur Respir J 10: 1683-1693, 1997.[CrossRef][ISI][Medline]
  30. Kharitonov SA, Yates D, Robbins RA, Logan-Sinclair R, Shinebourne EA, and Barnes PJ. Increased nitric oxide in exhaled air of asthmatic patients. Lancet 343: 133-135, 1994.[CrossRef][ISI][Medline]
  31. Kharitonov SA, Yates DH, and Barnes PJ. Inhaled glucocorticoids decrease nitric oxide in exhaled air of asthmatic patients. Am J Respir Crit Care Med 153: 454-457, 1996.[Abstract]
  32. Kharitonov SA, Yates DH, Chung KF, and Barnes PJ. Changes in the dose of inhaled steroid affect exhaled nitric oxide levels in asthmatic patients. Eur Respir J 9: 196-201, 1996.[Abstract]
  33. Klasen S, Hammermann R, Fuhrmann M, Lindemann D, Beck KF, Pfeilschifter J, and Racke K. Glucocorticoids inhibit lipopolysaccharide-induced up-regulation of arginase in rat alveolar macrophages. Br J Pharmacol 132: 1349-1357, 2001.[CrossRef][ISI]
  34. Knowles RG, Salter M, Brooks SL, and Moncada S. Anti-inflammatory glucocorticoids inhibit the induction by endotoxin of nitric oxide synthase in the lung, liver and aorta of the rat. Biochem Biophys Res Commun 172: 1042-1048, 1990.[CrossRef][ISI][Medline]
  35. Kostikas K, Papatheodorou G, Ganas K, Psathakis K, Panagou P, and Loukides S. pH in expired breath condensate of patients with inflammatory airway diseases. Am J Respir Crit Care Med 165: 1364-1370, 2002.[Abstract/Free Full Text]
  36. Kotaru C, Skowronski M, Coreno A, and McFadden ER Jr. Inhibition of nitric oxide synthesis attenuates thermally induced asthma. J Appl Physiol 91: 703-708, 2001.[Abstract/Free Full Text]
  37. Lehtimaki L, Kankaanranta H, Saarelainen S, Hahtola P, Jarvenpaa R, Koivula T, Turjanmaa V, and Moilanen E. Extended exhaled NO measurement differentiates between alveolar and bronchial inflammation. Am J Respir Crit Care Med 163: 1557-1561, 2001.[Abstract/Free Full Text]
  38. Lehtimaki L, Kankaanranta H, Saarelainen S, Turjanmaa V, and Moilanen E. Inhaled fluticasone decreases bronchial but not alveolar nitric oxide output in asthma. Eur Respir J 18: 635-639, 2001.[Abstract/Free Full Text]
  39. Lilly CM, Stamler JS, Gaston B, Meckel C, Loscalzo J, and Drazen JM. Modulation of vasoactive intestinal peptide pulmonary relaxation by NO in tracheally superfused guinea pig lungs. Am J Physiol Lung Cell Mol Physiol 265: L410-L415, 1993.[Abstract/Free Full Text]
  40. Lim S, Jatakanon A, John M, Gilbey T, O'Connor BJ, Chung KF, and Barnes PJ. Effect of inhaled budesonide on lung function and airway inflammation. Assessment by various inflammatory markers in mild asthma. Am J Respir Crit Care Med 159: 22-30, 1999.[Abstract/Free Full Text]
  41. Massaro AF, Gaston B, Kita D, Fanta C, Stamler JS, and Drazen JM. Expired nitric oxide levels during treatment of acute asthma. Am J Respir Crit Care Med 152: 800-803, 1995.[Abstract]
  42. Pietropaoli AP, Perillo IB, Torres A, Perkins PT, Frasier LM, Utell MJ, Frampton MW, and Hyde RW. Simultaneous measurement of nitric oxide production by conducting and alveolar airways of humans. J Appl Physiol 87: 1532-1542, 1999.[Abstract/Free Full Text]
  43. Redington AE, Meng QH, Springall DR, Evans TJ, Creminon C, Maclouf J, Holgate ST, Howarth PH, and Polak JM. Increased expression of inducible nitric oxide synthase and cyclo-oxygenase-2 in the airway epithelium of asthmatic subjects and regulation by corticosteroid treatment. Thorax 56: 351-357, 2001.[Abstract/Free Full Text]
  44. Shin HW, Rose-Gottron CM, Perez F, Cooper DM, Wilson AF, and George SC. Flow-independent nitric oxide exchange parameters in healthy adults. J Appl Physiol 91: 2173-2181, 2001.[Abstract/Free Full Text]
  45. Shin HW, Rose-Gottron CM, Sufi RS, Perez F, Cooper DM, Wilson AF, and George SC. Flow-independent nitric oxide exchange parameters in cystic fibrosis. Am J Respir Crit Care Med 165: 349-357, 2002.[Abstract/Free Full Text]
  46. Silkoff PE, McClean PA, Caramori M, Slutsky AS, and Zamel N. A significant proportion of exhaled nitric oxide arises in large airways in normal subjects. Respir Physiol 113: 33-38, 1998.[CrossRef][ISI][Medline]
  47. Silkoff PE, McClean PA, Slutsky AS, Furlott HG, Hoffstein E, Wakita S, Chapman KR, Szalai JP, and Zamel N. Marked flow-dependence of exhaled nitric oxide using a new technique to exclude nasal nitric oxide. Am J Respir Crit Care Med 155: 260-267, 1997.[Abstract]
  48. Silkoff PE, Sylvester JT, Zamel N, and Permutt S. Airway nitric oxide diffusion in asthma. Role in pulmonary function and bronchial responsiveness. Am J Respir Crit Care Med 161: 1218-1228, 2000.[Abstract/Free Full Text]
  49. Snyder AH, McPherson ME, Hunt JF, Johnson M, Stamler JS, and Gaston B. Acute effects of aerosolized S-nitrosoglutathione in cystic fibrosis. Am J Respir Crit Care Med 165: 922-926, 2002.[Abstract/Free Full Text]
  50. Springall D, Meng Q, Redington A, Howarth PH, Evans TJ, and Polak JM. Inducible nitric oxide synthase in asthmatic airway epithelium is reduced by corticosteroid therapy (Abstract). Am J Respir Crit Care Med 151: A833, 1995.
  51. Tsoukias NM and George SC. Impact of volume-dependent alveolar diffusing capacity on exhaled nitric oxide concentration. Ann Biomed Eng 29: 731-739, 2001.[CrossRef][ISI][Medline]
  52. Tsoukias NM and George SC. A two-compartment model of pulmonary nitric oxide exchange dynamics. J Appl Physiol 85: 653-666, 1998.[Abstract/Free Full Text]
  53. Tsoukias NM, Shin HW, Wilson AF, and George SC. A single-breath technique with variable flow rate to characterize nitric oxide exchange dynamics in the lungs. J Appl Physiol 91: 477-487, 2001.[Abstract/Free Full Text]
  54. Tsoukias NM, Tannous Z, Wilson AF, and George SC. Single-exhalation profiles of NO and CO2 in humans: effect of dynamically changing flow rate. J Appl Physiol 85: 642-652, 1998.[Abstract/Free Full Text]
  55. Wechsler ME, Grasemann H, Deykin A, Silverman EK, Yandava CN, Israel E, Wand M, and Drazen JM. Exhaled nitric oxide in patients with asthma: association with NOS1 genotype. Am J Respir Crit Care Med 162: 2043-2047, 2000.[Abstract/Free Full Text]
  56. West JB. Pulmonary Pathophysiology—The Essentials. Baltimore, MD: Williams & Wilkins, 1995.
  57. Yates DH, Kharitonov SA, Thomas PS, and Barnes PJ. Endogenous nitric oxide is decreased in asthmatic patients by an inhibitor of inducible nitric oxide synthase. Am J Respir Crit Care Med 154: 247-250, 1996.[Abstract]



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