![](https://assets.reachrx.ai/images/10.2147%E2%98%8Djaa.s555317/_page_0_Picture_2.jpeg) ![](https://assets.reachrx.ai/images/10.2147%E2%98%8Djaa.s555317/_page_0_Picture_3.jpeg) O R I G I N A L R E S E A R C H # Pulmonary Rehabilitation Reduces Airway Inflammation in Asthma Patients with High FeNO Levels Rainer Gloeckl [1](#page-0-0),[2](#page-0-0) , Daniela Kroll[1](#page-0-0),[2](#page-0-0) , Gaffar Abdulleyev[1](#page-0-0) , Tessa Schneeberger[1](#page-0-0),[2](#page-0-0) , Inga Jarosch[1,2](#page-0-0) , Christoph Nell[2](#page-0-0) , Andreas Rembert Koczulla [1–](#page-0-0)[3](#page-0-1) 1 Institute for Pulmonary Rehabilitation Research, Schön Klinik Berchtesgadener Land, Schönau am Königssee, Germany; ² Department of Pulmonary Rehabilitation, German Center for Lung Research (DZL), Philipps-University, Marburg, Germany; ³ Teaching Hospital, Paracelsus Medical University, Salzburg, Austria Correspondence: Andreas Rembert Koczulla, Department of Pulmonary Rehabilitation, German Center for Lung Research (DZL), Philipps-University Marburg, Biegenstrasse 10, Marburg, 35037, Germany, Email koczulla@med.uni-marburg.de **Purpose:** Pulmonary rehabilitation (PR) is a well-established non-pharmacological intervention for patients with asthma. While PR improves asthma control and exercise capacity, its impact on airway inflammation remains unclear. Fractional exhaled nitric oxide (FeNO) is a biomarker of type 2 airway inflammation, yet its response to PR has not been extensively studied. Therefore, aim of this study was to investigate diurnal variations in FeNO and how FeNO develops during PR in asthmatic patients with different levels of FeNO. **Patients and Methods:** This prospective, single-center study investigated FeNO changes in asthma patients undergoing a comprehensive three-week inpatient PR program. The study observation period covered 15 consecutive days of FeNO measurements. Patients were divided into three subgroups according to their initial FeNO measurement: low (<25 ppb), intermediate (25–50 ppb) and high (>50 ppb). FeNO was measured three times a day, along with assessments of asthma control, lung function, blood eosinophils and exercise capacity before and after PR. **Results:** Sixty-two patients were included. Only patients in the high FeNO group (n=22) showed a significant 40% decrease in FeNO from pre to post PR (93±29 ppb to 56±27 ppb, p<0.001), independent of medication changes. FeNO levels of patients with low (17±8 ppb to 16±10 ppb) and intermediate levels (39±12 ppb to 30±10 ppb) remained unchanged. Asthma control and exercise capacity improved across all three subgroups, with the greatest gains observed in patients with initially high FeNO. No significant FeNO changes occurred in the low and intermediate FeNO groups. **Conclusion:** PR significantly reduced FeNO levels in asthma patients with high baseline FeNO, suggesting an anti-inflammatory effect beyond pharmacological treatment. These findings highlight the role of PR as a complementary strategy in asthma management, particularly for patients with persistent airway inflammation despite optimized medication. Further randomized trials are needed to confirm these results and explore long-term effects. **Keywords:** fractional exhaled nitric oxide, type 2 inflammation, biomarker, eosinophil, asthma control, diurnal variation #### **Introduction** Asthma is a chronic inflammatory airway disease characterized by variable airflow obstruction and bronchial hyperresponsiveness.[1](#page-8-0) While inhaled corticosteroids (ICS) remain the cornerstone of anti-inflammatory therapy, a subset of patients continues to exhibit elevated markers of airway inflammation, highlighting the need for complementary non-pharmacological interventions[.2](#page-8-1) Pulmonary rehabilitation (PR) is a multidisciplinary intervention comprising structured exercise training, respiratory physiotherapy, patient education, behavioural modifications and others. Many studies have shown that PR improves exercise capacity, quality of life and asthma control.[3](#page-8-2) However, there is only limited evidence on the role #### **Graphical Abstract** ![](https://assets.reachrx.ai/images/10.2147%E2%98%8Djaa.s555317/_page_1_Figure_2.jpeg) of PR on airway inflammation.[4](#page-8-3) One possible mechanism by which PR influences airway inflammation may be related to the beneficial effects of exercise training in improving immune cell function, leading to reduced release of inflammatory mediators.[5](#page-8-4)[,6](#page-8-5) In addition, PR may reduce airway inflammation through improved airway clearance, reduced oxidative stress, and modulation of inflammatory signaling pathways independent of medication.[7](#page-9-0) To date, there is a lack of biomarkers to monitor the inflammatory response to PR, particularly in asthmatics with high levels of airway inflammation.[4](#page-8-3) Type 2 inflammation, which characterizes the Th2-high asthma endotype, plays a major role in driving airway eosinophilia and response to targeted therapies.[8](#page-9-1) Fractional exhaled nitric oxide (FeNO) is a well-established non-invasive biomarker of type 2 airway inflammation[.9](#page-9-2) Elevated FeNO levels are associated with eosinophilic airway inflammation and corticosteroid responsiveness, making it a valuable tool for assessing disease activity and guiding pharmacological treatment decisions.[10](#page-9-3) Furthermore, a recent meta-analysis has shown that FeNO-guided asthma treatment probably results in fewer exacerbations[.11](#page-9-4) Therefore, monitoring FeNO could be a valuable tool for managing asthma, both pharmacologically and non-pharmacologically. The role of FeNO in monitoring non-pharmacological interventions like PR remains unclear. Since PR may reduce airway inflammation through mechanisms independent of corticosteroids, FeNO could help assess these effects. Therefore, the aim of this study was to evaluate FeNO changes during a PR program in patients with asthma and different levels of airway inflammation. #### **Materials and Methods** #### Study Design This prospective, single-center study investigated changes in FeNO during PR in patients with asthma between May 2021 and December 2024. This observational study was approved by the responsible ethics committee of the Philipps-University Marburg, Germany and registered at clinicaltrials.gov (NCT06751550). Participants gave informed consent to participate in the study prior to enrolment and our study complies with the Declaration of Helsinki. #### Study Participants Patients with a confirmed diagnosis of bronchial asthma (all severity levels) were eligible for inclusion. Exclusion criteria comprised a primary diagnosis of COPD without an asthmatic component, lack of consent for study participation, and inability to independently use the FeNO measurement device. All asthma patients admitted to inpatient pulmonary rehabilitation at the Schön Klinik Berchtesgadener Land in Schönau, Germany, were screened for study eligibility and invited to participate. #### Pulmonary Rehabilitation Program All participants underwent a standardized, multidisciplinary, three-week inpatient PR program, as this is the standard setting in the German healthcare system. The comprehensive program included optimization of drug therapy (in accordance with current *Global Strategy for Asthma Management and Prevention* (GINA) guidelines, if necessary), respiratory physiotherapy, patient education on disease management, nutritional counseling, relaxation skills and exercise training (more details are described elsewhere).[12](#page-9-5) The exercise regimen consisted of a combination of endurance and resistance training (45 minutes total per session) on five to six days per week, complemented by group activities such as nordic walking and aqua jogging. ## FeNO Measurements FeNO levels were measured using the Vivatmo *me* device (Healthcare Solutions GmbH, Waiblingen, Germany). Each patient was provided with a personal device for self-administered measurements throughout the PR period. Daily FeNO assessments were performed at fixed time points (9 am, 1 pm, and 5 pm) using a new mouthpiece with every measurement, taking into account potential confounding factors such as recent medication use, dietary habits, and PR activities. Measurements were conducted according to manufacturer guidelines, requiring patients to exhale into the device with a steady flow and pressure for ten seconds ([eFigure 1\)](https://www.dovepress.com/article/supplementary_file/555317/555317%20Supplementary%20Data.docx). All valid measurements were digitally saved on the devices. The minimal clinically important difference (MCID) for FeNO in asthma is suggested to be a 10 ppb reduction for baseline levels between 25–50 ppb and a 20% reduction for levels above 50 ppb.[9](#page-9-2) #### Outcomes Comprehensive clinical assessments were performed at baseline and at the end of the PR program, including asthma control test (ACT), body plethysmography, blood sample parameters (eosinophils, total immune globulin, and C-reactive protein (CRP)) and exercise tests (six-minute walk test,[13](#page-9-6) incremental cycle test). ## Sample Size and Statistical Analyses A power analysis based on an average change of 4.5±10.4 ppb in the FeNO levels, observed during a training program in asthma patients, resulted in a sample size of 62 patients for an α=5% and β=5%.[14](#page-9-7) Baseline data are presented as mean ± SD for continuous variables and as frequencies and percentages for nominal data. Baseline differences between the three subgroups were assessed as follows. A chi-squared test was used to compare categorical variables. One-way ANOVA was used for metric data (if normally distributed). In the case of variance inhomogeneity (Levene's test), Welch's ANOVA was used. Missing data were handled by complete-case analysis, and no imputation methods were applied. Parameters before and after PR were analyzed by paired *t*-test if normally distributed. If normality was violated, a Wilcoxon signed-rank test was used. For variables measured at three or more time points, a repeated measures ANOVA or a mixed-effects model was used as appropriate. For significant results, post-hoc pairwise comparisons were performed with Bonferroni correction. A two-tailed significance level of α = 0.05 was used for all statistical tests. #### **Results** Sixty-two out of 73 eligible patients were included in the study. Eleven patients were excluded due to the following reasons: 9 refused to participate, and 2 were not able to perform the FeNO manoeuvre. #### Baseline Characteristics Patients were divided into subgroups according to their initial FeNO measurement on PR admission: low FeNO <25ppb (n=28), intermediate FeNO of 25–50ppb (n=12), or high FeNO >50ppb (n=22) [\(Table 1](#page-3-0)). General characteristics, asthma **Table 1** Baseline Characteristics | | All | Subgroup
with
Baseline
FeN0 <25ppb | Subgroup with
Baseline
FeN0 25–50ppb | Subgroup
with
Baseline
FeN0 >50ppb | p (Between
Subgroups) | |------------------------------------------------------|---------------|---------------------------------------------|--------------------------------------------|---------------------------------------------|--------------------------| | General Characteristics | | | | | | | n | 62 | 28 | 12 | 22 | | | Age, years | 55.5 ± 12.5 | 51.6 ± 12.0 | 60.3 ± 6.9 | 57.7 ± 14.4 | 0.078 | | Gender, female (%) | 37 (60.0%) | 11 (39.2%) | 6 (50.0%) | 8 (36.3%) | 0.730 | | BMI, kg/m2 | 27.2 ± 5.4 | 27.1 ± 5.1 | 27.0 ± 4.6 | 27.4 ± 6.2 | 0.977 | | Smoking status: never/ex-smoker/current/unknown, n | 50/1/8/3 | 23/1/2/2 | 9/0/3/0 | 18/0/3/1 | 0.290 | | Co-morbidities, n | 1.9 ± 1.6 | 1.5 ± 1.5 | 2.6 ± 1.7 | 2.0 ± 1.7 | 0.120 | | Asthma Phenotypes (according to GINA classification) | | | | | | | Allergic asthma | 26 (41.9%) | 14 (50.0%) | 4 (33.3%) | 8 (36.4%) | 0.50 | | Non-allergic asthma | 23 (37.1%) | 8 (28.6%) | 6 (50.0%) | 9 (40.9%) | 0.39 | | Late-onset asthma | 12 (19.4%) | 5 (17.9%) | 2 (16.7%) | 5 (22.7%) | 0.88 | | Obesity-associated asthma | 1 (1.6%) | 1 (3.6%) | 0 (0%) | 0 (0%) | 0.54 | | Asthma Control | | | | | | | Asthma control test, score | 15.9 ± 4.8 | 16.7 ± 4.5 | 15.8 ± 5.2 | 15.1 ± 4.9 | 0.561 | | Controlled asthma according to ACT, n (%) | 15 (24.2%) | 6 (21.4%) | 3 (25.0%) | 6 (27.2%) | 0.780 | | Airway Inflammation | | | | | | | FeNO, ppb | 47 ± 13 | 17 ± 8 | 39 ± 12 | 96 ± 64 | <0.001 | | Drug Therapy | | | | | | | ICS, yes | 60 (96.8%) | 27 (96.4%) | 12 (100%) | 21 (95.4%) | 0.77 | | OCS, yes | 4 (6.5%) | 1 (3.6%) | 1 (8.3%) | 2 (9.0%) | 0.70 | | LABA, yes | 54 (87,1% | 25 (89.3%) | 11 (91.7%) | 18 (81.8%) | 0.64 | | LAMA, yes | 32 (51.6%) | 13 (52.0%) | 5 (41.7%) | 14 (63.6%) | 0.36 | | Biological therapy, yes | 13 (21.0%) | 2 (7.1%) | 1 (8.3%) | 10 (45.4%) | 0.002 | | Lung Function | | | | | | | FEV1, %predicted | 76.7 ± 23.1 | 83.3 ± 20.7 | 67.1 ± 20.7 | 73.7 ± 25.6 | 0.094 | | FEV1/FVC, % | 73.2 ±17.3 | 75.1 ± 17.8 | 65.5 ± 17.3 | 74.9 ± 16.2 | 0.230 | | PEF, %predicted | 83.5 ± 27.2 | 90.8 ± 26.9 | 83.3 ± 29.8 | 74.3 ± 24.4 | 0.100 | | MEF25, %predicted | 117.9 ± 72.2 | 126.5 ± 74.1 | 83.1 ± 49.4 | 125.8 ± 77.1 | 0.180 | | Rtot, %predicted | 148.1 ± 115.3 | 100.3 ± 47.9 | 123.4 ± 62.7 | 123.8 ± 75.3 | 0.340 | | Blood Sample Parameters | | | | | | | Eosinophils, absolute | 261 ± 215 | 216 ± 120 | 287 ± 156 | 304 ± 311 | 0.320 | | Eosinophils, % | 3.9 ± 3.3 | 3.3 ± 1.9 | 4.2 ± 2.0 | 4.6 ± 4.9 | 0.410 | | IgE, IU/mL | 354 ± 484 | 322 ± 575 | 383 ± 342 | 385 ± 418 | 0.140 | | CRP | 6.1 ± 10.7 | 5.6 ± 10.2 | 3.7 ± 5.6 | 7.9 ± 13.2 | 0.530 | | Exercise Performance | | | | | | | | | | | | | | Peak work rate, W | 113 ± 53 | 116 ±50 | 89 ±24 | 121 ± 64 | 0.350 | **Abbreviations**: ACT, Asthma Control Test; BMI, Body Mass Index; FEV1, Forced Expiratory Volume in 1 Second; FVC, Forced vital capacity; GINA, Global Strategy for Asthma Management and Prevention; ICS, Inhaled Corticosteroids; IgE, immune globulin E; LABA, Long-Acting Beta-Agonist; LAMA, Long-Acting Muscarinic Antagonist; MEF25, Maximal Expiratory Flow at 25% of Forced Vital Capacity; OCS, Oral Corticosteroids; PEF, Peak Expiratory Flow; Rtot, Total Airway Resistance. phenotypes (according to GINA classification),[1](#page-8-0) asthma control, lung function, blood parameters, and exercise capacity did not differ significantly between the three subgroups. The only significant difference in our sample was that patients with high FeNO levels were significantly more likely to be treated with biological therapy. Baseline FeNO levels correlated significantly with baseline blood eosinophils [\(eFigure 2\)](https://www.dovepress.com/article/supplementary_file/555317/555317%20Supplementary%20Data.docx). ## Diurnal Variation of FeNO Levels To assess the influence of diurnal variation of FeNO levels, all available data from morning, noon, and evening measurements were pooled within the three subgroups [\(Figure 1\)](#page-4-0). FeNO levels were fairly stable throughout the day. Only the group with high FeNO baseline levels showed significantly lower FeNO levels in the evening (61±22ppb) compared to noon (68±24ppb; p=0.001).This difference of 10.3% is below the suggested MCID of a 20% change.[9](#page-9-2) # Changes in FeNO Over the Course of PR Although the study included patients over the course of a three-week PR programme, we were only able to collect FeNO data for 15 consecutive days. This was due to a time lag in the onboarding of the study participants. Patients with low and intermediate FeNO levels had stable FeNO levels over 15 days. However, patients with high baseline FeNO levels had a significant (p<0.001) 40% decrease in mean FeNO levels by from 93±29 ppb (day 1) to 56±27 ppb (day 15), with the greatest decrease occurring within the first few days of FeNO measurement ([Figure 2](#page-4-1)). This difference was not ![](https://assets.reachrx.ai/images/10.2147%E2%98%8Djaa.s555317/_page_4_Figure_6.jpeg) **Figure 1** Diurnal variation of FeNO levels between morning (9am), noon (12pm), and evening (5pm) in patients with asthma. Each point represents mean values (±95% CI) collected over 15 consecutive days during an in-patient pulmonary rehabilitation programme in patients with high (>50ppb, ▲), intermediate (25–50ppb, ■) or low baseline FeNO levels (<25ppb, ●), \*\*p=0.001. ![](https://assets.reachrx.ai/images/10.2147%E2%98%8Djaa.s555317/_page_4_Figure_8.jpeg) **Figure 2** Changes in FeNO levels over 15 consecutive days during an in-patient pulmonary rehabilitation programme. FeNO values are the average of all measurements per day (average of morning, noon, and evening data) in patients with high (>50ppb, ▲), intermediate (25–50ppb, ■) or low baseline FeNO levels (<25ppb, ●). Data are presented as mean values (±95% CI). \*\*\*p<0.001, ns = non-significant. **Table 2** Changes from Admission to Discharge of a 3-week Inpatient Pulmonary Rehabilitation Program in Asthmatic Patients | | Subgroup with Baseline FeN0 <25ppb
(n=28) | | | Subgroup with Baseline FeN0 25–50ppb
(n=12) | | | Subgroup with Baseline FeN0 >50ppb
(n=22) | | | |------------------------------------------------|----------------------------------------------|------------|-----------|------------------------------------------------|------------|-----------|----------------------------------------------|------------|------------| | | Pre | Post | Delta | Pre | Post | Delta | Pre | Post | Delta | | Airway Inflammation | | | | | | | | | | | FeNOa
, ppb | 17±8 | 16±10 | −1±7 | 39±12 | 30±10 | −9±6** | 93±29 | 56±27 | −37±18*** | | Asthma Control | | | | | | | | | | | ACT, score | 16.7±4.8 | 18.4±4.0 | 1.8±3.1* | 15.2±5.5 | 18.0±4.3 | 2.8±3.8* | 15.2±5.1 | 17.9±4.3 | 2.7±3.4** | | Uncontrolled asthma
according to ACT, n (%) | 17 (61%) | 13 (46%) | −4 (−15%) | 9 (75%) | 6 (50%) | −3 (−25%) | 15 (68%) | 10 (45%) | −5 (−23%) | | Lung Function | | | | | | | | | | | FEV1, %predicted | 81.4±20.8 | 83.9±21.7 | 2.5±8.2 | 65.5±21.0 | 69.3±19.2 | 3.8±4.9* | 73.1±26.0 | 76.8±24.1 | 3.7±16.1 | | FEV1/FVC, % | 77.3±11.5 | 76.7±10.7 | −1.4±6.8 | 64.7±18.6 | 65.3±15.9 | 0.6±5.6 | 74.6±16.5 | 76.7±13.6 | 2.1±7.0 | | PEF, %predicted | 87.8±25.7 | 92.6±27.2 | 4.8±13.9 | 83.2±32.0 | 86.1±31.6 | 2.9±10.7 | 73.6±24.8 | 84.3±24.7 | 10.7±20.2* | | MEF25, %predicted | 124.3±72.1 | 125.8±56.9 | 1.5±42.8 | 84.0±49.7 | 85.9±39.8 | 1.9±16.5 | 123.0±77.8 | 132.4±85.1 | 9.4±42.5 | | Rtot, %predicted | 105.4±48.1 | 115.3±64.7 | 9.9±33.8 | 125.8±68.1 | 122.3±54.4 | −3.4±37.7 | 126.2±76.3 | 111.0±56.5 | −15.3±90.6 | | Blood Sample Parameters | | | | | | | | | | | Eosinophils, absolute | 229±160 | 287±156 | 58±197 | 246±144 | 256±174 | 10±221 | 304±311 | 186±167 | −118±250* | | Eosinophils, % | 3.1±1.4 | 3.5±2.0 | 0.4±1.3 | 4.2±2.1 | 3.5±2.2 | −0.7±1.3 | 4.6±4.9 | 3.1±2.7 | −1.5±3.9* | | IgE | 241±352 | 207±282 | −34±131 | 457±377 | 314±239 | −143±304 | 386±432 | 377±442 | −9±42 | | Exercise Performance | | | | | | | | | | | 6MWD, m | 521±69 | 540±79 | 19±63 | 487±89 | 525±66 | 38±36* | 455±141 | 515±136 | 60±59*** | **Notes**: ^a Data represent pooled mean values (±SD) of morning, noon and evening FeNO measurements from 15 consecutive days during pulmonary rehabilitation. \*p<0.05, \*\*p<0.01, \*\*\*p<0.001 for intra-group pre-post comparisons; bold fonts highlight significant values. **Abbreviations**: 6MWD, 6-minute walk distance; ACT, Asthma Control Test; FeNO, fractional exhaled nitric oxide; FEV1, Forced Expiratory Volume in 1 Second; FVC, Forced vital capacity; IgE, Immunoglobulin E; MEF25, Maximal Expiratory Flow at 25% of Forced Vital Capacity; PEF, Peak Expiratory Flow; Rtot, Total Airway Resistance. significantly different whether patients received a change in medication (n=9) or remained on their existing drug therapy (n=13; [eFigure 3\)](https://www.dovepress.com/article/supplementary_file/555317/555317%20Supplementary%20Data.docx). Over the course of the PR, 4% of the patients in the low FeNO group, 42% in the intermediate FeNO group, and 82% in the high FeNO group reduced their FeNO levels more than the MCID. #### Changes in Other outcomes Over the Course of PR Some patients with uncontrolled asthma converted to controlled asthma (according to ACT scores) after PR in all three subgroups (with rates of 24% in the low baseline FeNO group, 33% in the intermediate group and 33% in the high baseline FeNO group, see [Table 2](#page-5-0)). Most lung function parameters did not change significantly with PR. However, exercise capacity as assessed by 6MWD improved most in the group with the highest FeNO level. In addition to the significant decrease in FeNO in the group with the highest FeNO levels, there was also a significant decrease in blood eosinophils. #### **Discussion** Our study demonstrated that PR significantly reduced FeNO in asthma patients with high baseline FeNO levels (>50 ppb), suggesting that PR may have anti-inflammatory effects in eosinophilic asthma. Previous studies on exercise-based PR programs have consistently shown significant and clinically relevant improvements in outcomes such as asthma control, quality of life, and exercise capacity[.3](#page-8-2) However, no significant effect of exercise training on markers of airway inflammation has been observed so far, as highlighted by a meta-analysis reporting a standardized mean difference of −0.03 (95% CI: −0.41 to 0.36, p=0.89)[.3](#page-8-2) While most studies have not demonstrated FeNO reduction following PR, some exceptions exist. For instance, Mendes et al reported a significant reduction in FeNO from 34 ppb to 27 ppb (p<0.05) after 12 weeks of aerobic endurance training (2x/week for 30 minutes).[15](#page-9-8) However, this change did not exceed the MCID of 10 ppb,[9](#page-9-2) limiting its clinical relevance. Similarly, in our study, no meaningful FeNO reduction was observed in patients with intermediate FeNO levels. Notably, most previous exercise training studies have focused on asthma patients with intermediate FeNO levels (19–45 ppb),[3–5](#page-8-2),[16](#page-9-9) potentially underestimating the anti-inflammatory effects of PR in patients with more pronounced eosinophilic inflammation. To the best of our knowledge, our study is the first to investigate PR-induced FeNO changes specifically in asthma patients with high baseline FeNO levels (>50 ppb) and to differentiate the response to PR according to FeNO baseline levels. This distinction is crucial, as patients with more pronounced eosinophilic inflammation may respond differently to non-pharmacological interventions than those with lower baseline FeNO levels. Patients with low or intermediate FeNO levels probably had less airway inflammation initially, which limited the potential for further reduction. It is also notable that patients in the low and intermediate FeNO level group did not increase their FeNO levels during the observational PR phase, which is an important safety finding. Surprisingly, the significant and clinically relevant FeNO reduction in patients with high baseline FeNO levels occurred regardless of whether they received medication adjustments (including biological therapy) or remained on their existing treatment ([eFigure 3](https://www.dovepress.com/article/supplementary_file/555317/555317%20Supplementary%20Data.docx)). This suggests that PR itself exerts an anti-inflammatory effect, independent of pharmacological interventions. Several mechanisms may explain this finding. First, patients were already participating in PR for several days before formal study enrolment, meaning that "day 1" in our graphs does not represent the very first day of PR. Acute reductions in FeNO after a single exercise session have been demonstrated in physically inactive adults with asthma, with even 30 minutes of treadmill walking leading to measurable decreases in airway inflammation.[17](#page-9-10) Thus, PR components such as exercise training and respiratory physiotherapy may have initiated anti-inflammatory effects by downregulating pro-inflammatory cytokines[5](#page-8-4) such as IL-4, IL-5, and IL-13, and increasing anti-inflammatory mediators, such as IL-10.[7](#page-9-0) These changes likely reduced eosinophilic airway inflammation and inhibit remodeling processes. Exercise training may also modulate immune cell function by promoting regulatory T cell activity and decreasing oxidative stress.[18](#page-9-11) Additionally, enhancing mucociliary clearance has been shown to improve asthma control and reduce airway inflammation.[19](#page-9-12) Learning an optimized inhalation technique may have further contributed to these improvements.[20](#page-9-13) Beyond these physiological effects, the inpatient PR setting itself may have played a role. Lower stress levels associated with the structured PR progra[m21](#page-9-14) and the environmental conditions of the PR center - located in a rural area of the German mountains with low allergen and air pollution levels[22](#page-9-15) - may have further contributed to FeNO reductions. A recent Cochrane review[4](#page-8-3) concluded that PR is likely to be associated with clinically meaningful improvements in functional exercise capacity and quality of life in adults with asthma (moderate certainty of evidence). Our study reinforces this by demonstrating improvements in asthma control and exercise capacity. Importantly, our findings suggest that FeNO may serve as a biomarker to identify patients with high FeNO levels that most likely benefit from PR in terms of reducing airway inflammation – particularly in cases where medication alone does not achieve sufficient reductions in FeNO levels. ## Clinical Implications Although PR led to a significant and clinically relevant reduction in FeNO levels in patients with elevated baseline FeNO, this reduction did not correlate significantly with improvements in asthma control (as measured by ACT scores). The relationship between FeNO and asthma control remains controversial, with some studies reporting significant inverse correlations (r = −0.648)[,23](#page-9-16) while others have found no significant association[.24](#page-9-17) These discrepancies may be attributed to differences in study design, patient characteristics, asthma phenotypes, medication use, and methodological approaches. However, despite these inconsistencies, the GINA guidelines recognize FeNO reduction as an important therapeutic target in asthma management[.1](#page-8-0) A prospective study of 304 patients with persistent asthma found that elevated FeNO levels predicted future uncontrolled asthma at one year.[25](#page-9-18) Additionally, FeNOguided management strategies have been shown to reduce exacerbation rates in adults with asthma.[26](#page-9-19)[,27](#page-9-20) Our study was among the first to use a home-use device for FeNO self-monitoring. Our study demonstrates the practicality and usability of patient self-monitoring, showing that it can easily be integrated into the daily routines of physicians and patients alike. While pharmacological therapy remains central to controlling airway inflammation, our findings support the integration of non-pharmacological interventions such as PR to further reduce airway inflammation – particularly in patients with persistently elevated FeNO levels despite standard treatment. Moreover, digital health interventions have recently demonstrated significant improvements in asthma control without pharmacotherapy.[28](#page-9-21) This highlights the potential of combining digital and rehabilitative strategies to optimize both symptom management and underlying inflammation. #### Diurnal Variability The potential influence of diurnal variability on FeNO measurements is frequently discussed in the literature. While some studies report significantly higher morning FeNO levels compared to evening levels,[29,](#page-9-22)[30](#page-9-23) others find no clinically relevant diurnal variation[.31](#page-9-24) In our study, we did also not observe meaningful diurnal fluctuations in FeNO [\(Figure 1](#page-4-0)). To account for potential variability, we used daily mean values from morning, noon, and evening measurements to analyze FeNO changes throughout the PR program. A sub-analysis confirmed that FeNO trends remained consistent regardless of whether only morning, noon, or evening values were used (see [eFigures 4–6](https://www.dovepress.com/article/supplementary_file/555317/555317%20Supplementary%20Data.docx)). However, we observed that 7 out of 22 patients (31%) with high baseline FeNO levels exhibited diurnal variations exceeding the MCID of 20% ([eFigure 7\)](https://www.dovepress.com/article/supplementary_file/555317/555317%20Supplementary%20Data.docx). While this suggests that individual variability exists, the overall FeNO reduction from baseline to PR discharge was 40%, making it unlikely that natural diurnal fluctuations alone accounted for this change. Instead, this substantial decrease is more plausibly attributed to the PR intervention itself, although the absence of a control group remains a limitation in fully confirming this causal relationship. #### Limitations This study has several limitations. The short duration of FeNO monitoring (15 days) limits conclusions regarding the long-term effects of PR on airway inflammation. While our findings demonstrate a significant FeNO reduction, it remains unclear whether these effects persist beyond the PR period. Additionally, the absence of a control group makes it difficult to definitively attribute the observed FeNO decrease to PR rather than natural variability over time. However, the persistence of significant FeNO reductions in patients who did not alter their medication suggests that PR played a role in this improvement. Another limitation is the potential influence of medication changes, which cannot be fully excluded as a confounding factor. Nevertheless, our subgroup analysis indicates that the reduction in FeNO was independent of treatment modifications. Further, FeNO is a marker of Type 2 inflammation and does not capture other inflammatory pathways, such as neutrophilic inflammation, which may also play a role in asthma pathophysiology. Future research should explore the long-term sustainability of PR-induced FeNO reductions and assess additional biomarkers, including sputum eosinophils, IL-5, and markers of systemic inflammation, to clarify the underlying mechanisms. Randomized controlled trials are also needed to establish a causal relationship between PR and airway inflammation reduction. #### **Conclusion** Our findings align with previous studies demonstrating the beneficial effects of PR on asthma control and exercise performance while providing novel insights into its impact on airway inflammation. To our knowledge, this is the first study to show that PR exerts an anti-inflammatory effect in asthma patients with high baseline FeNO levels, independent of medication changes. These results highlight the potential role of PR as a complementary strategy in asthma management, particularly for patients with persistent airway inflammation despite optimized pharmacological therapy. #### **Declaration of Generative AI and AI-Assisted Technologies in the Writing Process** During the preparation of this work the authors used DeepL Write in order to improve language and readability. After using this tool, the authors reviewed and edited the content as needed and takes full responsibility for the content of the publication. #### **Abbreviations** ACT, Asthma Control Test; ATS, American Thoracic Society; BMI, Body Mass Index; CG, Control Group; COPD, Chronic Obstructive Pulmonary Disease; CRP, C-reactive Protein; FENO, Fractional Exhaled Nitric Oxide; FEV1, Forced Expiratory Volume in 1 Second; FVC, Forced vital capacity; GINA, Global Initiative for Asthma; ICS, Inhaled Corticosteroids; IgE, immune globulin E; LABA, Long-Acting Beta-Agonist; LAMA, Long-Acting Muscarinic Antagonist; MCID, Minimal Clinically Important Difference; MEF25, Maximal Expiratory Flow at 25% of Forced Vital Capacity; OCS, Oral Corticosteroids; PEF, Peak Expiratory Flow; PR, Pulmonary Rehabilitation; Rtot, Total Airway Resistance; SD, Standard Deviation. #### **Data Sharing Statement** De-identified individual participant data and a data dictionary defining each field in the set can be made available to others on approval of a written request to the corresponding author. The request will be evaluated by a committee formed by a subset of co-authors to determine the research value. A data-sharing agreement will be needed. #### **Acknowledgments** This study did not receive any financial funding. However, FeNO measurement devices (Vivatmo me) and consumable materials (mouthpieces) were kindly provided by Bosch Healthcare Solutions GmbH, Waiblingen, Germany. Bosch Healthcare Solutions GmbH had no influence on the study protocol, data collection or data analysis. #### **Author Contributions** All authors made a significant contribution to the work reported, whether that is in the conception, study design, execution, acquisition of data, analysis and interpretation, or in all these areas; took part in drafting, revising or critically reviewing the article; gave final approval of the version to be published; have agreed on the journal to which the article has been submitted; and agree to be accountable for all aspects of the work. ## **Funding** Open access funding partially provided by the Open Access Publishing Fund of Philipps-Universität Marburg, Germany. #### **Disclosure** The authors report no conflicts of interest in this work. #### **References** - 1. GINA-Guideline. 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