Role of arterial hypoxemia and pulmonary mechanics in exercise limitation in adults with cystic fibrosis

High-intensity interval training accelerates oxygen uptake kinetics and improves exercise tolerance for individuals with cystic fibrosis

Background

Exercise training provides benefits for individuals with cystic fibrosis; however, the optimal program is unclear. High-intensity interval training is safe and effective for improving ‘functional capacity’ in these individuals with peak rate of O₂ uptake typically referenced. The ability to adjust submaximal rate of oxygen uptake (V̇O₂ kinetics) might be more important for everyday function because maximal efforts are usually not undertaken. Moreover, the ability of high-intensity training to accelerate V̇O₂ kinetics for individuals with cystic fibrosis could be enhanced with O₂ supplementation during training.

Methods

Nine individuals with cystic fibrosis completed incremental cycling to limit of tolerance followed by 8 weeks of high-intensity interval cycling (2 sessions per week x ~ 45 min per session) either with (n = 5; O2+) or without (AMB) oxygen supplementation (100%). Each session involved work intervals at 70% of peak work rate followed by 60 s of recovery at 35%. For progression, duration of work intervals was increased according to participant tolerance.

Results

Both groups experienced a significant increase in work-interval duration over the course of the intervention (O2+, 1736 ± 141 v. 700 ± 154 s; AMB, 1463 ± 598 v. 953 ± 253 s; P = 0.000); however, the increase experienced by O2+ was greater (P = 0.027). During low-intensity constant-work-rate cycling, the V̇O₂ mean response time was shortened post compared to pre training (O2+, 34 ± 11 v. 44 ± 9 s; AMB, 39 ± 14 v. 45 ± 17 s; P = 0.000) while during high-intensity constant-work-rate cycling, time to exhaustion was increased (O2+, 1628 ± 163 v. 705 ± 133 s; AMB, 1073 ± 633 v. 690 ± 348 s; P = 0.002) and blood [lactate] response was decreased (O2+, 4.5 ± 0.9 v. 6.3 ± 1.4 mmol^. L^− 1; AMB, 4.5 ± 0.6 v. 5.2 ± 1.4 mmol^. L^− 1; P = 0.003). These positive adaptations were similar regardless of gas inspiration during training.

Conclusion

Eight weeks of high-intensity interval training for patients with cystic fibrosis accelerated V̇O₂ kinetics and increased time to exhaustion. This provides some evidence that these patients may benefit from this type of exercise.

Trial registration

This study was retrospectively registered in the ISRTCN registry on 22/06/2019 (#ISRCTN13864650).

Exercise testing in patients with cystic fibrosis-importance of ventilatory parameters

BACKGROUND

Ventilatory parameters obtained during exercise predict survival in several chronic diseases; however, long-term changes in exercise ventilatory parameters in patients with cystic fibrosis (CF) have yet to be examined and potential differences between sexes in CF are unknown.

PURPOSE

We sought to examine the change in exercise ventilatory parameters over time in patients with CF and determine if the change is different between sexes.

METHODS

Exercise capacity (VO₂ peak) and exercise ventilatory parameters (V_E/VO₂ peak, V_E/VCO₂ peak, and V_E/VCO₂ slope) were determined from a maximal cardio-pulmonary test on a cycle ergometer on two visits separated by 39 ± 16 months in 20 patients with CF (10 female, 10 male).

RESULTS

No differences between sexes were observed at visit 1 (all p > 0.05). Overall, exercise ventilatory parameters significantly (p < 0.05) deteriorated between visits, with no change (p > 0.05) in VO₂ peak. Moreover, compared to males, female patients exhibited greater deteriorations in V_E/VO₂ peak (p = 0.001), V_E/VCO₂ peak (p = 0.002), and V_E/VCO₂ slope (p = 0.016) between visits.

CONCLUSIONS

These data in patients with CF indicate that exercise ventilatory parameters decline over time despite no change in VO₂ peak, and female patients exhibit a more rapid deterioration compared to males.

Prognostic significance of pulmonary hypertension in patients with cystic fibrosis: A systematic review and meta-analysis

BACKGROUND AND OBJECTIVE

Pulmonary hypertension (PH) is frequently found in advanced parenchymal lung diseases like cystic fibrosis (CF), but the role played by PH in the clinical outcome of CF patients remains unclear. The aim of this study is to determine the influence of PH on survival in the CF population by meta-analysis.

METHODS

Publications addressing the associations between PH and overall survival (OS) or other clinical characteristics in CF patients were selected from electronic databases. Odds ratios (ORs) or mean differences (MDs) were used to estimate the association between PH and the clinical characteristics. The hazard ratios (HRs) with 95% confidence interval (CI) were abstracted or calculated to evaluate the association between PH and CF survival outcome. Subgroup analyses were also conducted.

RESULTS

Seven studies including 2141 CF patients who met the inclusion criteria were included in our meta-analysis. With respect to clinical features, PH was significantly associated with lower PaO2 (P < .001), higher PaCO2 (P = .02), lower forced expiratory volume in 1 second percent (P < .001) and lower forced vital capacity percent (P < .001). However, PH had no significant impact on CF patients' OS (HR = 1.29, 95% CI 0.81 to 2.06, P = .283). Furthermore, subgroup analyses also showed no evidence of prognostic role of PH in CF patients (all P values >.05).

CONCLUSIONS

Our findings suggest that the presence of PH was strongly correlated with worse blood-gas parameters and worse lung function, but surprisingly had no significant prognostic value on survival among CF patients. Further large-scale and prospective studies are needed to confirm these findings.

Physiological markers of exercise capacity and lung disease severity in cystic fibrosis

BACKGROUND AND OBJECTIVE

Peak aerobic capacity (VO₂ peak) is an important outcome measure in cystic fibrosis (CF), but measurement is not widely available and can be influenced by patient motivation, pain and fatigue. Alternative markers of disease severity would be helpful. Neural respiratory drive, measured using parasternal intercostal muscle electromyography (EMGpara), reflects the load to capacity balance of the respiratory system and provides a composite measure of pulmonary function impairment in CF. The aim of the study was to investigate the relationship between exercise capacity, EMGpara and established measures of pulmonary function in clinically stable adult CF patients.

METHODS

Twenty CF patients (12 males, median (range) age: 22.3 (17.0-43.1) years) performed the 10-m incremental shuttle walk test (ISWT) maximally with contemporaneous measures of aerobic metabolism. EMGpara was recorded from second intercostal space at rest and normalized using peak electromyogram activity obtained during maximum respiratory manoeuvres and expressed as EMGpara%max (EMGpara expressed as a percentage of maximum).

RESULTS

VO₂ peak was strongly correlated with ISWT distance (r = 0.864, P < 0.0001). Lung gas transfer (T_L CO) % predicted was best correlated with VO₂ peak (r = 0.842, P < 0.0001) and ISWT distance (r = 0.788, P < 0.0001). EMGpara%max also correlated with VO₂ peak (-0.757, P < 0.0001), while the relationships between exercise outcome measures and forced expiratory volume in 1 s (FEV₁ ) % predicted and forced vital capacity (FVC) % predicted were less strong. A T_L CO% predicted of <70.5% was the strongest predictor of VO₂ peak <32 mL/min/kg (area under the curve (AUC): 0.96, 100% sensitivity, 83.3% specificity). ISWT distance and EMGpara%max also performed well, with other pulmonary function variables demonstrating poorer predictive ability.

CONCLUSION

T_L CO% predicted and EMGpara%max relate strongly to exercise performance markers in CF and may provide alternative predictors of lung disease progression.

Determinants of exercise capacity in cystic fibrosis patients with mild-to-moderate lung disease

Background

Adult patients with cystic fibrosis (CF) frequently have reduced exercise tolerance, which is multifactorial but mainly due to bronchial obstruction. The aim of this retrospective analysis was to determine the mechanisms responsible for exercise intolerance in patients with mild-to-moderate or severe disease.

Methods

Cardiopulmonary exercise testing with blood gas analysis at peak exercise was performed in 102 patients aged 28 ± 11 years: 48 patients had severe lung disease (FEV₁ < 50%, group 1) and 54 had mild-to-moderate lung disease (FEV₁ ≥ 50%, group 2). VO₂ peak was measured and correlated with clinical, biological, and functional parameters.

Results

VO₂ peak for all patients was 25 ± 9 mL/kg/min (65 ± 21% of the predicted value) and was < 84% of predicted in 82% of patients (100% of group 1, 65% of group 2). VO₂ peak was correlated with body mass index, C-reactive protein, FEV₁, FVC, RV, DLCO, V_E/VCO₂ peak, V_D/V_T, PaO₂, PaCO₂, P(A-a)O₂, and breathing reserve. In multivariate analysis, FEV₁ and overall hyperventilation during exercise were independent determinants of exercise capacity (R² = 0.67). FEV₁ was the major significant predictor of VO₂ peak impairment in group 1, accounting for 31% of VO₂ peak alteration, whereas excessive overall hyperventilation (reduced or absent breathing reserve and V_E/VCO₂) accounted for 41% of VO₂ alteration in group 2.

Conclusion

Exercise limitation in adult patients with CF is largely dependent on FEV₁ in patients with severe lung disease and on the magnitude of the ventilatory response to exercise in patients with mild-to-moderate lung disease.

Gas exchange in disease: asthma, chronic obstructive pulmonary disease, cystic fibrosis, and interstitial lung disease

Ventilation-perfusion (VA/Q) inequality is the underlying abnormality determining hypoxemia and hypercapnia in lung diseases. Hypoxemia in asthma is characterized by the presence of low VA/Q units, which persist despite improvement in airway function after an attack. This hypoxemia is generally attenuated by compensatory redistribution of blood flow mediated by hypoxic vasoconstriction and changes in cardiac output, however, mediator release and bronchodilator therapy may cause deterioration. Patients with chronic obstructive pulmonary disease have more complex patterns of VA/Q inequality, which appear more fixed, and changes in blood flow and ventilation have less benefit in improving gas exchange efficiency. The inability of ventilation to match increasing cardiac output limits exercise capacity as the disease progresses. Deteriorating hypoxemia during exacerbations reflects the falling mixed venous oxygen tension from increased respiratory muscle activity, which is not compensated by any redistribution of VA/Q ratios. Shunt is not a feature of any of these diseases. Patients with cystic fibrosis (CF) have no substantial shunt when managed according to modern treatment regimens. Interstitial lung diseases demonstrate impaired oxygen diffusion across the alveolar-capillary barrier, particularly during exercise, although VA/Q inequality still accounts for most of the gas exchange abnormality. Hypoxemia may limit exercise capacity in these diseases and in CF. Persistent hypercapnic respiratory failure is a feature of advancing chronic obstructive pulmonary disease and CF, closely associated with sleep disordered breathing, which is not a prominent feature of the other diseases. Better understanding of the mechanisms of hypercapnic respiratory failure, and of the detailed mechanisms controlling the distribution of ventilation and blood flow in the lung, are high priorities for future research.

Oxygen therapy for cystic fibrosis

BACKGROUND

The most serious complications of cystic fibrosis (CF) relate to respiratory insufficiency. Oxygen supplementation therapy has long been a standard of care for individuals with chronic lung diseases associated with hypoxemia. Physicians commonly prescribe oxygen therapy for people with CF when hypoxemia occurs. However, it is unclear if empiric evidence is available to provide indications for this therapy with its financial costs and often profound impact on lifestyle.

OBJECTIVES

To assess whether oxygen therapy improves the longevity or quality of life of individuals with CF.

SEARCH METHODS

We searched the Cochrane Cystic Fibrosis and Genetic Disorders Group Trials Register, comprising references identified from comprehensive electronic database searches and handsearches of relevant journals and abstract books of conference proceedings.Latest search of Group's Trials Register: 15 May 2013.

SELECTION CRITERIA

Randomized or quasi-randomized controlled trials comparing oxygen, administered at any concentration, by any route, in people with documented CF for any time period.

DATA COLLECTION AND ANALYSIS

Authors independently assessed the risk of bias for included studies and extracted data.

MAIN RESULTS

This review includes 11 published studies (172 participants); only one examined long-term oxygen therapy (28 participants). There was no statistically significant improvement in survival, lung, or cardiac health. There was an improvement in regular attendance at school or work in those receiving oxygen therapy at 6 and 12 months. Four studies examined the effect of oxygen supplementation during sleep by polysomnography. Although oxygenation improved, mild hypercapnia was noted. Participants fell asleep quicker and spent a reduced percentage of total sleep time in rapid eye movement sleep, but there were no demonstrable improvements in qualitative sleep parameters. Six studies evaluated oxygen supplementation during exercise. Again, oxygenation improved, but mild hypercapnia resulted. Participants receiving oxygen therapy were able to exercise for a significantly longer duration during exercise. Other exercise parameters were not altered by the use of oxygen.

AUTHORS' CONCLUSIONS

There are no published data to guide the prescription of chronic oxygen supplementation to people with advanced lung disease due to CF. Short-term oxygen therapy during sleep and exercise improves oxygenation but is associated with modest and probably clinically inconsequential hypercapnia. There are improvements in exercise duration, time to fall asleep and regular attendance at school or work. There is a need for larger, well-designed clinical trials to assess the benefits of long-term oxygen therapy in people with CF administered continuously or during exercise or sleep or both. However, we do not expect any new research to be undertaken in this area any time soon and do not plan to update this review again until any new evidence does become available.

[Daily physical activity in adult patients with cystic fibrosis]

Assessment of daily physical activity (DPA) is a major element in the development of respiratory rehabilitation. The aim of this study was to evaluate the DPA and the daily energy expenditure (DEE) in adult patients with cystic fibrosis. Thirty adult patients (16 women, 14 men, mean age 27.1±8.4 years) with cystic fibrosis (FVC: 69.2±19%, FEV1: 55.1±24%, BMI: 20.5±3.6kg/m(2)), in stable condition were included in the study. The questionnaire allowed DPA to estimate DEE from the intensity and type of activity. This tool divides the DEE into four intensity levels: less than 1 Metabolic Equivalent Task (MET), greater than 1 and less than 3 METs, 3-5 METs, and greater than 5 METs. A shuttle test for estimating VO(2) peak was performed in all patients. The DEE reached 9478.4±3224.3kJ24h(-1) and was significantly and highly correlated with calculated peak VO(2) (l/min) (r=0.876, P=<0.001); 22% DEE was less than 3 METs, 10% between 3 and 5 METs et 2,5% greater than 5 METs; 65% DEE was less than 1 MET. There was a strong correlation between DEE and BMI (r=0.860, P=<0.001), a low correlation with FVC (r=0.423, P=0.02). This questionnaire is relevant for estimating DPA and DEE in patients with cystic fibrosis and is an interesting test to be used to evaluate changes in patients after pulmonary rehabilitation.

The evolution of exercise capacity and its limiting factors in cystic fibrosis

The ability to perform exercise is an important determinant of both longevity and quality of life for patients with Cystic Fibrosis. There are a variety of physical and behavioural factors that contribute to exercise limitation. These, such as lung function or habitual physical activity, change over time. However, these factors can also be modified by treatments and interventions. This review discusses the various factors that contribute to exercise limitation in Cystic Fibrosis, and how these change with age.

Desempenho funcional de pacientes com fibrose cística e indivíduos saudáveis no teste de caminhada de seis minutos

OBJETIVO: Comparar pacientes com fibrose cística e indivíduos saudáveis quanto ao desempenho funcional no teste de caminhada de seis minutos (TC6). MÉTODOS: Estudo transversal e prospectivo, com indivíduos saudáveis e com fibrose cística de um hospital universitário de referência na cidade de Campinas (SP). O TC6 foi aplicado de acordo com as normas da American Thoracic Society e repetido após 30 min de repouso. Foram determinados FR, FC, SpO2 e escore da escala de Borg em todos os participantes, assim como o status nutricional e valores espirométricos para os pacientes com fibrose cística. Excluíram-se pacientes em exacerbação pulmonar. Foram utilizados coeficientes de correlação de Spearman e ANOVA para medidas repetidas. RESULTADOS: Foram incluídos 55 pacientes no grupo fibrose cística e 185 indivíduos saudáveis no grupo controle, com médias de idade de 12,2 ± 4,3 anos e 11,3 ± 4,3 anos, respectivamente. A distância percorrida no TC6 (DTC6) foi significativamente menor no grupo fibrose cística que no grupo controle em ambos os testes (547,2 ± 80,6 m vs. 610,3 ± 53,4 m no primeiro e 552,2 ± 82,1 m vs. 616,2 ± 58,0 m no segundo; p < 0,0001 para ambos). A DTC6 se correlacionou com idade, peso e altura somente no grupo fibrose cística. A SpO2 manteve-se estável durante o teste, com aumento da FC e da FR. CONCLUSÕES: Nesta amostra, os pacientes com fibrose cística apresentaram um menor desempenho funcional no TC6 quando comparados a indivíduos saudáveis de mesma faixa etária, sem a necessidade de repetição imediata do teste.

Impaired lung diffusing capacity for nitric oxide and alveolar-capillary membrane conductance results in oxygen desaturation during exercise in patients with cystic fibrosis

BACKGROUND

Exercise has been shown to be beneficial for patients with cystic fibrosis (CF), but for some CF patients there is a risk of desaturation, although the predicting factors are not conclusive or reliable. We sought to determine the relationship between the diffusion capacity of the lungs for nitric oxide and carbon monoxide (DLNO and DLCO) and the components of DLCO: alveolar-capillary membrane conductance (D(M)), and pulmonary capillary blood volume (V(C)) on peripheral oxygen saturation (SaO(2)) at rest and during exercise in CF.

METHODS

17 mild/moderate CF patients and 17 healthy subjects were recruited (age=26±7 vs. 23±8 years, ht=169±8 vs. 166±8 cm, wt=65±9 vs. 59±8 kg, BMI=23±3 vs. 22±3 kg/m(2), VO(2PEAK)=101±36 vs. 55±25%pred., FEV(1)=92±22 vs. 68±25%pred., for healthy and CF, respectively, mean±SD, VO(2PEAK) and FEV(1) p<0.001). Subjects performed incremental cycle ergometry to exhaustion with continuous monitoring of SaO(2) and measures of DLNO, DLCO, D(M) and V(C) at each stage.

RESULTS

CF patients had a lower SaO(2) at rest and peak exercise (rest=98±1 vs. 96±1%, peak=97±2 vs. 93±5%, for healthy and CF, respectively, p<0.01). At rest, DLNO, DLCO, D(M) were significantly lower in the CF group (p<0.01). The difference between groups was augmented with exercise (DLNO=117±4 vs. 73±3ml/min/mmHg; DLCO=34±8 vs. 23±8ml/min/mmHg; D(M)=50±1 vs. 34±1, p<0.001, for healthy and CF respectively). Peak SaO(2) was related to resting DLNO in CF patients (r=0.65, p=0.003).

CONCLUSIONS

These results suggest a limitation in exercise-mediated increases in membrane conductance in CF which may contribute to a drop in SaO(2) and that resting DLNO can account for a large portion of the variability in SaO(2).

High-intensity interval training in an adolescent with cystic fibrosis: a physiological perspective

Nutritional, musculoskeletal, and/or ventilatory status can lead to a decreased exercise capacity in children with cystic fibrosis (CF). Exercise training is already part of the usual care; however, the "optimal" intensity and volume of exercise training to improve exercise capacity is still unknown. Six weeks of high-intensity interval training (HIT) for a patient with CF with a ventilatory limitation was evaluated by a cardiopulmonary exercise test (CPET). Peak oxygen uptake and peak workload increased 19% and 16%, respectively, and there was a rise in peak ventilation from 50 L/min to 75 L/min, with an increase in both breathing depth and respiratory rate. A relative short period of HIT resulted in a significant increase in exercise capacity. In patients with CF, HIT might be an effective and efficient training regimen, especially in CF patients with a ventilatory limitation. Further research is necessary to investigate whether HIT is a better alternative than traditional aerobic training programs especially in ventilatory limited patients with CF.

Oxygen therapy for cystic fibrosis

BACKGROUND

The most serious complications of cystic fibrosis (CF) relate to respiratory insufficiency. Oxygen supplementation therapy has been a standard of care for individuals with chronic lung diseases associated with hypoxemia for decades. Physicians commonly prescribe oxygen therapy for people with CF when hypoxemia occurs. However, it is unclear if empiric evidence is available to provide indications for this therapy with its financial costs and often profound impact on lifestyle.

OBJECTIVES

To assess whether oxygen therapy improves the longevity or quality of life of individuals with CF.

SEARCH STRATEGY

We searched the Cochrane Cystic Fibrosis and Genetic Disorders Group Trials Register, comprising references identified from comprehensive electronic database searches and handsearches of relevant journals and abstract books of conference proceedings.Most recent search of Group's Trials Register: November 2008.

SELECTION CRITERIA

Randomized or quasi-randomized controlled trials comparing oxygen, administered at any concentration, by any route, in people with documented CF for any time period.

DATA COLLECTION AND ANALYSIS

Authors independently assessed study quality and extracted data.

MAIN RESULTS

Eleven published studies (172 participants) are included in this review, of which only one examined long-term oxygen therapy (28 participants). There was no statistically significant improvement in survival, lung, or cardiac health. There was an improvement in regular attendance at school or work in those receiving oxygen therapy at 6 and 12 months. Four studies examined the effect of oxygen supplementation during sleep by polysomnography. Although oxygenation improved, mild hypercapnia was noted. Participants took less time to fall asleep and spent a reduced percentage of total sleep time in rapid eye movement sleep, but there were no demonstrable improvements in qualitative sleep parameters. Six studies evaluated oxygen supplementation during exercise. Again, oxygenation improved, but mild hypercapnia resulted. Participants receiving oxygen therapy were able to exercise for a significantly longer duration during exercise. Other exercise parameters were not altered by the use of oxygen.

AUTHORS' CONCLUSIONS

There are no published data to guide the prescription of chronic oxygen supplementation to people with advanced lung disease due to CF. Short-term oxygen therapy during sleep and exercise improves oxygenation but is associated with modest and probably clinically inconsequential hypercapnia. There are improvements in exercise duration, time to fall asleep and regular attendance at school or work. There is a need for larger, well-designed clinical trials to assess the benefits of long-term oxygen therapy in people with CF administered continuously or during exercise or sleep or both.

Effect of Albuterol on Maximal Exercise Capacity in Cystic Fibrosis

BACKGROUND

Inhaled, short-acting beta-adrenergic agonists (SAbetaAs) are widely prescribed in cystic fibrosis (CF) subjects, despite a lack of convincing data for efficacy and the potential for these agents to result in airway instability. We tested the hypothesis that inhaled albuterol would improve maximal exercise performance in CF subjects with airflow obstruction, as a result of acute bronchodilation.

METHODS

Randomized, double-blind, placebo-controlled crossover study of the effect of inhaled albuterol on maximal exercise performance in 20 stable adult CF patients (mean +/- SD age, 23.3 +/- 6.1 years; FEV(1), 57.65 +/- 17.13% of predicted).

RESULTS

Ventilatory limitation to exercise was demonstrated in 16 subjects (80%). Significant bronchodilation occurred with exercise alone (end-exercise FEV(1), 2.24 +/- 0.8 L; vs preexercise FEV(1), 2.09 +/- 0.77 L; p < 0.0001), but albuterol resulted in significantly greater exercise-induced bronchodilation than placebo (change in FEV(1), 0.3 +/- 0.15 L vs 0.15 +/- 0.11 L; 95% confidence interval [CI], + 0.07 to + 0.23; p < 0.001). However, there was no difference in maximal workload achieved (albuterol, 158 +/- 46 W; vs placebo, 158 +/- 45 W; 95% CI, - 4.41 to + 4.71; p = 0.95), nor any other measure of exercise performance including maximal oxygen uptake.

CONCLUSIONS

Despite causing significant acute bronchodilation, inhaled albuterol did not improve maximal exercise performance in ventilatory-limited CF adults, adding to the body of literature that fails to show any clinical benefit of SAbetaAs in CF subjects. The current results provide further evidence to question the widespread use of these agents, although the potential for adrenergic beta-agonists to instead improve submaximal exercise performance merits further investigation.

Respiratory factors do not limit maximal symptom-limited exercise in patients with mild cystic fibrosis lung disease

To evaluate whether respiratory factors limit exercise capacity in patients with mild cystic fibrosis (CF) lung disease (mean FEV(1) = 76 +/- 7.7% predicted) we stressed the respiratory system of seven patients using added dead space (V(D)). Primary outcomes were exercise duration (Ex(dur)) and maximal oxygen uptake (VO(2max)). Dyspnoea/leg-discomfort were assessed at end-exercise. Ex(dur) was identical between control and V(D) studies (520 +/- 152 versus 511 +/ -166 s, p = NS) as was VO(2max)(1.6 +/- 0.5 versus 1.6 +/- 0.6 L/min, p = NS). Significant resting, sub-maximal and maximal workload increases in minute ventilation (V(E)) were detected (70.8 +/- 13.7 versus 79.5 +/- 16.9 L/min, p < 0.05). Analysis of breathing pattern revealed increases in V(E) were attributable to increases in tidal volume (2.0 +/- 0.5 versus 2.2 +/- 0.6 L, p < 0.05) with no change in respiratory frequency. There was no difference in dyspnoea/leg discomfort between tests. The increase in V(E) in response to V(D), with no change in [Exdur/VO(2max) suggests maximal symptom-limited exercise limitation is not primarily limited by respiratory factors in mild CF lung disease. Focused investigation and treatment of non-respiratory factors contributing to exercise limitation may improve exercise rehabilitation in this patient group.