Aerobic Exercise Training Improves Right- and Left Ventricular Systolic Function in Patients with COPD

Abstract

Objective: The aim of this study was to investigate the effects of moderate continuous training (MCT) and high intensity aerobic interval training (AIT) on systolic ventricular function and aerobic capacity in COPD patients. Methods: Seventeen patients with COPD (64 ± 8 years, 12 men) with FEV₁ of 52.8 ± 11% of predicted, were randomly assigned to isocaloric programs of MCT at 70% of max heart rate (HR) for 47 minutes) or AIT (∼90% of max HR for 4×4 minutes) three times per week for 10 weeks. Baseline cardiac function was compared with 17 age- and sex-matched healthy individuals. Peak oxygen uptake (VO_2-peak) and left (LV) and right ventricular (RV) function examined by echocardiography, were measured at baseline and after 10 weeks of training. Results: At baseline, the COPD patients had reduced systolic function compared to healthy controls (p < 0.05). After the training, AIT and MCT increased VO_2-peak by 8% and 9% and work economy by 7% and 10%, respectively (all p < 0.05). LV and RV systolic function both improved (p < 0.05), with no difference between the groups after the two modes of exercise training. Stroke volume increased by 17% and 20%, LV systolic tissue Doppler velocity (S’) by 18% and 17% and RV S’ by 15% after AIT and MCT, respectively (p < 0.05). Conclusion: Systolic cardiac function is reduced in COPD. Both AIT and MCT improved systolic cardiac function. In contrast to other patient groups studied, higher exercise intensity does not seem to have additional effects on cardiac function or aerobic capacity in COPD patients.

Keywords :

• stroke volume
• interval training
• echocardiography
• oxygen uptake

Introduction

Dyspnoea and impaired exercise tolerance are hallmarks of COPD resulting in reduced activity levels (1). In COPD patients exercise capacity has been shown to be a better predictor for mortality than FEV₁ (2). So far, exercise training has been the only way to increase aerobic capacity in COPD patients and endurance training has therefore become an essential component of pulmonary rehabilitation programmes (3, 4). These patients are also subject to increased risk of cardiovascular disease even after correcting for cofounders such as smoking (5, 6).

Right ventricular dysfunction has been regarded as the main cardiac abnormality in COPD patients, especially in patients with concomitant pulmonary hypertension and hypoxemia (7, 8). The left ventricular (LV) function has been less well studied and was long considered normal in COPD patients, except in patients with severe emphysema (9, 10). However, recent studies on otherwise healthy COPD patients have shown reduced LV function proportional to the degree of airway obstruction (11–13). A reduced stroke volume at all exercise intensities is also found in most COPD patients (14).

The effect of exercise training on cardiac function has not been extensively studied in COPD patients. Furthermore, the relationship between LV function and aerobic capacity in these patients is not known. Most exercise studies have investigated the effects on exercise capacity with moderate continuous training (MCT) and only a few aerobic interval training (AIT) with relatively high exercise intensity (15–17). In patients with chronic heart failure (CHF), coronary artery disease (CAD) and metabolic syndrome, AIT has been shown to be superior compared to MCT in improving aerobic capacity and cardiac function (18–22). In COPD patients it is evidence that higher training intensity results in higher gains in aerobic capacity (23).

The aim of the study was to determine the effect of these two exercise regimes on left and right ventricular function, as well as evaluate the effects on aerobic capacity and endothelial function. We hypothesized a) that reduced cardiac function in COPD could be improved by exercise training, and b) that AIT with the same regime proven to be superior to MCT in CHF, CAD and metabolic syndrome, also would be superior to MCT in COPD.

Materials and Methods

Subjects

We included 20 patients with stable COPD according to GOLD guidelines (24), no resting hypoxemia, age > 50 years and post bronchodilator FEV₁< 60%. Patient characteristics are shown in Table 1. Patients with known heart disease or any other medical condition limiting exercise training were excluded. EEG was recorded at baseline to exclude signs of ischemic heart disease, arrhythmias and conduction disorders and thereby ensure safe training for the patients. No ECG was recorded after finishing the exercise programs. Patients having participated in a pulmonary rehabilitation program during the last 3 months were excluded. All subjects were former smokers except two patients who were still smoking, and all subjects had a smoking history of at least 20 pack-years. The patients were recruited from outpatient ward of the Lung Department at St Olavs University Hospital of Trondheim, Norway.

Table 1.  Baseline study subjects characteristics

Measure	Controls n = 17	AIT group, n = 10	MCT grou n = 7	p-value
Age (years)	64.5 ± 6.4ns	64.8 ± 7.7	64.9 ± 5.3	0.99
Gender (♂/♀)	12/5	7/3	5/2	–
Height(cm)	174.1 ± 8.4ns	175 ± 11	171 ± 8	0.39
BMI (kg/m2)	26.1 ± 2.5ns	26.1 ± 4.4	23.5 ± 3.0	0.19
Resting HR(beats/min)	60 ± 8*	65 ± 8	70 ± 13	0.11
SBP (mmHg)	Nd	150 ± 28	138 ± 13	0.30
DBP /mmHg)	Nd	85 ± 12	81 ± 9	0.49
FEV1 (%pred)	Nd	54.8 ± 8.7	49.9 ± 14.5	0.39
FEV1 (liters)	Nd	1.83 ± 0.61	1.56 ± 0.57	0.37
FVC (liters)	Nd	4.07 ± 1.26	3.79 ± 1.17	0.65
FEV1%FVC (%)	Nd	46 ± 12	41 ± 8	0.35
DLCO (mmol/min/kPa)	Nd	5.64 ± 1.78	5.02 ± 0.75	0.48
VO2-peak (ml/kg/min)	Nd	21.3 ± 5.9	22.8 ± 4.7	0.73
Pack-Years (packs/year)	Nd	33.7 ± 9.7	33.6 ± 8.5	0.98
Compliance exercise program (number of training sessions)	Nd	29.0 ± 0.4	30.3 ± 0.6	0.11

AIT- aerobic interval training, MCT –moderate continuous training, BMI-body mass index, SBP- systolic blood pressure, DBP- diastolic blood pressure, HR-heart rate, FEV1 - Forced expiratory volume in 1 second, FVC-forced vital capacity, VO2 peak -peak oxygen consumption, Nd-no data, Ns- non significant.

p-values between AIT and MCT group are shown. The p-values for between control and the training groups comparisons are expressed as: *p < 0.05, † p < 0.01, ns –non significant (p > 0.05).

The criterion for completion of the training program was participation in at least 26 of 32 training sessions. Only the subjects completing the program were eligible for analysis.

In total 17 age- and sex-matched healthy individuals from the third Nord-Trøndelag Health Study (www.ntnu.edu/hunt) served as reference group for the baseline echocardiographic measurements but without participating in the training program (25). The study was approved by the Regional Ethics Committee (REK nr. 4.2008.754) and registered at ClinicalTrials.gov (NCT 00908765). Written informed consent was obtained from all patients.

Study design

The patients were randomised to either AIT or MCT stratified by FEV₁, age and gender. The randomization code was developed with a computer random-number generator to select random permuted blocks.

Exercise training

Patients randomized to isocaloric protocols of AIT or MCT, met for supervised training three days/week for 10 weeks. All training consisted of “uphill” treadmill walking. The AIT group warmed up for 10 minutes at 50% to 60% of peak oxygen uptake (VO_2-peak) (60% to 70% of peak heart rate (HR)) before exercising four 4-minute intervals at 90% to 95% of peak HR. Each interval was separated by 3-minute active pauses, walking at 50% to 70% of peak HR. The training session was terminated by a 3-minute cool-down at 50% to 70% of peak HR. Total exercise time was 38 minutes for the AIT group. To make the protocol isocaloric, patients in the MCT group exercised continuously at 70% of peak HR for 47 minutes each session. All subjects used a heart rate monitor (Polar Electro, Kempele, Finland) to obtain the assigned exercise intensity. The speed and inclination of the treadmill was adjusted continuously to ensure that every training session was carried out at the assigned heart rate throughout the training period (20, 22).

Pulmonary function

Flow volume spirometry and diffusion capacity measurements were performed with Vmax Spectra 20 (SensorMedics Corporation, Yorba Linda, CA). The better of two spirometry measurements with < 5% and 150 ml variation was reported. Measurements of reversibility were made at 10 min after administration 400 μg of Salbutamol from a pressurized metered-dose inhaler and volumatic device (26).

Lung diffusion capacity (DLCO) and membrane diffusion capacity (Dm) were performed by the”single-breath”-method. The mean of two acceptable tests that met the repeatability requirement of either being within 3 mL (STPD)·min⁻¹·mmHg⁻¹ or within 10% of the highest value was reported (27).

Exercise testing

Exercise testing was performed with an individual ramp protocol (individualized speed and inclination) and VO_2-peak was measured with MetaMax II (Cortex, Leipzig, Germany) as previously described (28). All patients underwent a 10 minute warm up prior to testing. Work economy (WE) was determined as oxygen uptake at a submaximal workload at a walking speed of 3 km/h and zero inclination (20).

Echocardiography

Two cardiologists were collecting and analyzing the data. Both were blinded to the treatment assignment of the patients. Echocardiography was performed with a Vivid 7 scanner (GE Vingmed Ultrasound, Horten, Norway) using a phased-array transducer (M3S) and analysed offline with EchoPAC PC^TM (GE Vingmed Ultrasound, Horten, Norway). Three cine loops from the three standard apical planes were recorded in tissue Doppler mode with simultaneous gray scale harmonic images. At rest, gray scale mean frame rate was 39.6/second and mean tissue Doppler frame rate was 157.2/second. Ejection fraction and LV volumes were determined using a biplane (apical four- and two chamber) modified Simpsons’ method. Stroke volume was determined by pulse-wave Doppler of the aortic outlet in the five chamber view. Peak early (E) and late (A) mitral inflow velocities, deceleration time (DT) and isovolumic relaxation time (IVRT) were measured according to ASE guidelines (29).

Peak mitral annular systolic (S’) and diastolic early (e’) and late (a’) tissue velocities were obtained in the pulsed-wave Doppler mode at rest. The ratio E/e’ was calculated, a measurement of left ventricular filling pressure (30). Pulsed wave tissue Doppler velocities were measured at the atrio-ventricular plane in four- and two chamber view, and the average of the four points was used. Peak systolic strain rate was calculated using tissue Doppler in the right ventricle dividing the free wall into two segments. Systolic pulmonary arterial pressure (SPAP) was a measurement of tricuspid regurgitate-velocity and the vena cava dimension (31).

Endothelial function

Endothelial function testing was performed pre-training and 72 hours after the last training session. Endothelium-dependent flow-mediated dilation (FMD) of the brachial artery was measured by a 14-MHz ultrasound Doppler probe (Vivid 7 system, GE Vingmed Ultrasound, Horten, Norway) as previously described (32). The% difference between the diameter measured after reactive hyperemia and the basal diameter was calculated as FMD.

Statistics

Data are presented as mean ± SD. The changes in physiologic variables were calculated at baseline and after training. Assumptions of normality were assessed by normal probability plots. The within group differences were analysed using paired t-test. Between groups comparisons were analysed using ANOVA. The level of significance was set at p < 0.05. Statistics were computed using SPSS version 15 and STATA version 10.

Results

Training program and exercise testing

In total, 10 patients in the AIT group (7 males) and 7 patients in the MCT group (5 males) completed the exercise program. Dropouts were due to lack of motivation and musculoskeletal pain (n = 3, all in the MCT group). After the training program, AIT and MCT increased VO_2-peak by 8% (p < 0.05) and 9% (p < 0.05), respectively, with no difference between the groups (Table 2). The work economy at submaximal workload was improved by 7% (p < 0.05) and 10% (p < 0.05) after AIT and MCT, with no difference between the groups (Table 2). The respiratory exchange ratio, peak heart rate and maximal ventilation was similar during pre- and post-training VO₂-peak testing suggesting similar exercise performance.

Table 2.  Aerobic capacity and exercise data

	AIT			MCT
	Baseline	10 weeks	p-value	Baseline	10 weeks	p-value	p-value between groups
VO2- peak (mL/kg/min)	21.3 ± 5.9	23.0 ± 7.2	0.02	22.8 ± 4.7	24.9 ± 4.9	0.003	0.57
Peak HR (beats/min)	146 ± 26	142 ± 28	0.23	160 ± 11	159 ± 7	0.88	0.22
RER	0.90 ± 0.02	0.90 ± 0.02	0.73	0.94 ± 0.03	0.94 ± 0.03	0.78	0.19
VE at VO2-peak (liters/min)	53 ± 21	56 ± 25	0.15	50 ± 14	50 ± 14	0.78	0.61
WE (mL/kg/min	11.3 ± 2.0	10.0 ± 1.6	0.02	11.1 ± 1.3	10.2 ± 1.1	0.007	0.64
VT (liters)	1.10 ± 0.34	1,15 ± 0.29	0.31	1.07 ± 0.29	1.07 ± 0.27	0.97	0.49
Heart rate at WE load (beats/min)	96 ± 13	98 ± 13	0.52	104 ± 12	104 ± 16	0.96	0.46

AIT- aerobic interval training, MCT- moderate continuous training, HR- heart rate, VO2-peak -peak oxygen consumption, RER-respiratory exchange rate, VT-Tidal Volume, VE- minute ventilation, WE- work economy. The within group p-values are shown to the right of each corresponding group. The between group differences are shown in the last column.

Lung function

No difference was found in dynamic lung volume measurements and no difference in DLCO or Dm was found after the training intervention neither in the MCT nor the AIT group.

Systolic and diastolic cardiac function

The AIT and MCT group had a 18.7% and 26.4% lower stroke volume, respectively, 19.4% and 18.4% lower ejection fraction, 15% and 13.8% lower LV S’ (p < 0.05) compared with healthy controls. There was no difference at baseline between the AIT or MCT group. After intervention EF increased by 11.9% (p < 0.01) in the AIT group and 4.9% (p < 0.05) in the MCT group (Table 3) and stroke volume by 16.3% (p < 0.05) in the AIT group and 20.5% (p < 0.05) in the MCT group (Table 3).

Table 3.  Resting echocardiography results

		AIT			MCT
	Control	Baseline	10 weeks	p-value	Baseline	10 weeks	p-value	p-value between groups
HR (beats/min)	60 ± 8*	65 ± 8	60 ± 7	0.034	70 ± 13	60 ± 11	0.04	0.108
EF (%)	65.5 ± 7.7‡	50.4 ± 7.1	56.4 ± 5.6	0.005	51.0 ± 4.5	53.5 ± 5.6	0.04	0.10
EDV (ml)	108.7 ± 19.5	109.6 ± 11.2	123 ± 10	0.008	101.1 ± 13.7	110.2 ± 11	0.035	0.063
FS (%)	41.0 ± 5.4†	36.1 ± 5.0	42.0 ± 6.8	0.003	35.0 ± 6.5	37.8 ± 6.7	0.033	0.181
SV (ml)	89.2 ± 17.0‡	69.9 ± 20.4	81.3 ± 20.6	0.038	63.3 ± 12.3	76.3 ± 18.0	0.043	0.786
LVOT max (m/s)	1.04 ± 0.12‡	0.93 ± 0.15	1.01 ± 0.15	0.033	0.85 ± 0.02	0.95 ± 0.07	0.018	0.553
LVOT VTI (cm)	22.5 ± 2.9*	19.9 ± 2.8	22.1 ± 3.4	0.001	18.6 ± 2.1	21.5 ± 3.3	0.028	0.25
LV S’ (cm/s)	8.2 ± 0.9‡	6.8 ± 0.97	8.0 ± 1.43	0.005	6.9 ± 1.2	8.1 ± 1.1	0.018	0.99
LV e’ (cm/s)	–9.1 ± 2.1	–7.6 ± 1.9	–8.3 ± 2.2	0.11	–8.9 ± 1.0	–9.8 ± 1.8	0.11	0.821
TAPSE (mm)	24.1 ± 3.2*	22.3 ± 3.8	25.0 ± 3.8	0.005	21.0 ± 4.9	24.4 ± 4.0	0.028	0.706
RV S’ (cm/s)	13.2 ± 2.1*	12.0 ± 1.6	14.4 ± 3.4	0.017	12.1 ± 1.4	13.9 ± 2.3	0.027	0.949
RV e’ (cm/s)	–12.3 ± 2.9	–11.2 ± 2.1	–13.2 ± 4.7	0.14	–13.1 ± 3.3	–14.2 ± 5.0	0.27	0.326
RV Global SR (s–1)	–1.59 ± 0.27*	–1.34 ± 0.20	–1.58 ± 0.24	0.026	–1.38 ± 0.17	–1.60 ± 0.12	0.028	0.719

AIT- aerobic interval training, MCT –moderate continuous training, SV-stroke volume, LVOT-left ventricular outflow tract, VTI-velocity time integral, LV-left ventricular, RV-right ventricular, S’- pulsed wave tissue Doppler systolic velocity (mean for septal, lateral, inferior and anterior annular plane in LV and lateral for RV), e’- tissue Doppler early diastolic velocity, TAPSE-tricuspid annular plane systolic excursion, SR-strain rate. The p-values shown are within group findings. The p-values for between group comparisons of controls and COPD patients at baseline and expressed as: *p < 0.05, † p < 0.01, ‡ p < 0.001.

RV S’ was 11.3% and 14.2% lower in AIT and MCT, respectively, (p < 0.05), whereas tricuspid annular plane systolic excursion (TAPSE) was 7.5% and 12.8% lower, respectively, compared to controls (p < 0.05) (Table 3). RV S’ increased by 15.2% in the AIT group and by 14.9% in the MCT group (Table 3) (p < 0.05). Patients and controls had similar left ventricular end diastolic volume (LVEDV, which increased in both exercise groups after the training period (Table 3). There was no difference in diastolic function either before or after the intervention (Table 3).

Systolic pulmonary arterial pressure

Estimated systolic pulmonary artery pressure (SPAP) was 23.3 mmHg in COPD patients at baseline and 20.2 mmHg after intervention (p = 0.21). There was no difference between the groups.

Resting heart rate and blood pressure

The exercise program gave a significant reduction in resting heart rate in both groups (p<0.05) (Table 3). There were no significant reductions in neither systolic nor diastolic blood pressure in neither within nor between the groups (Table 4).

Table 4.  Blood pressure and endothelial data

	AIT			MCT
	Baseline	10 weeks	p-value	Baseline	10 weeks	p-value	p-value between groups
Systolic BP (mmHg)	150 ± 28	146 ± 19	0.27	138 ± 13	134 ± 18	0.22	0.35
Diastolic BP (mm Hg)	85 ± 12	84 ± 13	0.84	81 ± 9	80 ± 7	0.78	0.47
Resting arterial diameter (mm)	3.49 ± 0.53	3.46 ± 0.51	0.40	3.48 ± 0.61	3.46 ± 0.51	0.55	0.85
FMD 1 minute post-occlusion(%)	2.88 ± 1.73†	2.76 ± 1.65	0.90	1.46 ± 1.14	1.66 ±1.93	0.82	0.25

AIT- aerobic interval training, MCT –moderate continuous training, FMD-flow mediated dilatation, BP-blood pressure. The p-values shown to the right of each group are within group findings. The p-values for between group are shown in the last column. The p-values for between group comparisons of controls and COPD patients at baseline and expressed as: *p < 0.05, † p < 0.01.

Endothelial function

Resting arterial diameter was 3.49 mm and 3.48 in the AIT and MCT group and did not change after the exercise program. Likewise, FMD in both groups remained unchanged (Table 4).

Discussion

The present study is the first to study effects of aerobic exercise training on cardiac function assessed by echocardiography, in COPD patients. We found that untrained COPD patients had impaired left- and right ventricular systolic function compared to healthy controls. After 10 weeks of exercise intervention, aerobic capacity and systolic cardiac function were markedly and similarly improved after both AIT and MCT. The main findings are presented in Figure 1.

Figure 1.  The change from baseline at peak performance after training, expressed as percent (mean and SD). White bars: AIT group, black bars: MCT group. All shown bars indicate a significant change within group (p < 0.05). AIT- aerobic interval training, MCT –moderate continuous training, HR- resting heart rate, SV-stroke volume, VO2-peak -peak oxygen consumption, WE-work economy.

Our finding of reduced LV function is in line with studies in patients with severe COPD (10,33). Recently, this has also been demonstrated in patients with mild emphysema, and is associated with the degree of emphysema (13). Even in early COPD there is evidence of subclinical LV and RV dysfunction with the RV dysfunction related to the severity of the airway obstruction (34). However, severe right ventricular dysfunction and cor pulmonale is usually found in patients with hypoxemia and pulmonary hypertension (8). Our study demonstrated significantly reduced RV and LV systolic function in COPD patients without overt heart disease, resting hypoxemia or significant pulmonary hypertension compared with that observed in healthy controls.

AIT has been shown to be superior to MCT with respect to both aerobic capacity and left ventricular function in healthy subjects and in patients with CHF, CAD and metabolic syndrome (18,20,22). Although being a main target for adaption to endurance training, the responses to exercise training on resting heart function have so far not been well studied in COPD patients. It has been suggested that ventilatory limitations in COPD prevent the cardio-circulatory system from being adequately taxed, thereby reducing the training effects (35,36). Our study shows that exercise training significantly improves cardiac function also in COPD patients despite their ventilatory constraints. We did not demonstrate a superior effect of AIT compared to MCT in COPD patients. This is consistent with a few other studies, although with somewhat different study design, (17,37,38).

Furthermore, the mode of training in COPD patients has almost exclusively been bicycle with only a very limited number of studies investigating walking or running. Due to a limited number of subjects, our study does not allow any firm conclusions with respect to possible differences in effects between AIT and MCT. However, the important finding of the present study is that these two different modalities of exercise training both have significant benefits with respect to cardiac function. Possibly, ventilatory constraints in COPD limit the substantial cardio-circulatory effects of AIT seen in other patients. Regarding the dominant role of exercise training in pulmonary rehabilitation program, it is of clinical importance that the cardiac effects can be obtained both with moderate and high-intensity exercise training.

Elucidating the causes of reduced cardiac function was beyond the scope of our study. Several mechanisms may explain this, among them systemic inflammation also affecting the myocardium. However, physical activity has been reported to be reduced in COPD, and reduced physical activity has also been shown to be associated with reduced left ventricular function in COPD (39,40). Reduced physical activity may, therefore, be an important factor explaining the reduced left ventricular function observed in the COPD patients in the present study. As our study shows that the cardiac impairment was improved by exercise training, this suggests that inactivity at least in part can be responsible for reduced cardiac function in COPD, and that the cardiac impairment to a large extent can be reversed. In our opinion, this finding further emphasises the role of exercise training in COPD.

COPD patients have an increased risk of cardiovascular events (5) and impaired endothelial function measured as reduced FMD (41). Reduced flow mediated dilatation (FMD) is proposed to be a predictor of cardiovascular events (42). A previous study has shown a significant reduction in FMD in COPD patients (41). We could however not demonstrate any significant beneficial effects on FMD after exercise as seen in other patient groups (43,44). A recent study has proposed that exercise induced reduction in systolic blood pressure could be a possible mechanism explaining the improved arterial stiffness seen after training in COPD-patients (45). We did not observe any effect of training on blood-pressure, and the lack of exercise-induced improvement in FMD after training in our study could thus be seen in relation to the unchanged blood pressure.

Limitations

The small sample size could prevent possible differences between the AIT and MCT group from being detected. The COPD patients included in our study had moderate to severe COPD with FEV1 < 60%, and our data may not be valid for patients with milder disease. Furthermore, the patients in the present study did not have resting hypoxaemia and pulmonary hypertension, and our data may not apply to those either. Overall echocardiographic measurements are sensitive to load changes; especially the right ventricle and results of improved RV function could be an effect of decreased load after intervention. However, there were no significant differences in blood pressure after exercise intervention.

Conclusion

Knowledge of cardiovascular effects of exercise training in COPD is essential as exercise training is an important part of pulmonary rehabilitation programmes. The present study shows that exercise training in COPD patients has important effects on resting cardiovascular function. Interestingly these effects are also achieved at moderate intensity training as well as high intensity aerobic interval training. Patients with COPD have dyspnoea, reduced exercise capacity and are at high risk of cardiovascular disease, and the cardiovascular benefits of exercise training may therefore be of great relevance in this group (5,46). The substantial effects of exercise training in COPD patients should in our opinion lead to an even more important role for exercise training in this patient group in the future.

Declaration of Interest Statement

The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.

Acknowledgements

The study was supported by grants from the National Heart and Lung Foundation, K.G. Jebsen Foundation, and the Norwegian Research Council Funding for Outstanding Young Investigators (U.W.). We are grateful to Anne Stine Fossum, Birgit Pedersen and Inger Lise Bjerkan at the Lung Department St. Olav's Hospital and to Ingerid Arbo (ISB). Funding: The study was supported by grants from the Norwegian Lung Medicine Society, the National Heart and Lung Foundation, K.G. Jebsen Foundation, and the Norwegian Research Council Funding for Outstanding Young Investigators (U.W.).