Rationale: Cardiovascular drugs may improve survival in chronic obstructive pulmonary disease (COPD). However, previous studies did not account for major sources of bias, and drug effects have not been evaluated in severe COPD.
Objectives: To estimate the time-dependent effects of cardiovascular drugs on survival in oxygen-dependent COPD, accounting for immortal and immeasurable time bias.
Methods: Prospective national study of patients starting long-term oxygen therapy for COPD in Sweden between 1 October 2005 and 30 June 2009. Effects on mortality were estimated using extended Cox regression adjusted for age, sex, PaO2, PaCO2, World Health Organization performance status, body mass index, comorbidity, and concomitant medications. Immortal and immeasurable time bias was addressed by analyzing all medications as time-dependent variables and accounting for hospitalized time, respectively.
Measurements and Main Results: Time-dependent effects of angiotensin-converting enzyme inhibitors or angiotensin receptor blockers, antiplatelet drugs, β-blockers, and statins on all-cause mortality were measured. Of the 2,249 included patients, 1,129 (50%) died under observation. No patient was lost to follow-up. The adjusted time-dependent model was compatible with reduced mortality for antiplatelet drugs (hazard ratio [HR], 0.86; 95% CI, 0.75–0.99; P = 0.030) and trends for angiotensin-converting enzyme inhibitors or angiotensin receptor blockers (HR, 0.90; 95% CI, 0.79–1.04; P = 0.166) and statins (HR, 0.86; 95% CI, 0.72–1.03; P = 0.105), whereas β-blockers increased mortality (HR, 1.19; 95% CI, 1.04–1.37; P = 0.010).
Conclusions: This study supports that antiplatelet drugs improve survival and β-blockers decrease survival in oxygen-dependent COPD.
Cardiovascular drugs may improve survival in chronic obstructive pulmonary disease (COPD), but previous studies did not account for major sources of bias, and drug effects have not been evaluated in very severe COPD.
This prospective study supports a beneficial effect of antiplatelet drugs on survival and beneficial trends for angiotensin-converting enzyme inhibitors, angiotensin receptor blockers, and statins. However, β-blockers may decrease survival in oxygen-dependent COPD.
As chronic obstructive pulmonary disease (COPD) is a leading cause of mortality worldwide, treatments that improve survival are needed (1).
Comorbidity is known to be highly prevalent in COPD (2) and is an important predictor of mortality (3, 4). A number of nonrandomized studies have suggested a beneficial effect on mortality for cardiovascular drugs: angiotensin-converting enzyme inhibitors (ACEI) (5–7), angiotensin receptor blockers (ARB) (5), antiplatelet drugs (7), β-blockers (7–12), and statins (5–7, 13–15).
However, the surprisingly large effect sizes in previous studies have raised the concern that the findings might have been biased. According to a recent analysis (16), the apparent drug effects in several of the studies (5, 13) were explained by immortal time bias and immeasurable time bias (17, 18). These biases might in fact have affected most previous cardiovascular drug studies in COPD (5–7, 9, 11, 13).
Immortal time bias may arise when exposure is assessed after the start of follow-up. All patients who become exposed during follow-up must, by definition, have lived until the date of exposure, whereas nonexposed patients must not; this creates a spurious association between drug exposure and improved survival (16). Immeasurable time bias can arise when drug exposure cannot be measured during hospitalizations. Therefore, hospitalized patients with shorter survival are more likely to be misclassified as unexposed, which also creates a spurious association between drug exposure and improved survival (16). Methods to account for both types of bias have been described (16).
In addition, previous studies evaluated the effects of drug exposure defined at baseline only, which might be inappropriate as drug exposure is likely to vary over time. The effects of cardiovascular drugs have not been previously studied in patients with severe COPD, who have high mortality, including from cardiovascular disease (19).
We therefore conducted a national prospective study of the time-dependent effects of cardiovascular drugs on mortality in severe COPD, accounting for immortal and immeasurable time bias. Some of the results of this study were previously presented in the form of an abstract (20).
This was a national prospective multicenter study of patients aged 45 years or older registered in the national Swedevox Register, who started long-term oxygen therapy (LTOT) for physician-diagnosed COPD between 1 October 2005 and 30 June 2009. The Swedevox Register is administered by the Swedish Society of Respiratory Medicine and prospectively includes patients starting LTOT for COPD in Sweden since 1987, with a population-based coverage of some 85% (21).
Only the latest treatment episode was included for patients who had started LTOT more than once (n = 62). Exclusion criterion was a diagnosis of lung cancer before the date of starting LTOT (baseline). Data were collected at baseline on resting PaO2 and PaCO2 breathing air and during oxygen therapy, FEV1, FVC, body mass index (BMI), smoking history, and World Health Organization performance status (22).
Comorbidity and in-hospital time within 4 years before baseline were obtained from the National Patient Register for in- and outpatient care, which covers more than 99% of all hospitalizations since 1987 and about 80% of all hospital-based outpatient care in Sweden since 2001 (23). The 4-year assessment period assured that all patients had the same assessment time for comorbidity, as data on outpatient care were available after 2001. Comorbidities were coded according to the tenth revision of the International Classification of Disease (ICD) (24). Definitions of all diagnosis entities are found in Table E1 in the online supplement.
Data on medications were obtained from the Swedish Prescribed Drug Register, which includes all dispensed prescriptions in outpatient care in Sweden after 1 July 2005 (25). Medications were categorized according to the Anatomical Therapeutic Chemical Classification System codes (26), as shown in Table E2. Patients were followed until withdrawal of LTOT, death, or 31 December 2009, whichever came first. Vital status was obtained from the Swedish Causes of Death Register. The underlying cause of death was available between 2005 and 2008 (n = 758; 67% of total deaths).
All patients gave their informed consent to participate. The study was approved by all the relevant ethics committees in Sweden, the Swedish National Board of Health and Welfare, and the Data Inspection Board.
Data were tabulated using frequencies and percentages for categorical variables, mean with SD, and median with range or interquartile range for continuous variables with normal and skewed distribution, respectively.
Dispensed prescribed drugs (prescriptions) were assessed during each full 91-day period (quarter) from the quarter before baseline until end of follow-up for each individual patient, as shown in Figure 1. A 91-day period was chosen, because the drugs in this study are usually prescribed for 3 months at a time. The probability of drug exposure for each quarter was set to 1 if there was at least one prescription during the quarter and set to 0 if there was no prescription during either this or the next quarter. To account for medications given in the hospital, the probability of exposure was set to the proportion of the current quarter spent in the hospital for quarters with no prescription but with a prescription during the next quarter. Exposure at baseline was defined as an exposure probability greater than 0 for the quarter before baseline. A more detailed description of the exposure assessment is provided in Section E1 in the online supplement.
The exposures of primary interest were treatment with ACEIs or ARBs (merged into the category ACEI/ARB owing to few cases of exposed to ARB and presumed similar effects), antiplatelet drugs, β-blockers, and statins. Secondary hypotheses were those of treatment effects of the combination of a long-acting β2 agonist and an inhaled corticosteroid (LABA + ICS), oral glucocorticoids, and tiotropium.
Time-dependent drug effects on all-cause mortality were estimated using an extension of the Cox model (27). The time-varying covariate was set to the exposure probability for the 91-day period before the quarter of the event for each drug (Figure 1). This lag-time effect allowed for the time it takes for the cardiovascular medications to achieve their therapeutic effects, and the model assumed that the therapeutic effects remained for no more than 3 months after the last prescription.
Missing elements were imputed for PaO2 air (n = 289), PaCO2 air (n = 301), and BMI (n = 701), as detailed in Section E2 in the online data supplement. The model estimates were robust to the imputations. FEV1, smoking history, the number of previous hospitalizations, and the presence of diabetes mellitus were not included in the final analysis, as they had no significant independent effects and did not affect any of the multiple regression estimates.
In the final model, the drug effects were adjusted for baseline differences in age, sex, PaO2 air, PaCO2 air, BMI, World Health Organization performance status (22), and comorbidities: anemia, renal failure, and cardiovascular diseases (cerebrovascular disease, heart failure, hypertension, ischemic heart disease, peripheral artery disease, pulmonary embolism, or other circulatory disease).
Time-dependent drug effects, expressed as hazard ratios (HRs) with 95% confidence intervals (CIs), are interpreted as, at any given time, the hazard for a patient who was exposed during the previous quarter as compared with the hazard for a patient who was not exposed, adjusted for all other covariates.
Data management of dispensed prescriptions was performed using Mimer SQL version 10.1 (Mimer Information Technology AB, Uppsala, Sweden), and analyses were conducted with Stata version 11.1 (StataCorp LP; College Station, TX) and SAS version 9.2 (SAS Institute, Inc., Cary, NC).
In total, 2,249 patients, 1,328 (59%) women and 921 men, were included after excluding 2 (0.09%) patients due to data irregularities and 39 (2%) patients with a lung cancer diagnosis at baseline. No patient was lost to follow-up. The cohort generated 3,118.1 person-years at risk and was followed for a median 1.1 years (first quartile–third quartile, 0.6–2.0 yr). During this time, 1,129 (50%) patients died. The main causes of death were respiratory disease (68%), cardiovascular disease (20%), and cancer (6%).
At baseline, ACEI/ARBs were used by 763 (34%) patients, antiplatelet drugs by 887 (39%), β-blockers by 829 (37%), and statins by 431 (19%). The number of patients treated with combinations of drugs and patient characteristics according to drug exposure at baseline are shown in Table 1. Patients treated with ACEI/ARB, antiplatelet drugs, β-blockers, or statins had higher BMI and more cardiovascular disease, diabetes mellitus, and renal failure than patients on none of these drugs (Table 1).
Characteristic | All Patients | ACEI/ARB | β-Blockers | Statins | Nontreated |
N (%) | 2,249 (100) | 763 (34) | 829 (37) | 431 (19) | 965 (43) |
Age, yr | 74.7 ± 8.2 | 75.3 ± 7.7 | 75.6 ± 7.8 | 73.5 ± 7.1 | 74.1 ± 8.6 |
Women, n (%) | 1,328 (59) | 423 (55) | 461 (56) | 216 (50) | 611 (63) |
PaO2 air, kPa | 6.5 ± 0.9 | 6.5 ± 0.9 | 6.5 ± 0.8 | 6.6 ± 0.8 | 6.6 ± 0.9 |
PaCO2 air, kPa | 6.3 ± 1.3 | 6.3 ± 1.3 | 6.3 ± 1.3 | 6.1 ± 1.3 | 6.2 ± 1.2 |
FEV1 % predicted | 37.7 ± 19.1 | 39.8 ± 18.9 | 40.5 ± 19.9 | 40.2 ± 20.7 | 35.9 ± 18.0 |
Known ever smoking, n (%) | 2,106 (93) | 726 (95) | 781 (94) | 408 (95) | 893 (93) |
Body mass index, n (%) | |||||
<18.5 | 279 (12) | 60 (8) | 81 (10) | 42 (10) | 152 (16) |
18.5–24.9 | 693 (31) | 203 (27) | 230 (28) | 115 (27) | 339 (35) |
25–29.9 | 338 (15) | 125 (16) | 132 (16) | 78 (18) | 120 (12) |
≥30 | 235 (10) | 112 (15) | 113 (14) | 58 (13) | 64 (7) |
WHO performance status, n (%) | |||||
0 | 132 (6) | 40 (5) | 37 (4) | 30 (7) | 63 (7) |
1 | 881 (39) | 292 (38) | 321 (39) | 169 (39) | 392 (41) |
2 | 714 (32) | 240 (31) | 268 (32) | 140 (32) | 302 (31) |
3 | 292 (13) | 101 (13) | 116 (14) | 48 (11) | 113 (12) |
4 | 31 (1) | 10 (1) | 11 (1) | 3 (1) | 16 (2) |
Hospitalizations* | 5 (2–8) | 5 (3–9) | 6 (3–9) | 5 (3–9) | 4 (2–8) |
Anemia, n (%) | 196 (9) | 79 (10) | 95 (11) | 36 (8) | 70 (7) |
Cardiovascular diagnoses, n (%) | |||||
0 | 755 (34) | 134 (18) | 116 (14) | 56 (13) | 510 (53) |
1 | 823 (37) | 266 (35) | 330 (40) | 160 (37) | 318 (33) |
2 | 449 (20) | 229 (30) | 245 (30) | 135 (31) | 98 (10) |
>2 | 222 (10) | 134 (18) | 138 (17) | 80 (19) | 39 (4) |
Diabetes mellitus, n (%) | 291 (13) | 159 (21) | 158 (19) | 120 (28) | 53 (5) |
Renal failure, n (%) | 97 (4) | 55 (7) | 62 (7) | 34 (8) | 13 (1) |
ACEI/ARB, n (%) | 763 (34) | 763 (100) | 415 (50) | 229 (53) | 0 |
Antiplatelet drugs, n (%) | 887 (39) | 402 (53) | 453 (55) | 321 (74) | 210 (22) |
β-Blockers, n (%) | 829 (37) | 415 (54) | 829 (100) | 256 (59) | 0 |
LABA + ICS, n (%) | 1,562 (69) | 536 (70) | 570 (69) | 315 (73) | 657 (68) |
Oral glucocorticoids, n (%) | 1,375 (61) | 437 (57) | 498 (60) | 282 (65) | 598 (62) |
Statins, n (%) | 431 (19) | 229 (30) | 256 (31) | 431 (100) | 0 |
Tiotropium, n (%) | 1,165 (52) | 412 (54) | 427 (52) | 233 (54) | 482 (50) |
The most commonly used ACEI/ARBs were enalapril and losartan, 98% of all β-blockers were cardioselective, and the antiplatelet and statin categories were dominated by aspirin and simvastatin, respectively. Dispensed drugs within each drug category are listed in Table E3. Of patients taking LABA or ICS, the majority (85%) used both drugs concomitantly; only few had monotherapy with LABA (5%) or ICS (10%). Table 2 shows changes in exposure status and drug adherence; of patients categorized as exposed to cardiovascular drugs at baseline, 10 to 14% changed status to unexposed during follow-up, and 5 to 10% of those unexposed at baseline were later exposed. During follow-up, patients were hospitalized for a median 7% (minimum, 0; maximum, 89%) of the time.
Drug | From Exposed to Unexposed, n (% of Exposed) | From Unexposed to Exposed, n (% of Unexposed) | Continuously Exposed, n (% of Total) | Median Adherence for Continuously Exposed (Min–Max), % |
ACEI/ARB | 100 (14) | 102 (7) | 637 (28) | 89 (25–100) |
Antiplatelet drugs | 93 (10) | 140 (10) | 759 (34) | 88 (18–100) |
β-Blockers | 83 (10) | 128 (9) | 714 (32) | 92 (41–100) |
LABA + ICS | 182 (12) | 173 (24) | 1,345 (60) | 87 (13–100) |
Oral glucocorticoids | 354 (27) | 288 (31) | 980 (44) | 84 (15–100) |
Statins | 55 (13) | 99 (5) | 365 (16) | 88 (38–100) |
Tiotropium | 211 (18) | 160 (14) | 934 (42) | 89 (18–100) |
The adjusted estimates were compatible with decreased mortality for antiplatelet drugs (HR, 0.86; 95% CI, 0.75–0.99; P = 0.030) and beneficial trends for ACEI/ARB (HR, 0.90; 95% CI, 0.79–1.04; P = 0.166) and statins (HR, 0.86; 95% CI, 0.72–1.03; P = 0.105), as shown in Table 3. In contrast, β-blockers were associated with increased mortality (HR, 1.19; 95% CI, 1.04–1.37; P = 0.010). In addition, oral glucocorticoids were significantly associated with higher mortality, and tiotropium tended to decrease mortality. The drug effects are depicted in Figure 2.
Parameter | Hazard Ratio | 95% CI | P Value |
Age, per yr | 1.04 | 1.03–1.05 | <0.001 |
Woman | 0.77 | 0.67–0.87 | <0.001 |
BMI | <0.001* | ||
<18.5 | 1.36 | 1.14–1.61 | <0.001 |
18.5–24.9 | Ref | — | — |
25–29.9 | 0.72 | 0.60–0.87 | <0.001 |
≥30 | 0.79 | 0.64–0.99 | 0.044 |
WHO performance status | <0.001* | ||
0 | Ref | — | — |
1 | 1.01 | 0.74–1.39 | 0.930 |
2 | 1.47 | 1.07–2.00 | 0.017 |
3 | 2.42 | 1.74–3.36 | <0.001 |
4 | 3.28 | 1.98–5.42 | <0.001 |
Missing | 1.37 | 0.96–1.96 | 0.082 |
PaO2 air, per 1 kPa | 0.91 | 0.85–0.98 | 0.009 |
PaCO2 air† | — | — | <0.001 |
Anemia | 1.24 | 1.02–1.50 | 0.034 |
Cardiovascular diagnoses | <0.001* | ||
0 | Ref | — | — |
1 | 1.25 | 1.08–1.46 | 0.003 |
2 | 1.41 | 1.18–1.69 | <0.001 |
>2 | 1.39 | 1.11–1.75 | 0.005 |
Renal failure | 1.34 | 1.03–1.74 | 0.027 |
ACEI/ARB | 0.90 | 0.79–1.04 | 0.166 |
Antiplatelet drugs | 0.86 | 0.75–0.99 | 0.030 |
β-Blockers | 1.19 | 1.04–1.37 | 0.010 |
LABA + ICS | 0.98 | 0.86–1.12 | 0.790 |
Oral glucocorticoids | 1.53 | 1.35–1.73 | <0.001 |
Statins | 0.86 | 0.72–1.03 | 0.105 |
Tiotropium | 0.90 | 0.79–1.03 | 0.126 |
There was no additional explanatory power in the model for interactions between ACEI/ARB and statins (P = 0.298), cardiovascular comorbidities and ACEI/ARB (P = 0.642), cardiovascular comorbidities and β-blockers (P = 0.909) or statins (P = 0.325), or between β-blockers and LABA + ICS or sex (P > 0.15 for both).
To explore the β-blocker effect, we estimated the time-dependent effects of β-blockers for subgroups of patients based on (1) the presence of cardiovascular disease, defined as at least one cardiovascular diagnosis at baseline; (2) exposure to LABA + ICS at baseline; and (3) sex. The negative impact of β-blockers tended to be stronger in patients without LABA + ICS, patients with cardiovascular disease, and in men (Figure E1). The increased mortality for β-blockers (HRs ≥ 1.20) was consistent across all subgroups except in women with LABA + ICS. However, there was no signs of interaction between β-blockers, sex, and LABA + ICS in the fully adjusted model (P = 0.782). Adjusting for ischemic heart disease in addition to the cardiovascular score did not change the β-blocker effect.
This study supports that antiplatelet drugs, and possibly ACEI/ARBs and statins, improve survival, whereas β-blockers decrease survival in patients with severe oxygen-dependent COPD.
This is the first study, to the authors’ knowledge, to evaluate the effects of cardiovascular drugs on mortality in very severe COPD and to account for immortal and immeasurable time bias as well as changes in drug exposure over time.
The improved survival for antiplatelet drugs and the beneficial trends for ACEI/ARBs and statins are consistent with studies of patients with less severe COPD (5–7, 11, 13–15). van Gestel and colleagues reported that statins reduced mortality (HR, 0.67; 95% CI, 0.52–0.86) (10, 14). The study by van Gestel and colleagues is interesting because it was likely free from immortal and immeasurable time bias (14) but was a single-center study of a selected population of patients with COPD undergoing vascular surgery. Improved survival for statins was also found by Rutten and colleagues (HR, 0.83; 95% CI, 0.65–1.08), but no effect was found for ACEI/ARB (HR, 1.01; 95% CI, 0.84–1.21) (11). Short and colleagues recently reported a reduced mortality for ACEI/ARB (HR, 0.79; 95% CI, 0.72–0.88), antiplatelet drugs (HR, 0.80; 95% CI, 0.73–0.88), and statins (HR, 0.89; 95% CI, 0.81–0.97) (7). Compared with previous findings, the effect sizes in the present study were smaller, which might be because we accounted for immortal and immeasurable time bias.
Despite initial concerns, cardioselective β-blockers have in recent years been considered safe and effective to use in COPD based on studies of patients with mostly mild to moderate airflow limitation (7–12, 28, 29). A recent meta-analysis reported that β-blockers were associated with a lower relative risk of mortality of 0.69 (95% CI, 0.62–0.78) but also indicated the presence of publication bias; studies reporting nonbeneficial effects of β-blockers were less likely to be published (12). Our finding of increased mortality in oxygen-dependent COPD is in line with a previous report that the positive effect of β-blockers in milder forms of COPD is lost in patients with more severe respiratory impairment (29).
Strengths of the present study are that it included a large representative national sample of patients with severe oxygen-dependent COPD with complete follow-up. The study could be specifically designed to address immortal and immeasurable time bias, as complete nationwide data was available on hospitalizations and dispensed outpatient drugs. The validity of our findings is supported by the fact that we found effects of LABA + ICS and tiotropium that were compatible with those of the randomized TORCH and UPLIFT trials (30, 31), in contrast to the dramatic effect shown in previous nonrandomized studies (7, 11, 13).
There were two main limitations to our study design, the first being that dispensed prescriptions do not necessarily imply consumption. Adherence to the cardiovascular drugs was, however, generally high among users, and most exposed patients received regular prescriptions, implying that they actually took their drugs. The second limitation, common to all observational designs, is possible confounding by indication owing to the lack of randomization (32). Previous studies have tried to compensate for this by using propensity score methods, which model with or match on the conditional probability of receiving the treatment using a high-dimensional set of pretreatment characteristics (6, 7, 9–11, 33). However, the patient characteristics in this study, as in all previous studies in this field, were collected at study start and not at the start of each drug treatment, implying that the use of propensity score methods might be inappropriate. We addressed confounding by indication by restricting the analysis to patients with very severe COPD and adjusting for markers of disease severity, concomitant medication, and the presence of underlying indicatory diseases. These approaches have been found to be as effective as propensity score methods in terms of reducing confounding by indication, but the absence of residual bias needs to be confirmed by further observational studies and, ultimately, by randomized trials (34).
The mechanism between ACEI/ARB and statins, respectively, and reduced mortality could be their well-documented effects on cardiovascular mortality (35, 36). Our trend of reduced mortality independent of documented cardiovascular disease may be explained either by an effect on undiagnosed disease, because cardiac disease is difficult to identify in severe COPD (37), or by an effect on the excess risk of cardiovascular morbidity and mortality that seems to be a systemic consequence of COPD (38). ACEI/ARBs and statins may also have direct effects on the respiratory system. Animal models recently found effects of ARB on emphysema, exercise capacity, and lung function (39), and effects of simvastatin on smoke-induced pulmonary artery remodeling and emphysema (40). Treatment with an ARB was also shown to decrease the lung hyperinflation in patients with COPD in a placebo-controlled randomized trial (41).
A link between antiplatelet drugs and improved survival could be the systemic antithrombotic effect, because COPD is associated with increased platelet activation (42).
The association between oral glucocorticoids and increased mortality is in line with previous reports and might reflect a causal relation or an association between taking oral steroids and more severe underlying disease and exacerbations (43–45).
Data are limited on adverse effect of β-blockers in severe COPD. A recent Cochrane review found that β-blockers had no negative effects on respiratory symptoms or FEV1 (46). However, this conclusion was based on a small number of relatively old studies with short follow-up, and these studies generally excluded patients with heart failure and other significant comorbidities (46, 47). In contrast, cardioselective β-blockers worsened the airway obstruction in patients with COPD in a randomized double-blind crossover trial (47, 48).
To further explore possible mechanisms behind the increased mortality, we studied the β-blocker effects in different patient subgroups. The adverse effect on survival was generally consistent across groups but tended to be lower for patients using LABA + ICS. This could indicate that long-acting bronchodilation counteracts some of the adverse respiratory effects of β-blockers (47), but subanalyses, including analyses of interaction, have low power and should be interpreted with caution. It cannot be precluded that β-blockers have adverse effects on lung function, airway responsiveness, or other parameters that could increase mortality in frail patients with very severe COPD.
In conclusion, the possible detrimental effect of β-blockers in patients with severe COPD is a novel and important finding that needs to be further validated. In addition, this study supports that antiplatelet drugs, and possibly ACEI/ARB and statins, have beneficial effects on survival in oxygen-dependent COPD.
The authors thank all physicians and nurses who included and cared for the patients.
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Funded by the Research Council of Blekinge, the Swedish Heart-Lung Foundation, and the Swedish National Board of Health and Welfare.
Author Contributions: M.P.E. had full access to all the data in the study and takes full responsibility for the integrity of the data and the accuracy of the data analysis. Conception and design: M.P.E., A.B.H., K.E.S. Acquisition of data: M.P.E., K.E.S. Analysis and interpretation of data: M.P.E., A.B.H. Drafting the article: M.P.E., A.B.H. Revising it for important intellectual content and approval of the version to be published: M.P.E., A.B.H., K.E.S.
This article has an online supplement, which is accessible from this issue’s table of contents at www.atsjournals.org
Originally Published in Press as DOI: 10.1164/rccm.201208-1565OC on January 17, 2013
Author disclosures