Emphysema presence, severity, and distribution has little impact on the clinical presentation of a cohort of patients with mild to moderate COPD

Diagnostic Performance of Dual-Energy Subtraction Radiography for the Detection of Pulmonary Emphysema: An Intra-Individual Comparison

PURPOSE/OBJECTIVES

To compare the diagnostic performance of dual-energy subtraction (DE) and conventional radiography (CR) for detecting pulmonary emphysema using computed tomography (CT) as a reference standard.

METHODS AND MATERIALS

Sixty-six patients (24 female, median age 73) were retrospectively included after obtaining lateral and posteroanterior chest X-rays with a dual-shot DE technique and chest CT within ±3 months. Two experienced radiologists first evaluated the standard CR images and, second, the bone-/soft tissue weighted DE images for the presence (yes/no), degree (1-4), and quadrant-based distribution of emphysema. CT was used as a reference standard. Inter-reader agreement was calculated. Sensitivity and specificity for the correct detection and localization of emphysema was calculated. Further degree of emphysema on CR and DE was correlated with results from CT. A p-value < 0.05 was considered as statistically significant.

RESULTS

The mean interreader agreement was substantial for CR and moderate for DE (kCR = 0.611 vs. kDE = 0.433; respectively). Sensitivity, as well as specificity for the detection of emphysema, was comparable between CR and DE (sensitivityCR 96% and specificityCR 75% vs. sensitivityDE 91% and specificityDE 83%; p = 0.157). Similarly, there was no significant difference in the sensitivity or specificity for emphysema localization between CR and DE (sensitivityCR 50% and specificityCR 100% vs. sensitivityDE 57% and specificityDE 100%; p = 0.157). There was a slightly better correlation with CT of emphysema grading in DE compared to CR (rDE = 0.75 vs. rCR = 0.68; p = 0.108); these differences were not statistically significant, however.

CONCLUSION

Diagnostic accuracy for the detection, quantification, and localization of emphysema between CR and DE is comparable. Interreader agreement, however, is better with CR compared to DE.

Imbalance Between Injury and Defense in the COPD Emphysematous Phenotype

The chronic obstructive pulmonary disease (COPD) emphysematous phenotype is characterized by destruction of lung tissue structure. Patients with this phenotype usually present with typical emphysema-like changes on chest computed Tomography CT, experience higher mortality and poorer prognosis, and are insensitive to routine pharmacological COPD therapy. However, the pathogenesis for the COPD emphysematous phenotype remains unclear, resulting in diagnostic and therapeutic challenges. The imbalance between injury and defense mechanisms is essential in the progression of many pulmonary diseases. Thus, in this review, we focus on the pathogenesis of the COPD emphysematous phenotype and discuss the pathophysiological processes involved in disease progression, from the perspective of injury and defense imbalance.

The Emerging Role of Quantification of Imaging for Assessing the Severity and Disease Activity of Emphysema, Airway Disease, and Interstitial Lung Disease

There has been an explosion of use for quantitative image analysis in the setting of lung disease due to advances in acquisition protocols and postprocessing technology, including machine and deep learning. Despite the plethora of published papers, it is important to understand which approach has clinical validation and can be used in clinical practice. This paper provides an introduction to quantitative image analysis techniques being used in the investigation of lung disease and focusses on the techniques that have a reasonable clinical validation for being used in clinical trials and patient care.

Integrative Genomics Analysis Identifies ACVR1B as a Candidate Causal Gene of Emphysema Distribution

Genome-wide association studies (GWAS) have identified multiple associations with emphysema apicobasal distribution (EABD), but the biological functions of these variants are unknown. To characterize the functions of EABD-associated variants, we integrated GWAS results with 1) expression quantitative trait loci (eQTL) from the Genotype Tissue Expression (GTEx) project and subjects in the COPDGene (Genetic Epidemiology of COPD) study and 2) cell type epigenomic marks from the Roadmap Epigenomics project. On the basis of these analyses, we selected a variant near ACVR1B (activin A receptor type 1B) for functional validation. SNPs from 168 loci with P values less than 5 × 10^-5 in the largest GWAS meta-analysis of EABD were analyzed. Eighty-four loci overlapped eQTL, with 12 of these loci showing greater than 80% likelihood of harboring a single, shared GWAS and eQTL causal variant. Seventeen cell types were enriched for overlap between EABD loci and Roadmap Epigenomics marks (permutation P < 0.05), with the strongest enrichment observed in CD4⁺, CD8⁺, and regulatory T cells. We selected a putative causal variant, rs7962469, associated with ACVR1B expression in lung tissue for additional functional investigation, and reporter assays confirmed allele-specific regulatory activity for this variant in human bronchial epithelial and Jurkat immune cell lines. ACVR1B expression levels exhibit a nominally significant association with emphysema distribution. EABD-associated loci are preferentially enriched in regulatory elements of multiple cell types, most notably T-cell subsets. Multiple EABD loci colocalize to regulatory elements that are active across multiple tissues and cell types, and functional analyses confirm the presence of an EABD-associated functional variant that regulates ACVR1B expression, indicating that transforming growth factor-β signaling plays a role in the EABD phenotype. Clinical trial registered with www.clinicaltrials.gov (NCT00608764).

Influence of Emphysema and Air Trapping Heterogeneity on Pulmonary Function in Patients with COPD

Purpose

To explore the influence of emphysema and air trapping heterogeneity on pulmonary function changes in patients with stable chronic obstructive pulmonary disease (COPD).

Patients and methods

One hundred and seventy-nine patients with stable COPD were enrolled in this prospective study. All patients underwent low-dose inspiratory and expiratory CT scanning and pulmonary-function tests. CT quantitative data for the emphysema index (EI) on full-inspiration and air trapping (AT) on full-expiration were measured for the whole lung, the right and left lungs, and the cranial-caudal lung zones. The heterogeneity index (HI) values for emphysema and air trapping were determined as the ratio of the difference to the sum of the respective indexes. The cranial-caudal HI and left–right lung HI were compared between mild-to-moderate (GOLD stage I and II) and severe (GOLD stage III and IV) disease groups. The associations between HI and pulmonary-function measurements adjusted for age, sex, height, smoking history, EI and AT of the total lung were assessed using multiple linear regression analysis.

Results

The absolute values for cranial-caudal HI (AT_CC_HI) and left–right lung HI (AT_LR_HI) on full-expiration were significantly larger in the mild-to-moderate group, while no significant intergroup differences were observed on full-inspiration. COPD patients with lower-zone and/or left-lung predominance showed significantly lower pulmonary function than those with upper-zone and/or right-lung predominance on full-expiration, whereas no significant differences were observed on full-inspiration. The absolute values of AT_CC_HI and AT_LR_HI significantly correlated with pulmonary-function measurements. Higher AT_CC_HI and lower AT_LR_HI absolute values indicated better pulmonary function, after adjusting for age, sex, height, smoking history, EI and AT of the total lung.

Conclusion

Subjects with more heterogeneous distribution and/or upper-zone predominant and/or right-lung predominant patterns on full-expiration tend to have better pulmonary function. Thus, in comparison with emphysema heterogeneity, AT heterogeneity better reflects the pulmonary function changes in COPD patients.

Complementary regional heterogeneity information from COPD patients obtained using oxygen-enhanced MRI and chest CT

Background

The heterogeneous distribution of emphysema is a key feature of chronic obstructive pulmonary disease (COPD) patients that typically is evaluated using high-resolution chest computed tomography (HRCT). Oxygen-enhanced pulmonary magnetic resonance imaging (OEMRI) is a new method to obtain information regarding regional ventilation, diffusion, and perfusion in the lung without radiation exposure. We aimed to compare OEMRI with HRCT for the assessment of heterogeneity in COPD patients.

Methods

Forty patients with stable COPD underwent quantitative HRCT, OEMRI, and pulmonary function tests, including arterial blood gas analysis. OEMRI was also performed on nine healthy control subjects. We measured the severity of emphysema (percent low attenuation volume; LAV%) in whole lungs and the standard deviations (SDs) of the LAV% values of 10 isovolumetric partitions (SD-LAV) as an index of cranial-caudal heterogeneity. Similarly, relative enhancement ratios of oxygen (RERs) in whole lungs from OEMRI and SD-RER were analyzed.

Results

COPD patients showed a lower mean RER than control subjects (12.6% vs 22.0%, p<0.01). The regional heterogeneity of the RERs was not always consistent with the LAV distribution. Both the HRCT (LAV% and SD-LAV) and the OEMRI (RER and SD-RER) indices were significantly associated with the diffusion capacity (DL_CO) and partial pressure of oxygen in arterial blood (PaO₂). The PaO₂ was associated only with the heterogeneity index of HRCT (SD-LAV) (R² = 0.39); however, the PaO₂ was associated with both the mean RER and heterogeneity (SD-RER) in the multivariate analysis (R² = 0.38).

Conclusions

OEMRI-derived parameters were directly associated with oxygen uptake in COPD patients. Although the OEMRI-derived parameters were not identical to the HRCT-derived parameters, the cranial-caudal heterogeneity in HRCT or OEMRI was complementary to that in evaluations of oxygen uptake in the lungs. Functional imaging seems to provide new insights into COPD pathophysiology without radiation exposure.

CT densitometry in emphysema: a systematic review of its clinical utility

BACKGROUND

The aim of the study was to assess the relationship between computed tomography (CT) densitometry and routine clinical markers in patients with chronic obstructive pulmonary disease (COPD) and alpha-1 anti-trypsin deficiency (AATD).

METHODS

Multiple databases were searched using a combination of pertinent terms and those articles relating quantitatively measured CT densitometry to clinical outcomes. Studies that used visual scoring only were excluded, as were those measured in expiration only. A thorough review of abstracts and full manuscripts was conducted by 2 reviewers; data extraction and assessment of bias was conducted by 1 reviewer and the 4 reviewers independently assessed for quality. Pooled correlation coefficients were calculated, and heterogeneity was explored.

RESULTS

A total of 112 studies were identified, 82 being suitable for meta-analysis. The most commonly used density threshold was -950 HU, and a significant association between CT density and all included clinical parameters was demonstrated. There was marked heterogeneity between studies secondary to large variety of disease severity within commonly included cohorts and differences in CT acquisition parameters.

CONCLUSION

CT density shows a good relationship to clinically relevant parameters; however, study heterogeneity and lack of longitudinal data mean that it is difficult to compare studies or derive a minimal clinically important difference. We recommend that international consensus is reached to standardize CT conduct and analysis in future COPD and AATD studies.

ADRB2 gene polymorphism and emphysema heterogeneity can modulate bronchodilator response in patients with emphysema

BACKGROUND

Genetic variation in the β₂-adrenergic receptor (ADRB2) gene has been thought to have an important role in the differential response to β₂-agonist therapy for asthma. However, previous studies have shown little evidence for an association between these ADRB2 variants and the bronchial dilator response (BDR) in chronic obstructive pulmonary disease (COPD) patients. This discrepancy could be explained by differences in the distribution and heterogeneity of pulmonary emphysema in COPD patients, since emphysema distribution and heterogeneity are thought to have a role in pulmonary function in COPD patients. We hypothesized that differences in emphysema distribution and heterogeneity may have masked significant alterations of the bronchodilator response among ADRB2 genotypes in COPD patients in previous studies.

METHODS

The BDR (induced by 20 μg of procaterol) was measured in 211 patients who had a smoking history of more than 10 pack/years and had undergone chest high resolution computed tomography examination. A low attenuations area (<960 Hounsfield Units) was identified and the emphysema heterogeneity index (EHI%) was calculated with a range in value from -100% to 100%. ADRB2 Arg16Gly genotyping was performed using polymerase chain reaction-restriction fragment length polymorphism analysis.

RESULTS

The BDR was augmented in patients with homogenous emphysema compared with those with upper-dominant emphysema. In patients carrying the AA genotype of ADRB2, the BDR was significantly increased in patients with upper-dominant emphysema, but not in patients with lower-dominant emphysema.

CONCLUSION

Combination analysis of ADRB2 Arg16Gly polymorphism and EHI% may predict the effectiveness of β₂-adrenergic receptor agonist treatment in patients with COPD and emphysema.

Lobar Emphysema Distribution Is Associated With 5-Year Radiological Disease Progression

BACKGROUND

Emphysema has considerable variability in its regional distribution. Craniocaudal emphysema distribution is an important predictor of the response to lung volume reduction. However, there is little consensus regarding how to define upper lobe-predominant and lower lobe-predominant emphysema subtypes. Consequently, the clinical and genetic associations with these subtypes are poorly characterized.

METHODS

We sought to identify subgroups characterized by upper-lobe or lower-lobe emphysema predominance and comparable amounts of total emphysema by analyzing data from 9,210 smokers without alpha-1-antitrypsin deficiency in the Genetic Epidemiology of COPD (COPDGene) cohort. CT densitometric emphysema was measured in each lung lobe. Random forest clustering was applied to lobar emphysema variables after regressing out the effects of total emphysema. Clusters were tested for association with clinical and imaging outcomes at baseline and at 5-year follow-up. Their associations with genetic variants were also compared.

RESULTS

Three clusters were identified: minimal emphysema (n = 1,312), upper lobe-predominant emphysema (n = 905), and lower lobe-predominant emphysema (n = 796). Despite a similar amount of total emphysema, the lower-lobe group had more severe airflow obstruction at baseline and higher rates of metabolic syndrome compared with subjects with upper-lobe predominance. The group with upper-lobe predominance had greater 5-year progression of emphysema, gas trapping, and dyspnea. Differential associations with known COPD genetic risk variants were noted.

CONCLUSIONS

Subgroups of smokers defined by upper-lobe or lower-lobe emphysema predominance exhibit different functional and radiological disease progression rates, and the upper-lobe predominant subtype shows evidence of association with known COPD genetic risk variants. These subgroups may be useful in the development of personalized treatments for COPD.

Influence of emphysema distribution on pulmonary function parameters in COPD patients

OBJECTIVE:

To evaluate the impact that the distribution of emphysema has on clinical and functional severity in patients with COPD.

METHODS:

The distribution of the emphysema was analyzed in COPD patients, who were classified according to a 5-point visual classification system of lung CT findings. We assessed the influence of emphysema distribution type on the clinical and functional presentation of COPD. We also evaluated hypoxemia after the six-minute walk test (6MWT) and determined the six-minute walk distance (6MWD).

RESULTS:

Eighty-six patients were included. The mean age was 65.2 ± 12.2 years, 91.9% were male, and all but one were smokers (mean smoking history, 62.7 ± 38.4 pack-years). The emphysema distribution was categorized as obviously upper lung-predominant (type 1), in 36.0% of the patients; slightly upper lung-predominant (type 2), in 25.6%; homogeneous between the upper and lower lung (type 3), in 16.3%; and slightly lower lung-predominant (type 4), in 22.1%. Type 2 emphysema distribution was associated with lower FEV1, FVC, FEV1/FVC ratio, and DLCO. In comparison with the type 1 patients, the type 4 patients were more likely to have an FEV1 < 65% of the predicted value (OR = 6.91, 95% CI: 1.43-33.45; p = 0.016), a 6MWD < 350 m (OR = 6.36, 95% CI: 1.26-32.18; p = 0.025), and post-6MWT hypoxemia (OR = 32.66, 95% CI: 3.26-326.84; p = 0.003). The type 3 patients had a higher RV/TLC ratio, although the difference was not significant.

CONCLUSIONS:

The severity of COPD appears to be greater in type 4 patients, and type 3 patients tend to have greater hyperinflation. The distribution of emphysema could have a major impact on functional parameters and should be considered in the evaluation of COPD patients.

Present and future utility of computed tomography scanning in the assessment and management of COPD

Computed tomography (CT) is the modality of choice for imaging the thorax and lung structure. In chronic obstructive pulmonary disease (COPD), it used to recognise the key morphological features of emphysema, bronchial wall thickening and gas trapping. Despite this, its place in the investigation and management of COPD is yet to be determined, and it is not routinely recommended. However, lung CT already has important clinical applications where it can be used to diagnose concomitant pathology and determine which patients with severe emphysema are appropriate for lung volume reduction procedures. Furthermore, novel quantitative analysis techniques permit objective measurements of pulmonary and extrapulmonary manifestations of the disease. These techniques can give important insights into COPD, and help explore the heterogeneity and underlying mechanisms of the condition. In time, it is hoped that these techniques can be used in clinical trials to help develop disease-specific therapy and, ultimately, as a clinical tool in identifying patients who would benefit most from new and existing treatments. This review discusses the current clinical applications for CT imaging in COPD and quantification techniques, and its potential future role in stratifying disease for optimal outcome.

The Impact of Homogeneous Versus Heterogeneous Emphysema on Dynamic Hyperinflation in Patients With Severe COPD Assessed for Lung Volume Reduction

Dynamic hyperinflation (DH) is a pathophysiologic hallmark of Chronic Obstructive Pulmonary Disease (COPD). The aim of this study was to investigate the impact of emphysema distribution on DH during a maximal cardiopulmonary exercise test (CPET) in patients with severe COPD.

This was a retrospective analysis of prospectively collected data among severe COPD patients who underwent thoracic high-resolution computed tomography, full lung function measurements and maximal CPET with inspiratory manouvers as assessment for a lung volume reduction procedure. ΔIC was calculated by subtracting the end-exercise inspiratory capacity (eIC) from resting IC (rIC) and expressed as a percentage of rIC (ΔIC %). Emphysema quantification was conducted at 3 predefined levels using the syngo PULMO-CT (Siemens AG); a difference >25% between best and worse slice was defined as heterogeneous emphysema.

Fifty patients with heterogeneous (62.7% male; 60.9 ± 7.5 years old; FEV₁% = 32.4 ± 11.4) and 14 with homogeneous emphysema (61.5% male; 62.5 ± 5.9 years old; FEV₁% = 28.1 ± 10.3) fulfilled the enrolment criteria. The groups were matched for all baseline variables. ΔIC% was significantly higher in homogeneous emphysema (39.8% ± 9.8% vs.31.2% ± 13%, p = 0.031), while no other CPET parameter differed between the groups. Upper lobe predominance of emphysema correlated positively with peak oxygen pulse, peak oxygen uptake and peak respiratory rate, and negatively with ΔIC%. Homogeneous emphysema is associated with more DH during maximum exercise in COPD patients.

Active relearning for robust supervised training of emphysema patterns

Radiologists are adept at recognizing the character and extent of lung parenchymal abnormalities in computed tomography (CT) scans. However, the inconsistent differential diagnosis due to subjective aggregation necessitates the exploration of automated classification based on supervised or unsupervised learning. The robustness of supervised learning depends on the training samples. Towards optimizing emphysema classification, we introduce a physician-in-the-loop feedback approach to minimize ambiguity in the selected training samples. An experienced thoracic radiologist selected 412 regions of interest (ROIs) across 15 datasets to represent 124, 129, 139 and 20 training samples of mild, moderate, severe emphysema and normal appearance, respectively. Using multi-view (multiple metrics to capture complementary features) inductive learning, an ensemble of seven un-optimized support vector models (SVM) each based on a specific metric was constructed in less than 6 s. The training samples were classified using seven SVM models and consensus labels were created using majority voting. In the active relearning phase, the ensemble-expert label conflicts were resolved by the expert. The efficacy and generality of active relearning feedback was assessed in the optimized parameter space of six general purpose classifiers across the seven dissimilarity metrics. The proposed just-in-time active relearning feedback with un-optimized SVMs yielded 15 % increase in classification accuracy and 25 % reduction in the number of support vectors. The average improvement in accuracy of six classifiers in their optimized parameter space was 21 %. The proposed cooperative feedback method enhances the quality of training samples used to construct automated classification of emphysematous CT scans. Such an approach could lead to substantial improvement in quantification of emphysema.

Finding the best thresholds of FEV1 and dyspnea to predict 5-year survival in COPD patients: the COCOMICS study

Background

FEV₁ is universally used as a measure of severity in COPD. Current thresholds are based on expert opinion and not on evidence.

Objectives

We aimed to identify the best FEV₁ (% predicted) and dyspnea (mMRC) thresholds to predict 5-yr survival in COPD patients.

Design and Methods

We conducted a patient-based pooled analysis of eleven COPD Spanish cohorts (COCOMICS). Survival analysis, ROC curves, and C-statistics were used to identify and compare the best FEV₁ (%) and mMRC scale thresholds that predict 5-yr survival.

Results

A total of 3,633 patients (93% men), totaling 15,878 person-yrs. were included, with a mean age 66.4±9.7, and predicted FEV₁ of 53.8% (±19.4%). Overall 975 (28.1%) patients died at 5 years. The best thresholds that spirometrically split the COPD population were: mild ≥70%, moderate 56–69%, severe 36–55%, and very severe ≤35%. Survival at 5 years was 0.89 for patients with FEV₁≥70 vs. 0.46 in patients with FEV₁ ≤35% (H.R: 6; 95% C.I.: 4.69–7.74). The new classification predicts mortality significantly better than dyspnea (mMRC) or FEV₁ GOLD and BODE cutoffs (all p<0.001). Prognostic reliability is maintained at 1, 3, 5, and 10 years. In younger patients, survival was similar for FEV₁ (%) values between 70% and 100%, whereas in the elderly the relationship between FEV₁ (%) and mortality was inversely linear.

Conclusions

The best thresholds for 5-yr survival were obtained stratifying FEV₁ (%) by ≥70%, 56–69%, 36–55%, and ≤35%. These cutoffs significantly better predict mortality than mMRC or FEV₁ (%) GOLD and BODE cutoffs.

High correlation of the response of upper and lower lobe small airway epithelium to smoking

The distribution of lung disease induced by inhaled cigarette smoke is complex, depending on many factors. With the knowledge that the small airway epithelium (SAE) is the earliest site of smoking-induced lung disease, and that the SAE gene expression is likely sensitive to inhaled cigarette smoke, we compared upper vs. lower lobe gene expression in the SAE within the same cigarette smokers to determine if the gene expression patterns were similar or different. Active smokers (n = 11) with early evidence of smoking-induced lung disease (normal spirometry but low diffusing capacity) underwent bronchoscopy and brushing of the upper and lower lobe SAE in order to compare upper vs lower lobe genome-wide and smoking-responsive gene expression by microarray. Cluster and principal component analysis demonstrated that, for each individual, the expression of the known SAE smoking-responsive genes were highly correlated in upper and lower lobe pairs, although, as expected, there were differences in the smoking-induced changes in gene expression from individual to individual. These observations support the concept that the heterogeneity observed among smokers in the anatomic distribution of smoking-induced disease are not secondary to the topographic differences in the effects of cigarette smoke on the airway epithelium.

Distribution and prognostic validity of the new Global Initiative for Chronic Obstructive Lung Disease grading classification

BACKGROUND

The new Global Initiative for Chronic Obstructive Lung Disease (GOLD) update includes airflow limitation, history of COPD exacerbations, and symptoms to classify and grade COPD severity. We aimed to determine their distribution in 11 well-defined COPD cohorts and their prognostic validity up to 10 years to predict time to death.

METHODS

Spirometry in all 11 cohorts was postbronchodilator. Survival analysis and C statistics were used to compare the two GOLD systems by varying time points.

RESULTS

Of 3,633 patients, 1,064 (33.6%) were in new GOLD patient group A (low risk, less symptoms), 515 (16.3%) were B (low risk, more symptoms), 561 (17.7%) were C (high risk, less symptoms), and 1,023 (32.3%) were D (high risk, more symptoms). There was great heterogeneity of this distribution within the cohorts ( x (2) , P < .01). No differences were seen in the C statistics of old vs new GOLD grading to predict mortality at 1 year (0.635 vs 0.639, P = .53), at 3 years (0.637 vs 0.645, P = .21), or at 10 years (0.639 vs 0.642, P = .76).

CONCLUSIONS

The new GOLD grading produces an uneven split of the COPD population, one third each in A and D patient groups, and its prognostic validity to predict time to death is no different than the old GOLD staging based in spirometry only.

Comparison of clinico-physiologic and CT imaging risk factors for COPD exacerbation

To date, clinico-physiologic indices have not been compared with quantitative CT imaging indices in determining the risk of chronic obstructive pulmonary disease (COPD) exacerbation. We therefore compared clinico-physiologic and CT imaging indices as risk factors for COPD exacerbation in patients with COPD. We retrospectively analyzed 260 COPD patients from pulmonary clinics at 11 hospitals in Korea from June 2005 to November 2009 and followed-up for at least one year. At the time of enrollment, none of these patients had COPD exacerbations for at least 2 months. All underwent clinico-physiologic and radiological evaluation for risk factors of COPD exacerbation. After 1 yr, 106 of the 260 patients had at least one exacerbation of COPD. Multiple logistic regression analysis showed that old age, high Charlson Index, and low FEV(1) were significant in a clinico-physiologic model, with C-statistics of 0.69, and that increased age and emphysema index were significant in a radiologic model, with C-statistics of 0.64. The difference between the two models was statistically significant (P = 0.04 by bootstrap analysis). Combinations of clinico-physiologic risk factors may be better than those of imaging risk factors in predicting COPD exacerbation.