American Journal of Respiratory and Critical Care Medicine

Rationale: There is limited knowledge of the prognostic value of quantitative computed tomography (CT) measures of emphysema and airway wall thickness (AWT) on mortality.

Objectives: To examine 8-year mortality in relation to CT-measured emphysema and AWT, and assess if potential impact of these predictors remained after adjustment for lung function.

Methods: In the Norwegian GenKOLS study of 2003–2005, 947 ever-smokers (49% with COPD) aged 40–85 years performed spirometry and CT examination. Mortality data from 2003–2011 were gathered from the Norwegian Cause of Death Registry. CT emphysema % low-attenuation areas (%LAA) and standardized measure for AWT (AWT-Pi10) were main predictors. We performed Laplace regression for survival data, estimating survival time for specified population percentiles within each emphysema category. Models were adjusted for sex, FEV1, COPD status, age, body mass index, smoking, and inflation level.

Measurements and Main Results: During 8-year follow-up all-cause mortality rate was 15%. Although 4% of the subjects with %LAA less than 3 died, 18% with %LAA 3–10 and 44% with %LAA greater than or equal to 10 died. After adjustment, the comparable percentile subjects with medium and high emphysema had 19 months shorter survival than subjects who died in the lowest emphysema category. Subjects with %LAA greater than or equal to 10 had 33 and 37 months shorter survival than the lowest emphysema category with regard to respiratory and cardiovascular mortality, respectively. No significant associations were found between %LAA and cancer and lung cancer mortality. AWT did not predict mortality independently, but a positive interaction with emphysema was observed.

Conclusions: AWT affected mortality with increasing degree of emphysema, whereas CT measure of emphysema was a strong independent mortality predictor.

Scientific Knowledge on the Subject

Chest computed tomography makes it possible to subgroup patients with chronic obstructive pulmonary disease (COPD) into predominantly emphysematic and airways disease phenotypes. So far, however, most patients with COPD are assessed through spirometry alone. No data are available as to how level and distribution of emphysema predicts mortality in subjects with moderate or no COPD, and no data are available on mortality prediction of airway wall thickness.

What This Study Adds to the Field

Level of emphysema predicts all-cause mortality as well as respiratory and cardiovascular mortality in a population-based sample of subjects with and without COPD. Airway wall thickness is associated with respiratory mortality in those with severe emphysema. Given the magnitude of CT examinations performed worldwide each year, predictive effects of such measures on mortality risks are of substantial importance.

Computed tomography (CT) of the thorax is increasingly used to assess subjects with chest symptoms, patients suffering from various lung diseases and coronary heart disease, and in lung cancer screening programs. In chronic obstructive pulmonary disease (COPD) spirometry is crucial in diagnosis and staging. Yet, it is well known that spirometry far from explains the whole picture of COPD (1). Chest CT offers an anatomic correlate of the disease, enabling subgrouping the disease into predominantly emphysematic and airways disease phenotypes.

In the United States some 10 million chest CTs were taken in 2007, representing a huge increase since 1980 (2). Little is known of the prediction of CT-derived emphysema in subjects with and without normal lung function. In heavy ever-smoking men (35) and women (5) it has recently been shown that emphysema predicts increased decline in lung function (3, 4).

Even more scarce data are available on the ability of emphysema to predict mortality (6, 7). One study was based on a sample of predominantly male ever-smokers with extreme smoking consumption (6) and the other was based on patients with α1-antitrypsin deficiency (7). Martinez and coworkers found no association between level of emphysema and risk of death in a large study of patients with severe COPD (8). Emphysema has been related to increased mortality in cancer screening cohorts (9). However, their representativity to the population at large may be questioned.

No data are available as to how level and distribution of emphysema predicts mortality in subjects with moderate or no COPD, and no data are available on mortality prediction of airway wall thickness (AWT). Such data would enhance the risk assessment of subjects examined with chest CT.

We therefore examined the extent of emphysema and AWT versus total and cause-specific mortality in the GenKOLS study, which is a large Norwegian community-based cohort of subjects including patients with and without COPD followed for 8 years. We hypothesized that a greater level of emphysema and AWT would predict total mortality and respiratory, cardiovascular, and lung cancer mortality. We further hypothesized that these associations would be independent of sex, age, lung function, smoking, and body mass index (BMI).

Some of the results of this study have been previously reported in the form of an abstract at the European Respiratory Society annual congress (10).

The GenKOLS study has been described in detail elsewhere (11). Briefly, the study is a community-based study of 462 COPD cases and 485 subjects without COPD examined with a quantitative CT scan. All were aged 40–85 years at study start in 2003–2005 and had greater than or equal to 2.5 pack-years of smoking history. They answered extensive questionnaires and performed spirometry before and after inhaling 400 μg salbutamol (11). COPD cases had post-bronchodilator FEV1/FVC less than 0.70 and FEV1 less than 80% predicted. Subjects without COPD had post-bronchodilator FEV1/FVC greater than 0.70 and FEV1 greater than 80% predicted.

Quantitative CT

CT scans were performed using a GE LightSpeed Ultra CT scanner (120 kVp, 200 mA; GE Healthcare, Milwaukee, WI), at suspended full inspiration (apex to base) using 1-mm slice thickness at 20-mm intervals. Details of the CT assessment have been presented elsewhere (11). Briefly, the extent of emphysema was assessed using the percentage of lung voxels with X-ray attenuation values less than (low-attenuation areas [%LAA]) −950 Hounsfield units. Percent emphysema for the whole lung was calculated, as was the difference between the upper and lower lung regions in percent emphysema (8). To reduce technical errors associated with very small airways, only airways with an internal perimeter greater than 6 mm were included. AWT is presented as the square root of the wall area for a standardized airway with an internal perimeter of 10 mm (1113). To correct for effects of lung inflation on our CT measurements, CT-derived total lung volume was divided by the predicted total lung capacity to obtain a proxy for inflation level (14).

Outcome Variables

We obtained permission from the Regional Ethics Committee in Western Norway. Information on all-cause mortality and on four cause-specific mortality outcomes (respiratory mortality, cardiovascular mortality, cancer mortality, and lung cancer mortality) was obtained from the National Cause of Death Registry in Norway from 2003 through June 2011 (91 mo). For details of the definitions, see the online supplement.

Statistical Analyses

The main predictors of interest in our analyses were emphysema measured as %LAA and AWT (AWT-Pi10). We categorized degree of emphysema after inspecting the distribution in a quantile plot, in which it was clear that most (60%) had very low %LAA, whereas a rise in %LAA occurred between the 60th and the 80th percentile, followed by a sharp rise from the 80th percentile upward (see Figure E1 in the online supplement). Thus, we categorized degree of emphysema into three categories in our analyses: (1) low (<3%LAA); (2) medium (3–10%LAA); and (3) high (>10%LAA). When looking at non-COPD cases who remained alive throughout the study period, the upper limit of normal %LAA (95th percentile) was 4 (95% confidence interval, 3.5–6). Subsequently, all subjects in the less than 3%LAA group were considered well within the normal range. Medium and high emphysema are both considered emphysematous categories, although we acknowledge that subjects with %LAA greater than 3 and less than 4 in the medium category are below the upper limit of normal in the study population. AWT (AWT-Pi10) was analyzed as a continuous variable.

Other covariates that were considered as adjustment factors were sex, age, case-control status, post-bronchodilator FEV1, smoking status, age of smoking onset, pack-years, BMI, and inflation level. All variables that were significant in univariate analyses were adjusted for in the multivariate analyses.

We constructed Kaplan-Meier plots for all mortality outcomes and we performed Laplace regression to examine associations between risk factors and mortality. Laplace regression is an alternative to Cox regression for analysis of survival data (15, 16). It models survival percentiles, instead of hazard ratios. Although the Kaplan-Meier curves tell the proportion of subjects who are still alive at the end of follow-up in each emphysema category, the Laplace regression estimates tell how many months it took before a specified percentile proportion of the study population died within each emphysema category.

Evaluating percentiles of survival time has numerous advantages. Survival percentiles are easier to interpret than hazard ratios, whose limitations are well known (17). The percentiles measure the time it takes for any given proportion of individuals to die and allow comparisons across groups of individuals while adjusting for potential confounders. Easy interpretability may help convey research findings to lay readers.

When interpreting Laplace results, we compared the same percentile across groups. For example, when 4% died in the lowest-degree emphysema category (<3%LAA), we examined the fourth percentile also in the middle (≥3 to <10%LAA) and high (≥10%LAA) emphysema categories, even if more subjects died in the middle and high categories. This ensured a valid comparison across groups. We used bootstrapping to estimate the standard errors of the Laplace regression coefficients. In addition, we estimated Harrell C concordance statistic, which assesses the prognostic ability of a variable (18).

Finally, we tested for interactions between sex and degree of emphysema on the various mortality outcomes, and for interactions between FEV1 and degree of emphysema. We also tested for interaction between AWT and degree of emphysema. The significance level for the interaction effects was set to 0.01 to avoid type 2 error.

The study population consisted of 947 subjects (Table 1). Male sex, COPD, lower FEV1, increasing age, being an ex-smoker, increasing pack-years, lower BMI, and increasing emphysema severity were risk factors associated with mortality status (Table 1). Although most of the subjects who remained alive throughout the study period had low %LAA, only a minority of those who died during the follow-up period had equally low %LAA. Whereas only 13% of those who were still living at the end of the follow-up period had %LAA greater than or equal to 10, as many as 58% of those who died during the study period had high %LAA. Also, there was a statistically nonsignificant tendency for increasing AWT to be related to mortality risk.

TABLE 1. BASELINE CHARACTERISTICS BY MORTALITY STATUS OF PARTICIPANTS IN THE GENKOLS STUDY 2003–2005

CharacteristicAlive (n = 803)Deceased (n = 144)Significance*Total (n = 947)
Women, %352 (44)40 (28)<0.01392 (41)
Men, %451 (56)104 (72)555 (59)
Subjects without COPD, %468 (58)17 (12)<0.01485 (51)
COPD cases, GOLD Stage II, %228 (28)49 (34)277 (29)
COPD cases, GOLD Stage III, %83 (10)46 (32)129 (14)
COPD cases, GOLD Stage IV, %24 (3)32 (22)56 (6)
FEV1, mean (SD)82 (24)51 (23)<0.0177 (26)
Age, mean (SD)58 (10)70 (8)<0.0160 (10)
Ex-smokers, %425 (53)92 (64)0.02517 (55)
Current smokers, %378 (47)52 (36)430 (45)
Age of smoking onset, mean (SD)18 (5)19 (6)0.0318 (5)
Pack-years, mean (SD)23 (15)34 (21)<0.0125 (17)
Body mass index, mean (SD)26.2 (4.4)24.8 (4.6)<0.0126.0 (4.4)
%LAA <3542 (68)26 (18)<0.01568 (60)
%LAA ≥3 to <10155 (19)35 (24)190 (20)
%LAA ≥10106 (13)83 (58)189 (20)
AWT-Pi10, mm, mean (SD)4.84 (0.3)4.88 (0.3)0.234.85 (0.3)

Definition of abbreviations: AWT = airway wall thickness; COPD = chronic obstructive pulmonary disease; GOLD = Global Initiative for Chronic Obstructive Lung Disease; LAA = low-attenuation area.

*Two-sided P value from Monte Carlo permutation test after 100 random permutations.

Percent of predicted post-bronchodilator.

Of the subjects without COPD, 90% (n = 434) had %LAA below 3%; 9% (n = 45) had %LAA between 3% and 10%; and 1% (n = 6) had %LAA 10 and above. The corresponding figures for COPD cases were 29% (n = 134); 31% (n = 145); and 40% (n = 183), respectively.

Of the 144 deaths that occurred in the study population from baseline through June 2011, 28% were respiratory deaths, 10% died of cardiovascular causes, 11% died of cancer other than lung cancer, and 17% died of lung cancer.

Kaplan-Meier plots show survival estimates by degree of emphysema for respiratory mortality, cardiovascular mortality, cancer mortality, lung cancer mortality, and all-cause mortality (Figure 1). Although 4% of the subjects in the lowest emphysema category died, the corresponding figure was 18% for subjects in the middle emphysema category and 44% in the highest emphysema degree category (Figure 1E). The tendency was the same for all cause-specific outcomes, in that mortality increased with increasing degree of emphysema, being most pronounced for respiratory mortality (Figure 1A).

Univariate Risk Factors for Mortality

Figure 1E shows that 4% of those with low %LAA, 18% of those with medium %LAA, and 44% of those with high %LAA died (all-cause mortality) during the follow-up period. Correspondingly, Table 2 for all-cause mortality shows that whereas the 4% who died in the low %LAA category all died within 79 months of follow-up, the first 4% who died in the medium %LAA category died within 23 months (79 minus 56), and the first 4% who died in the high %LAA category died within 12 months of follow-up (79 minus 67). Figure E1 in the online supplement further illustrates these survival estimates. A similar trend was present when looking at Global Initiative for Chronic Obstructive Lung Disease (GOLD) severity stages and number of survival months (see Figure E2). Whereas the first 4% who died of all causes in GOLD stage 0 died within the first 91 months, the first 4% who died in GOLD stage 2 died within the first 21 months of follow-up, and the first 4% who died in GOLD stages 3 and 4 died within the first 11 months (see Figure E2). AWT did not influence all-cause mortality. Other significant risk factors in the univariate analyses of all-cause mortality were having COPD, decreased FEV1 % predicted, increased age, and being an ex-smoker versus a current smoker.

TABLE 2. UNIVARIATE ANALYSES OF PREDICTORS OF MORTALITY FROM BASELINE THROUGH JUNE 2011, LAPLACE REGRESSION

All-Cause (95% CI)Respiratory (95% CI)Cardiovascular (95% CI)Cancer (95% CI)Lung Cancer (95% CI)
Women37 (21 to 52)11 (−8 to 30)11 (−26 to 48)30 (10 to 50)43 (3 to 83)
Men−9 (−25 to 8)−6 (−28 to 16)1 (−43 to 45)3 (−27 to 34)−12 (−49 to 25)
Subjects without COPD93 (84 to 101)92 (43 to 142)93 (84 to 102)72 (49 to 95)99 (77 to 122)
COPD cases−73 (−83 to −63)−88 (−141 to −36)−86 (−93 to −79)−49 (−76 to −21)−76 (−103 to −50)
FEV1 % predicted (post-bronchodilator)1.3 (1.0 to 1.6)1.6 (0.7 to 2.5)1.6 (1.1 to 2.2)0.8 (0.0 to 1.6)1.1 (0.6 to 1.7)
Age−3 (−4 to −3)−2 (−4 to −1)−5 (−6 to −3)−3 (−4 to −2)−3 (−5 to −2)
Ex-smokers24 (18 to 31)4 (−3 to 11)7 (−18 to 32)31 (23 to 39)31 (11 to 51)
Current smokers12 (3 to 21)25 (7 to 43)14 (−11 to 40)17 (−9 to 42)1 (−31 to 34)
Age of smoking onset0 (−1 to 1)1 (1 to 2)1 (1 to 2)1 (−1 to 2)1 (−2 to 4)
Pack-years−0.6 (−1.1 to −0.1)0.1 (−0.5 to 0.7)0 (−1 to 1)−0.6 (−1.1 to −0.3)−0.9 (−1.5 to −0.3)
Body mass index1 (−1 to 3)5 (2 to 9)3 (−1 to 7)3 (−0 to 7)1 (−4 to 6)
%LAA <379 (63 to 95)78 (58 to 97)76 (57 to 96)75 (56 to 93)79 (67 to 92)
%LAA ≥ 3 to <10−56 (−71 to −41)−69 (−90 to −48)−67 (−94 to −41)−47 (−77 to −17)−52 (−85 to −18)
%LAA ≥10−67 (−88 to −46)−78 (−102 to −54)−72 (−91 to −53)−52 (−72 to −31)−56 (−83 to −30)
AWT-Pi10, mm−71 (−312 to 170)211 (−134 to 556)154 (−2 to 309)−107 (−481 to 267)−240 (−508 to 28)

Definition of abbreviations: AWT = airway wall thickness; CI = confidence interval; COPD = chronic obstructive pulmonary disease; LAA = low-attenuation area.

N = 947 subjects from the GenKOLS study 2003–2005.

Estimated percentiles: 4 percentile for all-cause mortality, 0.3 percentile for respiratory and cardiovascular mortality, and 1.5 percentile for cancer and lung cancer mortality. For reference categories in categorical variables, coefficient represents number of months survival and for the other categories, coefficients represent number of months more or less survival relative to the reference category. For continuous variables, coefficients represent number of months survival with every increase in the continuous variable. Numbers in bold font type differ from reference categories with statistical significance (P < 0.05).

In addition, inflation level was significantly related to all-cause mortality and lung cancer mortality, but not to respiratory, cardiovascular, or cancer mortality.

Emphysema was a significant predictor of all the cause-specific mortalities (Table 2, see Figure E1), with increasing level of emphysema being related to shorter survival. The greatest gradient by emphysema was seen for respiratory mortality, the lowest for cancer mortality. The same pattern was observed for GOLD severity stages (see Figure E2). Post hoc estimations of Harrell C concordance statistic for %LAA and for GOLD severity classification with regard to the different mortality outcomes (Table 3) showed that emphysema predicted respiratory mortality with excellent discrimination (C = 0.85) and all-cause mortality, cardiovascular mortality, and cancer mortality with acceptable discrimination (C between 0.7 and 0.8). Emphysema predicted respiratory and cardiovascular mortality more accurately than did the GOLD severity classification. AWT predicted none of the cause-specific mortalities in the crude analyses.

TABLE 3. ESTIMATIONS OF HARRELL C CONCORDANCE STATISTIC FOR %LAA AND GOLD SEVERITY CLASSIFICATION WITH REGARD TO MORTALITY OUTCOMES FROM BASELINE THROUGH JUNE 2011 AFTER UNIVARIATE LAPLACE REGRESSION ANALYSES

C Statistic %LAAC Statistic GOLD Severity
All-cause mortality0.760.77
Respiratory mortality0.850.72
Cardiovascular mortality0.770.71
Cancer mortality0.700.63
Lung cancer mortality0.660.75

Definition of abbreviations: GOLD = Global Initiative for Chronic Obstructive Lung Disease; LAA = low-attenuation area.

N = 947 subjects from the GenKOLS study 2003–2005.

Of the other predictors examined increasing age and having COPD were related to all the specific outcomes, whereas decreased FEV1 in percent predicted was related to all the outcomes except cancer mortality. Being a current compared with an ex-smoker was related to increased respiratory and cardiovascular survival, earlier onset of smoking was related to reduced respiratory and cardiovascular survival, and increasing pack-years of smoking was related to reduced survival in the cancer and lung cancer mortality (Table 2). In addition, inflation level was significantly related to all-cause mortality and lung cancer mortality, but not to respiratory, cardiovascular, or cancer mortality.

CT Measures of Emphysema in Multivariate Analyses of Mortality

In the multivariate analyses of all-cause mortality the middle and high emphysema groups had 19 months shorter survival than the lowest emphysema group, after adjustment for other significant risk factors (Table 4). Similarly, increasing degree of emphysema was associated with higher respiratory mortality and higher cardiovascular mortality (Table 4). CT measures of emphysema were not significantly associated with cancer and lung cancer mortality in the multivariate analyses (Table 4).

TABLE 4. ASSOCIATIONS BETWEEN %LAA AND MORTALITY OUTCOMES FROM BASELINE THROUGH JUNE 2011, ADJUSTED FOR UNIVARIATE SIGNIFICANT PREDICTORS*

Survival Difference in Months (95% CI)P Value
All-cause mortality
 %LAA < 3Reference
 3 ≤ %LAA < 10−19 (−40 to 2)0.08
 %LAA ≥ 10−19 (−37 to 1)0.05
Respiratory mortality
 %LAA < 3Reference
 3 ≤ %LAA < 10−60 (−83 to −38)<0.01
 %LAA ≥ 10−33 (−56 to −11)<0.01
Cardiovascular mortality
 %LAA < 3Reference
 3 ≤ %LAA < 10−39 (−82 to 5)0.08
 %LAA ≥ 10−37 (−64 to −9)0.01
Cancer mortality
 %LAA < 3Reference
 3 ≤ %LAA < 10−23 (−67 to 22)0.32
 %LAA ≥ 10−13 (−50 to 23)0.48
Lung cancer mortality
 %LAA < 3Reference
 3 ≤ %LAA < 10−12 (−48 to 24)0.50
 %LAA ≥ 109 (−27 to 44)0.63

Definition of abbreviations: CI = confidence interval; COPD = chronic obstructive pulmonary disease; LAA = low-attenuation area.

Laplace regression. N = 947 subjects from the GenKOLS study 2003–2005.

*The model for all-cause mortality is adjusted for sex, interaction between sex and %LAA, COPD status, post-bronchodilator FEV1 % predicted, age, smoking status, and inflation level. The model for respiratory mortality is adjusted for COPD status, post-bronchodilator FEV1 % predicted, smoking status, and body mass index. The model for cardiovascular mortality is adjusted for COPD status, post-bronchodilator FEV1 % predicted, and age. The model for cancer mortality is adjusted for COPD status, age, and post-bronchodilator FEV1 % predicted. The model for lung cancer mortality is adjusted for COPD status, post-bronchodilator FEV1 % predicted, age, pack-years, and inflation level.

Coefficient estimated for 4 percentile (all-cause mortality), 0.3 percentile (respiratory and cardiovascular mortality), and 1.5 percentile (cancer and lung cancer mortality). Numbers in bold font type differ from reference categories with statistical significance (P < 0.05).

The multivariate analyses were then repeated excluding those with GOLD stadium 3 and 4, and also repeated once more excluding those without COPD (see Table E1). The regression coefficients remained unchanged; emphysema was associated with shorter survival time for the healthier part of the population (non-COPD and GOLD stage 2) and the subjects with COPD (GOLD stage 2–4) with regard to respiratory mortality and cardiovascular mortality.

Interaction Analyses

We then tested the a priori decided interactions. The only one that reached level of significance in the multivariate analyses was AWT and degree of emphysema on respiratory mortality. Although AWT was not associated with respiratory mortality in itself, increased AWT reduced survival time in subjects with more severe emphysema (Figure 2).

We also examined the association between emphysema and GOLD stages on all-cause mortality (Figure 3). Although not statistically significant as an interaction term, there was a clear pattern that for those in the lowest emphysema category, GOLD severity stage influenced survival time. For subjects without COPD and also subjects with COPD in GOLD stage 2, degree of emphysema affected survival time.

This study has shown that level of emphysema predicts all-cause mortality, and respiratory and cardiovascular mortality in a population-based sample of subjects with and without COPD. There also seemed to be an association between AWT and respiratory mortality, but only in those with severe emphysema. These associations are independent of sex, age, smoking, lung function, and BMI. To our knowledge this is the first study to examine the predictive ability of emphysema on mortality in a community-based sample, and also the first study to examine the association between AWT and mortality. Given the magnitude of CT examinations performed worldwide each year, predictive effects of such measures on mortality risks are of substantial importance. The fact that our study comprised also subjects without COPD and that it focused on several mortality outcomes makes the results from this study relevant across medical disciplines.

Emphysema and Mortality

Our data on mortality by emphysema extend previous findings in male COPD cases with extreme smoking consumption (6) or subjects suffering from α1-antitrypsin deficiency (7). Our results contradict those of the NETT study showing no relationship between emphysema level and mortality (8). The discrepancy between our study and the NETT study could be caused by the latter comprising highly selective subjects of severe COPD without significant comorbidities, whereas the current was a community-based study of mostly moderate or no COPD.

There may be several explanations why emphysema predicts respiratory mortality. First, recent longitudinal studies of COPD cases with GOLD stage II–IV have shown a relationship between increased level of emphysema and subsequent greater FEV1 decline (35). The mechanisms behind this association are yet to be clarified. Emphysema may be the cause of the FEV1 decline, but could also be part of the disease process. Second, level of emphysema is related to lung-specific inflammatory markers, such as surfactant protein D (19), and systemic inflammatory markers including C-reactive protein, total white blood count, and neutrophils, which all in turn are predictors of mortality. Hence, lung-specific and systemic inflammation may cause disease progression and increased mortality. Third, we and others have shown cross-sectionally that emphysema is related to low muscle mass and osteoporosis, which are related to increased mortality (2022). However, it is yet to be determined if emphysema proceeds low skeletal muscle mass and osteoporosis or vice versa (20).

Emphysema independently predicted cardiovascular mortality. It has long been known that terminal stages of COPD are associated with cor pulmonale (23, 24). However, we observed an impact of emphysema on cardiovascular mortality also when excluding those with severe and very severe COPD. This is in line with recent findings of a population-based study of 2,816 subjects aged 40–86 years, in which emphysema was linearly related to impaired left ventricular filling, reduced stroke volume, and lower cardiac output without changes in the ejection fraction (25). Potential mechanisms in early, mild emphysema may be the subclinical loss of lung parenchyma and the pulmonary capillary bed (26). Another mechanism may be endothelial and microvascular dysfunction of the pulmonary and systemic capillary system observed cross-sectionally in subjects with mild emphysema (27). To date it is not known if endothelial damage may contribute to emphysema or vice versa. Experimental data indicate both directions (28). Finally, the emphysema-cardiovascular mortality association may work through common risk factors not adjusted for (i.e., airborne pollutants other than smoking) (29).

Increased level of emphysema and impaired lung function in terms of FEV1 in percent predicted were both univariately related to increased mortality of lung cancer. However, after including them in a multivariate model together with smoking habits (Table 4), only lung function remained a significant predictor. This is in line with a recent metaanalysis finding that quantitative emphysema assessment was not a predictor, whereas qualitative emphysema assessment predicted mortality from lung cancer (30).

AWT and Mortality

We observed that increased AWT was associated with respiratory mortality in those with severe emphysema. This is in line with a recent micro-CT examination of lung specimen from patients with COPD GOLD stage 4 showing that narrowing and loss of terminal bronchioles were related to emphysematous destruction (31). This study also found by multidetector CT that number of airways 2–2.5 mm in diameter was increasingly reduced with increasing GOLD stage (31). Our study expands these results by showing that the burden of airway remodeling as reflected by AWT on the respiratory system may be enough to affect mortality, but only in those with severe emphysema. Although respiratory mortality in the highest emphysema category ranged from 13% in those with low AWT to 22% in those with high AWT (Figure 2), AWT did not affect mortality rate in the mild and medium emphysema groups. However, it is difficult to fully disentangle the meaning of the observed interaction between AWT and emphysema on respiratory mortality. Studies with more statistical power and more robust AWT measurements are needed to examine this association further.

Strengths and Limitations

There are several methodologic strengths in this study. First, the study population is large and includes subjects with and without COPD. Second, all CT scans were performed with the same scanner. Third, the present study had a long follow-up period (8 yr), which is of the essence when examining mortality in a middle-aged population such as this. Fourth, a thorough validation of all the death certificates was undertaken (see online supplement).

Certain methodologic weaknesses need further discussion. First, the method for measuring AWT prevented us from measuring airways smaller than 2 mm in diameter. Given that COPD is mainly a small-airway disease, this is suboptimal. However, it has been shown that measurements of larger airways can be used as a surrogate for the processes in the smaller airways (32), and that the inflammatory response in smaller airways can also be seen in larger airways (33). Second, the extent of emphysema is represented as the percentage of the lung occupied by low X-ray attenuating regions. However, level of inspiration during the scan, cardiac congestion, fibrosis, and increased phlegm production reduce %LAA, whereas image noise increases the level of %LAA thereby influencing the specificity of %LAA as an indicator of emphysema. However, subjects with severe fibrosis were excluded from analysis, and the level of inspiration was taken into account by adding the ratio of CT-assessed lung volume to predicted total lung capacity. We also adjusted for phlegm production without this affecting the observed coefficients overtly (additional analyses, results not shown). The large number of subjects in this study works to decrease the effect of outliers on the associations between emphysema and mortality.

Implication of the Findings

Chest CT is increasingly used in the clinical evaluation of subjects with and without COPD (34). Emphysema is frequently observed also in subjects with moderate COPD and in ever-smokers without COPD (11). The present study offers the first data on how emphysema level predicts mortality in these groups, and may be used in their risk assessment. Accurate prediction of mortality is important because it helps identify patients in whom the implementation of specific therapeutic measures may improve outcome.

In conclusion, this population-based study of subjects with and without COPD has shown that level of emphysema is related to increased all-cause, respiratory, and cardiovascular mortality. Also, increased AWT is related to increased respiratory mortality in those with severe emphysema, although further studies are needed to examine this association more closely.

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Correspondence and requests for reprints should be addressed to Ane Johannessen, Ph.D., Centre for Clinical Research, Haukeland University Hospital, 5021 Bergen, Norway. E-mail:

Author Contributions: A.J. took part in the data collection, performed the statistical analyses, drafted and revised the paper. T.D.S., H.C., E.O., and A.G. took part in the data collection and revised the draft paper. M.B. provided statistical guidance and revised the draft paper. P.B. took part in the data collection, drafted and revised the paper. T.B.G. and A.D. revised the draft paper. R.M.N. revised the draft paper and constructed Figures 2 and 3.

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.201209-1722OC on January 17, 2013

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