Respiratory physiology Prevalence of reduced lung diffusing capacity and CT scan findings in smokers without airflow limitation: a population-­based study Francisco Garcia-­Rio ‍ ‍,1,2 Marc Miravitlles ‍ ‍,2,3 Joan B Soriano ‍ ‍,2,4 Borja G Cosío,2,5 Juan José Soler-­Cataluña ‍ ‍,2,6 Ciro Casanova,2,7 Pilar de Lucas,8 Inmaculada Alfageme,9 José Miguel Rodríguez González-­Moro,10 María Guadalupe Sánchez Herrero ‍ ‍,11 Julio Ancochea,2,4 EPISCAN II study To cite: Garcia-­Rio F, ABSTRACT Miravitlles M, Soriano JB, et al. Background Population distribution of reduced diffusing WHAT IS ALREADY KNOWN ON THIS TOPIC Prevalence of reduced lung capacity of the lungs for carbon monoxide (DLCO) in ⇒ Diffusing capacity of the lungs for carbon monox- diffusing capacity and CT scan smokers and main consequences are not properly ide (DLCO) provides information on the effective findings in smokers without recognised. The objectives of this study were to describe alveolar-­ capillary surface area available for gas airflow limitation: a population-­ the prevalence of reduced DLCO in a population-­based transfer in the lungs. based study. BMJ Open Resp Res 2023;10:e001468. sample of current and former smoker subjects without airflow limitation and to describe its morphological, WHAT THIS STUDY ADDS doi:10.1136/ bmjresp-2022-001468 functional and clinical implications. ⇒ Reduced DLCO is a frequent finding among smokers Methods A sample of 405 subjects aged 40 years or without airflow limitation. ► Additional supplemental older with postbronchodilator forced expiratory volume in ⇒ This functional disorder is associated with a limited material is published online 1 s/forced vital capacity (FVC) >0.70 was obtained from exercise tolerance and reduced daily physical activ- only. To view, please visit the a random population-­based sample of 9092 subjects ity, as well as a larger pulmonary artery diameter. journal online (http://​dx.​doi.​ evaluated in the EPISCAN II study. Baseline evaluation org/​10.​1136/​bmjresp-​2022-​ HOW THIS STUDY MIGHT AFFECT RESEARCH, 001468). included clinical questionnaires, exhaled carbon monoxide (CO) measurement, spirometry, DLCO determination, 6 min PRACTICE OR POLICY walk test, routine blood analysis and low-­dose CT scan ⇒ Absence of differences in the lung parenchyma at- Received 21 September 2022 Accepted 22 November 2022 with evaluation of lung density and airway wall thickness. tenuation suggests that DLCO may be a surrogate Results In never, former and current smokers, prevalence marker of smoking-­ induced early vascular lung of reduced DLCO was 6.7%, 14.4% and 26.7%, damage. respectively. Current and former smokers with reduced DLCO without airflow limitation were younger than the subjects with normal DLCO, and they had greater levels of dyspnoea and exhaled CO, greater pulmonary artery that is, usually identified by spirometry, the diameter and lower spirometric parameters, 6 min walk most common lung function test. distance, daily physical activity and plasma albumin levels Diffusing capacity of the lungs for carbon (all p<0.05), with no significant differences in other chronic monoxide (DLCO), a complex test that respiratory symptoms or CT findings. FVC and exhaled CO provides a quantitative measure of the effec- were identified as independent risk factors for low DLCO. tive alveolar-­ capillary surface area available Conclusion Reduced DLCO is a frequent disorder for gas transfer in the lungs, is also currently among smokers without airflow limitation, associated used in the clinical setting for the characteri- with decreased exercise capacity and with CT findings suggesting that it may be a marker of smoking-­induced sation of subjects with COPD.3 DLCO is useful early vascular damage. in distinguishing COPD phenotypes4 5 and for © Author(s) (or their Trial registration number NCT03028207. predicting increased symptoms, poor quality employer(s)) 2023. Re-­use of life, decreased exercise tolerance and permitted under CC BY-­NC. No commercial re-­use. See rights severe exacerbations in patients with COPD.3 and permissions. Published by Moreover, among patients with airflow limita- BMJ. INTRODUCTION tion, DLCO negatively correlates with the For numbered affiliations see Cigarette smoke has been identified as a major extend of emphysema on lung CT scans.6 end of article. risk factor for chronic obstructive pulmonary Moreover, DLCO seems to have a role in the Correspondence to disease (COPD), a progressive destructive evaluation of at-­risk subjects, in early stages of Dr Francisco Garcia-­Rio; lung disease with persistent chronic inflam- lung disease or when they have not yet devel- ​fgr01m@​gmail.​com mation characterised by airflow limitation,1 2 oped it.7 Thus, some evidence suggests that a Garcia-­Rio F, et al. BMJ Open Resp Res 2023;10:e001468. doi:10.1136/bmjresp-2022-001468    1 Open access reduced DLCO predicts a future decline in forced expi- Study subjects ratory volume in 1 s (FEV1),8 9 and it has been reported Subjects from the general population, who resided in that, in smokers with normal spirometry, a low DLCO the postal code areas nearest to the participating hospi- increases the risk of developing COPD.10 Even a DLCO tals, were selected. A list of random telephone numbers below 85% predicted has been found to be a significant was obtained, stratified according to these postal codes predictor of all-­cause mortality.11 and quotas for sex and age groups. We selected men or This is particularly important, since DLCO could women aged 40 years or more, with no physical or cogni- provide more integrated information about the lung tive difficulties that would prevent them from completing abnormalities induced by cigarette smoking. Although spirometry or any of the study procedures. To evaluate most interest has focused on the contributions of ciga- the secondary outcomes, the first 35 subjects with airflow rette smoke towards injuring the extracellular matrix and limitation and the first 35 subjects without airflow limita- pulmonary epithelial cells, critical interdependence has tion from 12 preselected sites were consecutively invited been recognised between alveolar epithelial and micro- to undergo a DLCO test and thoracic CT scan, with the vascular endothelial cells to maintain the airspace struc- aim to recruit a total of approximately 400 individuals for ture, and loss of endothelial cells within the lung directly each group. contributes to emphysematous remodelling.12 In fact, Patients or the public were not involved in the design, along with persistent inflammation, protease-­antiprotease or conduct, or reporting, or dissemination plans of our imbalance and oxidative stress, smoking-­induced endo- research. thelial damage has been linked to pulmonary lesions and For the purposes of this prespecified secondary objec- systemic comorbidities, including pulmonary hyperten- tive of EPISCAN II, a current smoker was defined as an sion and atherosclerosis.13 individual who smoked at least one cigarette, cigar or Although DLCO provides insight into respiratory phys- pipe a day and a former smoker was defined as an indi- iology beyond that obtained with spirometry, including vidual who had discontinued using any form of tobacco indirect measurement of pulmonary vascular abnor- at least 6 months before the study visit. Current or former malities, no previous information is available about the smoker subjects were included in the analysis if they had population prevalence of low DLCO in subjects with a a postbronchodilator FEV1/forced vital capacity (FVC) history of smoking without airflow limitation as well as >0.70 and a cumulative tobacco use of at least 10 pack-­ its clinical and physiological consequences.14 The second years. As a control group, never-­smoker subjects with a Epidemiology of COPD in Spain (EPISCAN II) study, a postbronchodilator FEV1/FVC >0.7 were included. population-­ based epidemiological study designed with the main objective of determining the prevalence of COPD in Spain through the analysis of a representative Procedures and measurements sample of adults from all Spanish regions,15 provides an Demographic information on anthropometric charac- ideal opportunity to analyse these aspects. This manu- teristics, level of education, family conditions, smoking script presents the results of a secondary objective which history and comorbidities were collected. Comorbidities was to assess the prevalence of DLCO alterations in a were assessed using the Charlson and COPD-­ specific population-­based sample of current or former smoker co-­morbidity test (COTE) indices.17 18 Health status was subjects without airflow limitation, to identify its deter- assessed by the COPD Assessment Test (CAT) question- minants and to evaluate the physiological and clinical naire,19 and the respective questions of the European parameters related to DLCO in these subjects. Community for Coal and Steel Questionnaire (ECSC)20 were used to identify respiratory symptoms (chronic cough, chronic bronchitis, chronic expectoration, dysp- noea or wheezing). The degree of dyspnoea was evalu- METHODS ated by the modified Medical Research Council dyspnoea EPISCAN II is a national, multicentre, cross-­sectional, scale.21 Physical activity was measured by the Yale Physical population-­based epidemiological study, whose Activity Survey questionnaire validated for the Spanish protocol, fieldwork and methods have been previously population and the elderly population, providing a described.15 16 Briefly, the study was conducted at 20 summary of the physical activity level, as well as the time teaching hospitals throughout Spain from April 2017 to and energy cost of weekly physical activity.22 February 2019. Prebronchodilator and postbronchodilator spirom- etry was performed using a pneumotachograph (Vyntus Spiro, Carefusion, Germany), according to American Patient and public involvement Thoracic Society (ATS)/European Respiratory Society Patients and public were not involved in the develop- (ERS) standardisation23 and using the Global Lung ment of the research question, design or recruitment; no Initiative equations as reference values.24 DLCO was patient advisors were required, and data were analysed measured by the single-­breath method with the same anonymously. The results will be disseminated to the equipment in all study centres (MasterScreen diffusion, scientific community in academic writing. Carefusion, Germany) in accordance with ATS/ERS 2 Garcia-­Rio F, et al. BMJ Open Resp Res 2023;10:e001468. doi:10.1136/bmjresp-2022-001468 Open access Figure 1 Flow chart of study participants. DLCO, diffusing capacity of the lungs for carbon monoxide. recommendations.5 The DLCO was corrected to body image using mediastinal windows.30 PA enlargement was temperature, pressure, water vapour saturated condi- defined as a PA diameter ≥29 mm in men and ≥27 mm tions and a minimum of two acceptable manoeuvres in women.31 that matched within 10% or less of the alveolar volume (VA) and DLCO was required. We excluded tests with a Statistical analysis breath-­holding time <9 or >11 s, an inspiratory capacity Categorical variables were presented as numbers with less than 85% of the largest previously measured vital percentages, and continuous variables as mean with SD or capacity, expiration in >4 s, or with evidence of leaks or median (IQR), according to their distribution. Compari- Valsalva or Müller manoeuvres. Adjustments were made sons between subgroups have been performed using the for atmospheric pressure, haemoglobin levels and CO Student’s t-­test, analysis of variance, Mann-­Whitney U test back pressure using non-­invasive estimation of carboxy- or χ2 test. The relationship between variables has been haemoglobin by a CO-­oximeter according to equations assessed with the Pearson correlation analysis. Multiple recommended in current standardisation.25 Cotes equa- logistic regression analysis has been used to investigate tions were used as reference values,26 and values below factors associated with a reduced DLCO. Only variables the lower limit of normal were considered reduced. with a level of significance <0.01 in the bivariate analysis The 6 min walk test was performed in accordance with were included in the multiple regression model. The ATS guidelines,27 and the Body Obstructive Dyspnoea ORs and the coefficient of determination (r2) were calcu- Exercise (BODE) index28 was calculated accordingly. lated for the model. Data were analysed with the SPSS From each participant, 20 mL of venous blood were V.25.0 software, considering a p value of 0.05 statistically collected for routine blood analysis, C reactive protein, significant for all tests. fibrinogen and albumin. CT images were acquired during maximal inspiration, without contrast and with low-­dose radiation (120 kVp RESULTS as acquisition voltage). The images obtained under- The EPISCAN II population included 9092 subjects went semiautomatic postprocessing for determination who were able to perform a valid spirometry. Among of the percentage of emphysema, areas of low attenua- those, 8015 had no airflow limitation, and 405 of them tion and bronchiolar airway wall thickness, as previously were invited to a longer office visit with DLCO deter- described.16 29 The pulmonary artery (PA) diameter mination and a CT scan. This subgroup was similar to was measured at the level of the PA bifurcation, and the remaining patients without airflow limitation in the average of two perpendicular measurements of the terms of demographic characteristics, smoking inten- ascending aorta diameter were taken on the same CT sity, comorbidities and spirometric parameters (online Garcia-­Rio F, et al. BMJ Open Resp Res 2023;10:e001468. doi:10.1136/bmjresp-2022-001468 3 Open access Table 1 Demographic and clinical characteristics of the study subjects* Current smokers Former smokers Never-­smokers (n=75) (n=97) (n=165) P value Females, n (%) 46 (61.3) 43 (44.3) 115 (69.7) <0.001 Age, years 56±8 62±10 61±12 <0.001 BMI, kg/m2 26.1±4.6 28.1±3.9 27.4±5.2 0.023 Level of studies, n (%) 0.051  No studies 0 1 (1.0) 2 (1.2)  Primary education 7 (9.3) 24 (24.7) 33 (20.0)  Secondary education 23 (30.7) 14 (14.4) 32 (19.4)  University 44 (58.7) 58 (59.8) 98 (59.4) Lives alone, n (%) 14 (18.7) 22 (22.7) 22 (13.3) 0.143 Pack-­years 32±14 32±23 – 0.839 Charlson Comorbidity Index 0.3±0.7 0.5±1.1 0.3±0.8 0.294 COTE index 1.4±2.6 0.8±1.8 1.05±2.4 0.212 CAT score 9.2±6.6 6.8±5.4 6.2±5.6 0.001 Chronic cough, n (%) 27 (36.5) 12 (12.5) 18 (11.2) <0.001 Chronic bronchitis, n (%) 10 (15.2) 3 (3.2) 4 (2.5) <0.001 Chronic expectoration, n (%) 21 (28.0) 14 (14.7) 14 (8.5) <0.001 Dyspnoea, n (%) 6 (8.2) 16 (16.7) 26 (16.5) 0.209 Wheezing, n (%) 42 (56.8) 27 (27.8) 41 (24.8) <0.001 Baseline SaO2, % 96±1 96±1 96±2 0.638 Exhaled CO, ppm 25±16 3±4 4±5 <0.001 Prebronchodilator FVC, z-­score 0.07±0.96 0.21±1.00 0.26±0.93 0.365 Prebronchodilator FEV1, z-­score −0.17±1.04 0.05±1.09 0.21±1.01 0.031 Prebronchodilator FEV1/FVC, z-­score −0.46±0.77 −0.31±0.85 −0.13±0.83 0.014 Postbronchodilator FVC, z-­score 0.10+0.85 0.24+0.96 0.32+0.99 0.164 Postbronchodilator FEV1, z-­score −0.15+0.98 0.06+1.11 0.19+1.12 0.011 Postbronchodilator FEV1/FVC, z-­score −0.45+0.80 −0.32–0.91 −0.15+1.07 0.002 6MWD, m 508±102 523±94 510±101 0.539 DLCO, z-­score −0.80±0.60 −0.30±0.71 −0.26±0.68 <0.001 DLCO/VA, z-­score −0.10±0.12 −0.02±0.12 −0.01±0.15 0.005 VA, z-­score −0.46±0.66 −0.31±0.68 −0.25±0.66 0.086 Haemoglobin, g/L 148±14 146±14 140±14 <0.001 Eosinophils, % 2.1±1.5 2.3±1.6 2.0±1.6 0.440 Fibrinogen, g/L 3.9±0.8 3.8±0.9 3.8±1.0 0.910 C reactive protein, mg/L 1.4±2.8 0.9±1.5 1.5±2.8 0.134 Albumin, g/l 12.1±16.2 17.1±18.4 12.5±16.3 0.717 BODE index 1.0±0.5 1.1±0.5 1.1±0.6 0.195 YPAS—physical activity summary 55±22 62±24 57±21 0.236 Total emphysema volume, % 1.9±4.4 2.6±3.7 1.7±3.4 0.276 Low density volume, % 47±20 51±18 41±18 0.005 P15, HU −910±31 −919±26 −904±26 0.003 Secondary bronchi WA, % 25.4±5.7 25.3±5.7 24.5±5.4 0.392 Pulmonary artery diameter, mm 26±4 27±4 24±4 0.074 Pulmonary artery:aorta ratio 0.78±0.11 0.78±0.11 0.79±0.12 0.811 Comparison between groups using ANOVA or chi-­squared test. *Data are mean±SD or number (percentage). ANOVA, analysis of variance; BMI, body mass index; BODE, Body Obstructive Dyspnoea Exercise; CAT, chronic obstructive pulmonary disease assessment test; CO, carbon monoxide; COTE, Chronic obstructive pulmonary disease-­specific co-­morbidity test; DLCO, lung diffusing capacity of carbon monoxide; FEV1, forced expiratory volume at 1 s; FVC, forced vital capacity; HU, Hounsfield units; 6MWD, 6 min walk distance; P15, 15th percentile; SaO2, oxygen saturation; VA, alveolar volume; WA, wall area; YPAS, Yale Physical Activity Survey. 4 Garcia-­Rio F, et al. BMJ Open Resp Res 2023;10:e001468. doi:10.1136/bmjresp-2022-001468 Open access Figure 2 Frequency of a reduced pulmonary diffusion capacity among the different subgroups of the study, considering all the subjects analysed and only those who reported chronic respiratory symptoms. DLCO, diffusing capacity of the lungs for carbon monoxide. supplemental table 1). According to previous definitions, (assessed by the distance walked for 6 min), and the these subjects were classified as never-­smokers (n=165), level of daily physical activity. Finally, a negative rela- former smokers (n=151) or current smokers (n=89). Last, tionship was identified between both DLCO and the in the two subsamples of smokers only those subjects with DLCO/VA ratio with the diameter of the PA, although a cumulative use of tobacco greater than 10 pack-­years without differences in its ratio with the ascending aorta were selected (figure 1). Table 1 summarises the main diameter. No other differences in DLCO or DLCO/VA characteristics of the three study groups. ratio were detected based on the presence of chronic respiratory symptoms (online supplemental table 2). The analysis of parameters related to DLCO and Prevalence of lung diffusing capacity reduction DLCO/VA ratio carried out separately in current and In the subjects selected for the analysis, the frequency former smoker subjects (online supplemental tables 3 of a DLCO below its lower limit of normal was higher and 4) showed a trend similar to those described in in current smokers (26.7%; 95% CI 16.7% to 36.7%) the joint analysis of both subgroups. than in former smokers (14.4%; 95% CI 7.4% to 21.4%) or never-­ smokers (6.7%; 95% CI 2.9% to 10.5%) (p<0.001). When the analysis was limited to Differences of smoker subjects with reduced or normal DLCO subjects with self-­reported chronic respiratory symp- The comparison of current or former smokers with a toms using the ECSC questionnaire, the frequencies DLCO below their lower limit of normal versus subjects obtained in the three subgroups showed a similar with normal DLCO (table 3) shows that DLCO reduc- pattern (figure 2). tion occurs in younger subjects with a higher level of exhaled CO. In addition, smokers with reduced Clinical and functional variables related to DLCO DLCO have lower spirometric parameters, higher In current and former smoker subjects, women levels of dyspnoea, lower exercise tolerance and less presented lower z-­scores than men, both for DLCO daily physical activity. In fact, subjects with reduced (−0.64±0.57 vs −0.34±0.80, p=0.001) and DLCO/VA DLCO perform less physical activity (median (IQR)) (−0.11±0.12 vs 0.01±0.12, p<0.001). Table 2 shows the (35 (24–48) vs 57 (30–82) hour/week, p=0.005) relationship of both the DLCO and the DLCO/VA and reach a lower energy cost (117 (82–120) vs 187 ratio with the main clinical, functional and morpho- (99–281) MET × hour/week, p=0.004). Moreover, logical variables in all subjects with previous or current smokers with low DLCO also have worse BODE scores tobacco use. In them, DLCO presented an inversely and lower plasma albumin levels. In turn, larger PA proportional relationship with nicotine dependence diameters were found in smoker subjects with reduced assessed by the Fagerström test, health status and DLCO compared with those with normal DLCO. In exhaled CO levels. In turn, DLCO was directly propor- fact, subjects with reduced DLCO showed a tendency tional to spirometric parameters, exercise tolerance towards PA enlargement, although without reaching Garcia-­Rio F, et al. BMJ Open Resp Res 2023;10:e001468. doi:10.1136/bmjresp-2022-001468 5 Open access Table 2 Relation between diffusion parameters and clinical, functional and morphological variables in subjects with smoking history of more than 10 pack-­years* DLCO, z-­score DLCO/VA, z-­score R P value R P value Age, years 0.017 0.829 −0.079 0.303 BMI, kg/m2 0.127 0.097 0.295 <0.001 Pack-­years −0.054 0.478 −0.041 0.595 Fagerström test −0.291 0.010 −0.270 0.017 Charlson index 0.020 0.799 0.103 0.179 COTE index −0.115 0.134 0.010 0.892 CAT total score −0.194 0.011 −0.084 0.271 mMRC dyspnoea scale −0.299 0.565 0.041 0.938 Baseline SaO2, % 0.093 0.227 0.046 0.549 Exhaled CO, ppm −0.370 <0.001 −0.302 <0.001 Prebronchodilator FVC, z-­score 0.352 <0.001 −0.136 0.074 Prebronchodilator FEV1, z-­score 0.393 <0.001 −0.023 0.7666 Prebronchodilator FEV1/FVC, z-­score 0.148 0.053 0.194 0.011 6MWD, m 0.206 0.007 0.020 0.793 Haemoglobin, g/L 0.087 0.261 0.117 0.133 Eosinophils, % 0.041 0.599 0.154 0.047 Fibrinogen, g/L −0.073 0.390 −0.098 0.248 C reactive protein, mg/L −0.079 0.326 −0.020 0.800 Albumin, g/L 0.295 0.182 −0.114 0.613 BODE index −0.020 0.791 0.174 0.023 YPAS—physical activity summary 0.218 0.043 0.148 0.175 Total emphysema volume, % 0.020 0.812 −0.064 0.440 Low density volume, % 0.050 0.547 −0.021 0.801 P15, HU −0.089 0.280 −0.038 0.650 Secondary bronchi WA, % 0.002 0.978 0.008 0.926 Pulmonary artery diameter, mm −0.294 <0.001 −0.381 <0.001 Pulmonary artery:aorta ratio 0.027 0.748 0.082 0.321 Bold values highlight significant relationships. *Pearson correlation coefficient (r) and p value are shown. BMI, body mass index; BODE, Body Obstructive Dyspnoea Exercise; CAT, Chronic Obstructive Pulmonary Disease Assessment Test; CO, carbon monoxide; COTE, Chronic obstructive pulmonary disease-­specific co-­morbidity test; DLCO, lung diffusing capacity of carbon monoxide; FEV1, forced expiratory volume at 1 s; FVC, forced vital capacity; HU, hounsfield units; mMRC, modified Medical Research Council; 6MWD, 6 min walk distance; P15, 15th percentile; SaO2, oxygen saturation; VA, alveolar volume; WA, wall area; YPAS, Yale Physical Activity Survey. the level of statistical significance (26.7 vs 12.6%, multiple logistic regression models separately for current p=0.083). Online supplemental tables 5 and 6 present and former smokers (online supplemental tables 7 and the comparison between subjects with reduced and 8). normal DLCO, evaluating current smokers and former smokers separately. Finally, from all those variables that reached statis- tical significance in the comparison between reduced DISCUSSION and normal DLCO in current or former smokers, the The main finding of our study is that reduced DLCO multiple logistic regression model only retained prebron- is present in more than a quarter of current smokers chodilator FVC and exhaled CO as independent predic- without airflow limitation, being associated with lower tors of reduced DLCO (table 4), although the model spirometric parameters, as well as reduced exercise contribution to explained variance in DLCO was modest tolerance and less daily physical activity. Furthermore, (r2=0.153, p<0.001). The same variables were retained in a reduced FVC and increased exhaled CO have been 6 Garcia-­Rio F, et al. BMJ Open Resp Res 2023;10:e001468. doi:10.1136/bmjresp-2022-001468 Open access Table 3 Comparison between current or former smoker subjects according to the lung diffusing capacity* Reduced DLCO Normal DLCO (n=34) (n=138) P value Females, n (%) 21 (61.8) 68 (49.3) 0.133 Age, years 54 (48–64) 60 (54–67) 0.028 BMI, kg/m2 26.7 (21.6–29.1) 27.1 (24.4–30.0) 0.237 Level of studies 0.070  No studies 1 (2.9) 0  Primary education 7 (20.6) 24 (17.4)  Secondary education 6 (17.6) 31 (22.5)  University 19 (55.9) 83 (60.1) Lives alone 5 (14.7) 31 (22.5) 0.227 Pack-­years 25 (20–39) 30 (20–40) 0.529 Fagerström test 4 (3–6) 4 (2–5) 0.682 Charlson Comorbidity Index 0.4±0.8 0.4±1.0 0.861 COTE index 1.9±2.7 0.9±2.0 0.066 CAT total score 5.0 (4.5–15.5) 4 (2-­8) 0.531 mMRC dyspnoea level 0.037 0   23 (67.6) 105 (76.1) 1   8 (23.5) 30 (21.7) 2   1 (2.9) 3 (2.2) 3   2 (5.9) 0 Chronic cough, n (%) 8 (24.2) 31 (22.6) 0.503 Chronic bronchitis, n (%) 2 (6.9) 11 (8.5) 0.565 Chronic expectoration, n (%) 7 (20.6) 28 (20.6) 0.604 Wheezing, n (%) 15 (44.1) 54 (39.4) 0.378 Baseline oxygen saturation, % 97 (96–97) 96 (96–97) 0.863 Exhaled CO, ppm 31 (29–32) 12 (2–31) 0.006 Prebronchodilator FVC, z-­score −0.43 (−1.00 to 0.40) 0.36 (−0.29 to 0.95) <0.001 Prebronchodilator FEV1, z-­score −0.44 (−1.32 to −0.10) 0.03 (−0.54 to 0.91) 0.001 Prebronchodilator FEV1/FVC, z-­score −0.48 (−0.85 to −0.06) −0.34 (−0.99 to 0.28) 0.501 Reduced VA, n (%) 3 (8.8) 2 (1.4) 0.053 6MWD, m 465 (450–480) 555 (533–605) 0.047 Haemoglobin, g/L 147 (144–149) 153 (148–170) 0.821 Eosinophils, % 0.1 (0.1–0.1) 0.2 (0.1–1.9) 0.435 Fibrinogen, mg/dL 4.4 (3.9–4.9) 3.5 (2.9–4.1) 0.698 C reactive protein, mg/L 0.7 (0.2–1.3) 0.4 (0.1–3.5) 0.404 Albumin 4.2 (4.2–4.3) 4.7 (4.6–39.6) <0.001 BODE index 0.006  0–2 points (survival: 80%) 30 (88.2) 137 (99.3)  3–4 (survival: 67%) 4 (11.8) 1 (0.7) Physical activity summary 29 (24–62) 53 (34–69) 0.048 Total emphysema volume, % 0.69 (0.18–2.80) 0.34 (0.17–2.79) 0.261 Low density volume, % 53 (34–63) 48 (24–54) 0.210 P15, HU −911 (−917 to −886) −905 (−930 to −889) 0.127 Secondary bronchi WA, % 25.8 (22.8–27.5) 25.3 (20.2–29.8) 0.928 Pulmonary artery diameter, mm 25 (24–27) 23 (21–24) 0.002 Continued Garcia-­Rio F, et al. BMJ Open Resp Res 2023;10:e001468. doi:10.1136/bmjresp-2022-001468 7 Open access Table 3 Continued Reduced DLCO Normal DLCO (n=34) (n=138) P value Pulmonary artery:aorta ratio 0.77 (0.72–0.79) 0.70 (0.65–0.71) 0.578 Pulmonary artery enlargement 8 (26.7%) 16 (13.4%) 0.095 Comparisons were performed by Student’s t-­test, Mann-­Whitney U or χ2 tests. Bold values highlight significant differences. *Data are mean±SD, median (IQR) or number (frequency) according to their type and distribution. BMI, body mass index; BODE, body obstructive dyspnoea exercise; CAT, chronic obstructive pulmonary disease assessment test; CO, carbon monoxide; DLCO, lung diffusing capacity for carbon monoxide; FEV1, forced expiratory volume at 1 s; FVC, forced vital capacity; LLN, lower limit of normal; mMRC, modified Medical Research Council; 6MWD, 6 min walk distance; P15, 15th percentile; VA, alveolar volume; WA, wall airway. identified as independent predictors of reduced DLCO Although some previous studies have described a weak in smokers without airflow limitation. relationship between the DLCO of heavy smokers (>20 As previous methodological comments, it is worth pack-­years) and lung attenuation,6 38 they included a mentioning that the procedure followed for study notable percentage of patients with airflow limitation. subject selection generated a sample with a prevalence Thus, in a subsample of smokers or former smokers from of current and former smokers that are the same as in the GenKOLS study, DLCO was related to the percentage the general population of our country (22% and 25%, of low-­attenuation areas and to the standardised airway respectively).32 In fact, to our knowledge, this is the first wall thickness only in patients with COPD, while in subjects article that shows a population approach to the evalua- without airflow limitation the same relationship was not tion of DLCO with simultaneous measurements of low significant.39 Similarly, in 38 former smokers without dose CT scan in smokers without airflow limitation. airflow limitation, no differences in low-­ attenuation The population prevalence of DLCO below the LLN areas or bronchial wall thickness were identified between identified in our study in current and former smokers those with normal or low DLCO.40 This suggests that, (19.8%) is consistent with the 17% previously described in early phases of smoking-­induced lung damage, the in active smokers selected from advertisements to assess decrease in DLCO is not attributable to emphysema-­like lung health10 and 24% identified in a retrospective anal- changes. As DLCO decreases in a wide variety of patho- ysis of a lung function database.33 In addition, our data logic conditions, including reduction in alveolar surface confirm the relationship of DLCO with some previously area, decreased perfusion, even ventilation or inflam- identified variables, such as male gender or spirometric mation or fibrosis of the alveolar wall impairing alveolar parameters.34–36 However, the sample analysed has not diffusion,5 impaired gas exchange may not necessarily identified a relationship between DLCO and the pres- reflect early emphysema, since it may be due to other ence of chronic respiratory symptoms. This finding is also smoking-­related changes, including altered membrane consistent with a small study of 58 smokers without airflow diffusion or pulmonary vascular changes.41 The contribu- limitation, in which subjects who reported non-­specific tion of peripheral airways to DLCO reduction has been respiratory problems, chronic bronchitis or wheezing suggested in never-­smokers from the COPDGene cohort, had lower spirometric parameters, but no differences in which small airway dysfunction correlated signifi- were found in DLCO or with the single-­breath nitrogen cantly with lower DLCO among both non-­ obstructed method, suggesting that symptoms are more related to and GOLD 1–2 subjects.42 This might justify that, in our physiological changes in the central airways than in the smoker subjects, FVC was retained as an independent peripheral airways.37 predictor of low DLCO, instead of FEV1, since the former Table 4 Multiple logistic regression model to detect decreased DLCO in current and former smoker subjects* Variable B SE P value OR 95% CI Prebronchodilator FVC, z-­score −0.815 0.225 <0.001 0.443 0.285 to 0.688 Exhaled CO, ppm 0.030 0.013 0.018 1.030 1.005 to 1.056 Constant −1.910 0.301 – – – *Exhaled CO level, prebronchodilator FVC, prebronchodilator FEV1, serum albumin level and pulmonary artery diameter were entered into the model. The BODE index was not entered because it was considered redundant and not strictly applicable to subjects without airflow limitation. B, regression coefficient; BODE, body obstructive dyspnoea exercise; CI, confidence interval; CO, carbon monoxide; DLCO, diffusing capacity of the lungs for carbon monoxide; FEV1, forced expiratory volume in 1 s; FVC, forced vital capacity; OR, odds ratio. 8 Garcia-­Rio F, et al. BMJ Open Resp Res 2023;10:e001468. doi:10.1136/bmjresp-2022-001468 Open access better represents the contribution of the whole bronchial several hypoxia-­ independent pathways that promote tree, including its most distal portions. K+channel dysregulation in the absence of clinically In this study, the other risk factor independently asso- established COPD.50 ciated with the presence of reduced DLCO was increased Our study has several limitations that are worth exhaled CO. The DLCO correction for exhaled CO discussing. First, the cross-­ sectional design does not suggests that its contribution does not seem to be exclu- allow for cause-­and-­effect relationships to be established, sively dependent on CO backpressure, so it is interesting although the data reveal that reduced DLCO is associated to consider that exhaled CO is a recognised indicator of with differences in dyspnoea level, exercise tolerance and oxidative stress,43 one of the main causes of endothelial PA diameter. Second, the sample size may be limiting for damage. This possibility is reinforced by the identification some subanalyses, despite providing a representative in smokers with reduced DLCO of low levels of albumin, a sample of smokers at the population level and achieving negative acute-­phase reactant with anti-­inflammatory and statistical significance in most comparisons. Third, other antioxidant properties.44 Indeed, it has been suggested spirometric parameters suggestive of peripheral airway that albumin level may be a marker of susceptibility to disease have not been evaluated, such as the FEV3/FEV6 the oxidative response resulting from smoking, while ratio, whose decrease is highly prevalent in smokers with albuminuria should be a non-­invasive marker of arterial preserved pulmonary function but impaired indices of stiffness.45 Although merely speculative, these findings physical function and quality of life.51 52 And fourth, the are in agreement with some current evidence suggesting EPISCAN II study did also not include specific measure- that peripheral airway destruction might be initiated, in ments of small airway function or pulmonary micro- part, by early smoking-­induced damage to the pulmonary circulation to allow for the specific assessment of their vascular endothelium mediated by plasma endothelial relationship with a reduced DLCO. microparticles with apoptotic features, which are elevated In conclusion, this study shows that reduced DLCO is a in smokers with normal spirometry and reduced DLCO.46 frequent finding among smokers without airflow limita- It is particularly attractive to consider that, in the absence tion and is associated with a limited exercise tolerance of lung parenchymal damage, DLCO provides a window and reduced daily physical activity, as well as a larger into the microvasculature of smoker subjects. Thus, PA diameter. These findings, together with increased the lower DLCO of smokers probably reflects the pres- exhaled CO and reduced plasma albumin, in the absence ence of greater ventilation-­perfusion inequalities, lower of differences in the lung parenchyma attenuation, allow carbon dioxide elimination efficiency and, consequently, us to speculate that, in these subjects, DLCO may be a greater ventilatory stimulation,47 which, in turn, would surrogate marker of smoking-­induced early vascular lung justify the higher degree of dyspnoea reported by our damage. smokers with reduced DLCO. This basic physiological proposal is in line with studies about exercise response in Author affiliations 1 smokers at risk of COPD, in which exercise dyspnoea is Servicio de Neumología, Hospital Universitario La Paz-­IdiPAZ, Universidad Autónoma de Medicina, Madrid, Spain mainly explained by increased inspiratory neural drive.47 2 Centro de Investigación Biomédica en Red en Enfermedades Respiratorias Furthermore, this could also explain their lower exercise (CIBERES), Madrid, Spain tolerance, consistently with the previous description of a 3 Pneumology Department, Hospital Universitary Vall d'Hebron/Vall d'Hebron correlation between low DLCO and reduced 6 min walk Institut de Recerca (VHIR), Barcelona, Spain 4 distance in former smokers with normal chest CT scans Servicio de Neumología, Hospital Universitario La Princesa; Universidad Autónoma de Madrid, Madrid, Spain and spirometry.40 However, since the EPISCAN study 5 Servicio de Neumología, Hospital Universitario Son Espases-­IdiSBa, Palma protocol did not include the determination of functional de Mallorca, Spain residual capacity or inspiratory capacity, it is not possible 6 Servicio de Neumología, Hospital Arnau de Vilanova-­Lliria, Departamento de to completely rule out the existence of a certain degree of Medicina, Universitat de València, Valencia, Spain 7 hyperinflation not detected by lung imaging tests, which Pulmonary Deparment-­Research Unit, Hospital Universitario Nuestra Señora de Candelaria, Universidad de La Laguna, Tenerife, Spain is a main determinant of exercise tolerance in patients 8 Servicio de Neumología, Hospital General Gregorio Marañón, Madrid, Spain with airflow limitation.48 9 Unidad de Gestión Clínica de Neumología, Hospital Universitario Virgen de Finally, one last aspect of our study to highlight is Valme, Universidad de Sevilla, Sevilla, Spain 10 the larger diameter of the PA found in smokers with Servicio de Neumología, Hospital Univesitario de Alcalá de Henares, Alcalá reduced DLCO compared with those with normal de Henares, Spain 11 Departamento Médico, GSK, Tres Cantos, Spain DLCO. Although this difference does not lead to signifi- cant differences in the PA enlargement between the two Twitter Francisco Garcia-­Rio @FranGarciaRio, Marc Miravitlles @MiravitllesMarc smoker subgroups or in the relationship between the and Borja G Cosío @BorjaCos70 diameter of the PA and the ascending aorta, which is the Acknowledgements The collaboration of Monica Sarmiento and Neus Canal of most consistent indicator of pulmonary hypertension,49 IQVIA, and of Maria Victoria Pardo, Miguel Pascual and David Bañas from GSK is they might also reflect an early impact of smoking on the explicitly acknowledged. pulmonary vascular bed. In this regard, it is interesting Collaborators EPISCAN II study: Elena García Castillo; Claudia Valenzuela; Ana Pueyo Bastida; Lourdes Lázaro Asegurado; Luis Rodríguez Pascual; Mª José Mora; to note that cigarette smoking has recently been shown Luis Borderias Clau; Lourdes Arizón Mendoza; Sandra García; Juan Antonio Riesco to contribute to pulmonary arterial remodelling through Miranda; Julián Grande Gutiérrez; Jesús Agustín Manzano; Manuel Agustín Sojo Garcia-­Rio F, et al. BMJ Open Resp Res 2023;10:e001468. doi:10.1136/bmjresp-2022-001468 9 Open access González; Miguel Barrueco Ferrero; Milagros Rosales; José Alberto Fernández Villar; and license their derivative works on different terms, provided the original work is Cristina Represas; Ana Priegue; Isabel Portela Ferreño; Cecilia Mouronte Roibás; properly cited, appropriate credit is given, any changes made indicated, and the Sara Fernández García; Rocío Cordova Díaz; Nuria Toledo Pons; Margalida Llabrés; use is non-­commercial. See: http://creativecommons.org/licenses/by-nc/4.0/. José María Marín Trigo; Marta Forner; Begoña Gallego; Pablo Cubero; Elisabet Vera; Mª Begoña Picurelli Albero; Noelia González García; Agustín Valido Morales; ORCID iDs Carolina Panadero; Cristina Benito Bernáldez; Maria Velarde; Antonio Santa Cruz Francisco Garcia-­Rio http://orcid.org/0000-0002-7227-2238 Siminiani; Carlos Castillo Quintanilla; Rocío Ibáñez Meléndez; José Javier Martínez Marc Miravitlles http://orcid.org/0000-0002-9850-9520 Garcerán; Desirée Lozano Vicente; Pedro García Torres; Maria del Mar Valdivia; Joan B Soriano http://orcid.org/0000-0001-9740-2994 Juan Pablo de Torres Tajes; Montserrat Cizur Girones; Carmen Labiano Turrillas; Juan José Soler-­Cataluña http://orcid.org/0000-0003-1134-0783 Carlos Ruiz Martínez; Elena Hernando; Elvira Alfaro; José Manuel García; Jorge María Guadalupe Sánchez Herrero http://orcid.org/0000-0003-4352-3509 Lázaro; David Bravo; Laura Hidalgo; Silvia Francisco Terreros; Iñaki Zorrilla; Ainara Alonso Colmenero; Cristina Martínez González; Susana Margon; Rosirys Guzman Taveras; Ramón Fernández; Alicia Álvarez; José Ramón Agüero Balbín; Juan Agüero Calvo; Jaume Ferrer Sancho; Esther Rodríguez González; Eduardo Loeb; José Luis Izquierdo Alonso; Mª Antonia Rodríguez García; Juan Abreu González; Candelaria REFERENCES Martín García; Rebeca Muñoz; Haydée Martín García; Miguel Angel Martínez Muñiz; 1 Singh D, Agusti A, Anzueto A, et al. 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