1 Chair of Geriatrics, University Campus Biomedico of Rome, Italy

Correspondence to:
Dr Filippo Luca Fimognari
Centro per la Salute dell’Anziano, Università Campus Biomedico di Roma, Via dei Compositori 130, 00128, Rome, Italy; filippo.fimognari@virgilio.it

Keywords: chronic obstructive pulmonary disease; restrictive lung disease

In a recent issue of Thorax Gan et al1 published a systematic review and meta-analysis of 14 reports which confirmed the strong association between COPD and biological markers of systemic inflammation. In six reports COPD was diagnosed by the presence of a ratio of forced expiratory volume in 1 second to forced vital capacity (FEV1/FVC ratio) lower than 0.7. However, in the remaining eight studies this measure was not available, and the authors assumed that all participants in the lowest quartile of FEV1% (and, in one study,2 of FVC%) had a diagnosis of COPD. In these cases the corresponding highest quartile group served as control. Since a COPD diagnosis based on a decreased FEV1/FVC ratio was lacking in eight reports, the possibility cannot be excluded that a certain number of patients included in the meta-analysis did not have COPD but, rather, a restrictive ventilatory defect. This could be particularly true for participants in the study by Engstrom et al2 who were characterised only by a low FVC.

According to the current GOLD guidelines,3 only an FEV1/FVC ratio lower than 0.7 indicates airflow obstruction, thus allowing a COPD diagnosis. Indeed, in the absence of particular pulmonary diseases, many subjects show a homogenous decrease in all dynamic lung volumes (FEV1, FVC, PEF) without any change in the FEV1/FVC ratio, and are thus considered to have "impaired lung function". The occurrence of respiratory symptoms,4 systemic inflammation,2 and the increased risk of cardiovascular disease5 are the only features that subjects with restrictive disease share with COPD. In fact, whereas COPD is characterised by a decrease in body mass index and blood lipids, subjects with restrictive disease often have abdominal obesity, insulin resistance, and other metabolic risk factors.6

Although we believe that most of the included patients were affected by COPD, the possible inclusion of patients with restrictive lung disease may have altered the statistical conclusions of the meta-analysis.

In addition, the decision to select patients in the lowest quartile of FEV1 and FVC prevented the authors from confirming the absence of inflammation in mild COPD (GOLD stage I and II), a finding previously reported by the same group in a study not included in this meta-analysis.7

Because patients with restrictive lung disease and those with COPD have different features, the generic term "impaired lung function" should not be used. Future studies of the role of inflammation and other cardiovascular risk conditions in patients with respiratory disease, and those investigating the outcome in these subjects, should clearly distinguish between these two groups of patients.