In the present real-world study, BMI, serum albumin, and pulmonary functions including VC, FVC, FEV1.0, FEV1.0/FVC, %FEV1.0, and DLco were significantly lower, and the cardiovascular disease comorbidity rate, COPD assessment test score, mMRC dyspnea scale, and use of long-term oxygen therapy, LABAs, ICS, and macrolides were significantly higher in COPD patients who experienced exacerbations than in COPD patients who never experienced severe exacerbations. Of patients who experienced exacerbations, frequent exacerbators had significantly lower BMI, FEV1.0, and %FEV1.0, and higher risk of critical exacerbations, blood eosinophils, history of mechanical ventilation use, use of long-term oxygen therapy, LAMAs, and macrolides than infrequent exacerbators. Multivariate analysis showed that the parameter most correlated with exacerbation frequency was the percentage of blood eosinophils.
Exacerbation is one of the independent phenotypes in COPD patients who develop worse health conditions, including symptoms and pulmonary function, which is supported by many previous reports (21, 22). For example, there was an 8% increase in the risk of exacerbations per unit increase in CAT (23), and additional exacerbation was associated with a greater decline of 6–7 ml in FEV1.0 in a huge cohort study (24, 25), consistent with the results of the current study (Table 1). As for comorbidities, cardiovascular diseases were more common in patients who had exacerbations with significantly higher pulmonary artery pressure (Table 1). Cardiovascular events are common in COPD patients, and 20.5% of stable COPD patients had unrecognized heart failure (26, 27); they were also significantly associated with long-term mortality in COPD patients with exacerbations (28).
Exacerbation frequency is also an important factor related to the prognosis of COPD patients. Donaldson et al analyzed 109 patients with COPD who experienced 757 exacerbations and suggested that FEV1.0 was decreased at 40.1 ml/year in frequent exacerbators compared to 32.1 ml/year in infrequent exacerbators (25). In a 1-year prospective observational trial of Japanese COPD patients, Tomioka et al reported that frequent exacerbators had lower BMIs and higher CAT scores than infrequent exacerbators (11). Moreover, the retrospective UPLIFT study of 6000 patients with COPD and the 3-year observational ECLIPS study of 2000 patients with COPD reported that the patients with higher exacerbation rates had more rapid lung function decline (29, 30). Interestingly, more than 20% of COPD patients with GOLD stage 2 (50% < FEV1.0 < 80% predicted) had two or more annual exacerbations, and the frequency in the previous year predicted the future frequency (30). These data suggest that frequency is also an independent phenotype of COPD patients with exacerbation.
The present results suggest that the percentage of blood eosinophils in the stable period is the factor most correlated with exacerbation frequency compared to other variables, including the critical exacerbation rate, %FEV1.0, history of mechanical ventilation use, use of long-term oxygen therapy, and use of macrolides (Table 4). A previous paper reported that COPD patients with a blood eosinophil count > 500 cells/µL had a 1.43 times higher risk of exacerbation in a huge database analysis (31), and others reported that patients with a blood eosinophil count > 340 cells/µL also had a 1.76-fold increased risk of severe exacerbation (32), which is consistent with the present result. Notably, the blood eosinophil was not significantly different between COPD patients who experienced exacerbations and those who never experienced exacerbations in the present study, which supported the association of blood eosinophils with exacerbation frequency, but not exacerbation itself (Table 1).
To decrease frequent exacerbations, ICS treatment was considered for suppression of airway inflammation. Recently, two randomized trials showed that ICS therapy suppressed moderate to severe exacerbations in COPD patients with improvement of pulmonary function (33, 34). In that study, the COPD patients with blood eosinophil counts ≥ 310 cells/µL showed a greater reduction of the exacerbation rate with improvement of FEV1.0, symptoms, and QOL score than those with blood eosinophil counts < 90 cells/µL (15). Importantly, the present results showed that the rate of patients treated by ICS was no different between frequent exacerbators and infrequent exacerbators (Table 3), which might contribute to the current prognostic results.
There are three limitations in this study. First, it was not possible to completely exclude patients with asthma-COPD overlap from the current study population. Because such patients had frequent exacerbations with higher blood eosinophil levels, the current results in frequent exacerbators might be affected by this population. Second, unreported exacerbations were also reported to be important for health status, as much as reported exacerbations, in COPD patients (35). The effect of unreported exacerbations, which might have impacted the current results, was not evaluated. Third, the present study involved patients at a single hospital with limited ethnic diversity. To confirm the validity of the present results, multicenter prospective studies with a larger number of patients should be performed.