*Overall Growth and Essential Characteristics of Research*

Table 2 described the basic characteristics of publications. The first seven papers related to this health issues in dataset were published in 1991. There has been a gradual raise in the annual number of papers on intervention to improve the QoL of patients with COPD within the period 1991–2018, contributing to a total of 3242 papers. The papers in 2018 showed the reading interests of readers in last six month with the average times of download (mean use rate) was 1.8; meanwhile, the papers in 2013 received the highest concern in last five years with average times of download (the mean use rate) was 2.1. The papers in the year 2000 had the highest average citation with 6.9 citations per paper.


**Table 2.** General characteristics of publications.

The paper having the highest influence was the second report entitled Global Strategy for the Diagnosis, Management, and Prevention of COPD published in 2007 with 3456 citations [46].

Figure 1 shows countries collaboration network. In total, there were 89 countries contributing for the research field (automatically calculated by VOSviewer). In figure there were 64 countries with minimum of 5 papers. Of those, the United States of America led in the number of studies with 786 papers (24.2%), followed by England (452 papers, 13.9%), the Netherlands (322 papers, 9.9%), and Canada (268 papers, 8.3%). Although people living in low-and middle-income countries (LMICs) are more vulnerable to developing COPD [47], there was only China in the list of top 10 countries having the highest volume. As can ben seen, there were four main clusters in this countries network (1) Asia with the leadership of China in collaboration with two East European countries (Czech and Romania) (red cluster); (2) the U.S and South American countries (yellow cluster); (3) Canada, South Africa, New Zealandm and European countries (turquoise cluster); (4) European countries with three subgroups with the lead of France, the Netherlands, and England (the rest).

By analyzing abstracts and titles, the most co-occurrence terms were found to discover the scope of COPD research (Figure 2). Three major clusters were formed by 279 most common terms in title and abstract with the minimum appearance of 95 times. The three significant clusters are: Cluster 1 (red) refers to comorbidity and COPD, among which mental health illness (depression and anxiety) was

most frequently mentioned. Cluster 2 (blue) focuses on interventions and treatment to increase QoL of people with COPD. Cluster 3 (yellow) points out the risk and mortality of exacerbation of COPD.

**Figure 1.** Countries collaboration network. Note: four main clusters, including (1) red cluster: Asia countries and two East European countries (Czech and Romania); (2) yellow cluster: the U.S and South American countries; (3) turquoise cluster Canada and South Africa, New Zealand and European countries; (4) the rest: European countries with three subgroups with the lead of France, the Netherlands and England.

**Figure 2.** Text mining using VOSviewer (titles and abstracts). Note: the colors of each node were automatically assigned by VOSviewer based on its score; the node size was based on the frequency of each term; the length and thickness of the lines reflected the association between two terms. Cluster 1 (red) refers to comorbidity and COPD; cluster 2 (blue) focuses on interventions and treatment to increase QoL of people with COPD, cluster 3 (yellow) points out the risk and mortality of exacerbation of COPD.

Table 3 shows the most cited papers. Each had more than 100 citations during the study period. Based on the list, three main topics which have been recently attracted the attention of researchers were: (1) The Global Initiative for chronic obstructive lung disease (GOLD) reports and other national reports. GOLD was a consensus report published periodically since 2001. It included the latest evidence for diagnosis and prevention from experts, which were as "strategy documents" for adequate care for COPD at a global level [48] (paper 1, paper 7, paper 4, paper 9, paper 28, paper 36 ); (2) Exacerbations in patients with COPD (paper 2, paper 3, paper 12, paper 25, paper 29, paper 32); (3) Treatments and interventions of COPD (paper 5, paper 6, paper 11, paper 14, paper 15, paper 17, paper 19, paper 20, paper 23, paper 24, paper 27, paper 33, paper 34, paper 35, paper 39, paper 40), (4) QoL, health-related QoL and COPD (paper 26, paper 30, paper 31, and paper 41), others topic (rehabilitation—paper 8; COPD and comorbidity—paper 10; COPD and its effects to patient health and life—paper 22 and paper 37).

Applying latent dirichlet allocation in title and abstracts, ten major research topics were formed (Table 4). Topic 2 (*n* = 468 papers), Topic 1 (*n* = 436 papers), and topic 3 (*n* = 355 papers) were three topics with the highest volume of publications. Pulmonary rehabilitation has been a rapidly developed field in the last decades [49]. Further, improving QoL of patients living with COPD by pharmacological therapies (topic 6) or non-pharmacological therapies (topic 3, topic 10) has been a major area of focus. Notably, the domain of mental health received frequent attention from the scientific community with 436 papers. The reason for it could be that about 85% of people living with COPD were at high risk of developing anxiety disorders compared with healthy people [50].

Figure 3 shows the changes in the development of topics. Topic 1 in the last five years (2014–2018) had the highest number of published papers (*n* = 237), followed by topic 2 (*n* = 186) and topic 3 (*n* = 141).

**Figure 3.** Changes in research topics development.



Note: \* The inhaled steroids in obstructive lung disease in Europe (ISOLDE).



Figure 4 shows the cluster of research areas in QoL of patients with COPD. The horizontal axis represents the distance between clusters, while the vertical axis displays the research areas [51]. The red lines show the depth for the cut-off of the analysis [52]. Research landscapes were divided into three main parts. The root (first group) in the top of the dendrogram included (a) respiratory system and (b) critical care medicine. This cluster had a close relationship with (1) intervention and health care such as general & internal medicine, pharmacy, nursing, and cardiovascular system (second group); (2) comorbidities, for instance, psychiatry, clinical psychology, clinical neurology (third group). However, the first group did not have a strong relatedness to the cluster in the bottom, such as rehabiliation or the integration of public health, environmental and occupational health (health care science & services; health policy & services, occupational &public, environment, interdiscriplinary social sciences).

**Figure 4.** Dendrogram of coincidence of research areas.
