Since 2009, the prevalence of chronic obstructive pulmonary disease (COPD) in Korea has been increasing, and this trend is expected to continue, especially with an aging population and increased risk factors such as air pollution [1]. In Korea, the disability-adjusted life year, which represents social loss due to disease, is also increasing, contrary to the global trend of stable or declining COPD cases, indicating an underestimated social disease burden [2]. Furthermore, due to the complexity of COPD, its diagnosis is often delayed. The irreversible nature of the disease [3], which progressively worsens over time, often leads to a vicious cycle of worsening symptoms, decreased performance of daily activities, and deterioration of the overall functional status, significantly affecting patients' quality of life [4]. In this context, the Global Initiative for Chronic Obstructive Lung Disease (GOLD) guidelines [3] and Korean COPD guidelines [5], which establish the standard of care for COPD, emphasize the importance of a comprehensive assessment. This assessment includes evaluation of the patient's physical functional status and self-reported indicators of subjective symptoms, conditions, and quality of life.

In patients with COPD, dyspnea reduces their ability to engage in sustained physical activity, leading to limitations in daily life and overall functioning. Physical functioning also decreases as COPD severity increases [1]. Consequently, both domestic and international COPD guidelines [3,5] emphasize the importance of maintaining physical function in patients with COPD. This is crucial for preventing disease progression, delaying exacerbations, and assessing the extent to which patients can continue walking, as determined by the walking ability assessment. Similarly, it is crucial to keep patients actively engaged in their daily lives in order to maintain their functional status. To achieve this, significant emphasis has been placed on enhancing self-motivation [6] and treatment adherence [7] to both pharmacological and nonpharmacological interventions. Self-efficacy (SE), defined as an individual's confidence in successfully performing certain behaviors, plays a key role in COPD management [8]. Promoting SE is particularly crucial in ongoing respiratory rehabilitation, which is essential for preventing exacerbations of COPD [3,5]. Indeed, previous research on patients with COPD has demonstrated a strong association between higher SE and several positive health outcomes, such as fewer complaints related to dyspnea and fewer limitations in daily activities [9]. Additionally, higher SE has been linked to an improved overall quality of life in patients with COPD [8]. These findings suggest that SE positively affects the overall health outcomes of patients with COPD, indicating the need to explore SE and disease severity in this population.

Several variables are involved in disease progression of COPD patients. In particular, forced expiratory volume in one second (FEV₁) [7], which is a key indicator for diagnosing COPD and measuring the degree of airflow limitation, has been suggested by the GOLD as an important criterion for stratifying the severity of COPD [3]. However, a previous study [7] that stratified COPD patients based on their FEV₁ and examined their relationship with patient symptoms found no significant associations. In addition, a lower FEV₁ in COPD patients is associated with an increased frequency of acute exacerbations, symptoms, and physical disability [10]. Moreover, greater physical disability and psychological stress are linked to a poorer quality of life and less favorable long-term prognosis [11,12]. These findings underscore the importance of considering a comprehensive picture that includes patient-reported outcomes and psychological indicators when assessing patients with COPD and determining appropriate therapeutic approaches [13]. Relying solely on a single metric such as disease severity may not provide a complete understanding of a patient's condition. Emphasizing the significance of a comprehensive assessment is crucial because it allows the exploration of interventions that can aid in preventing exacerbations of COPD [2,7].

As mentioned above, COPD is a complex disease, and several variables are associated with a patient's condition and treatment outcomes. It is crucial to adopt a therapeutic approach based on a comprehensive assessment that includes subjective reports and the patient's physical condition. However, in Korea, there is a lack of studies that assess multiple variables along with COPD severity in patients and identify the differences between these variables based on disease severity. Therefore, this study aimed to comprehensively explore respiratory rehabilitation SE, health-related quality of life (HRQoL), and physical functioning of COPD patients, while considering the impact of airflow limitation, which is a commonly used indicator of disease severity. The goal was to provide insights into patient characteristics that should be considered in a comprehensive assessment of patients with COPD and to offer implications for nursing practice and future research.

The objectives of this study were as follows: first, to identify the characteristics of patients with COPD, and second, to determine the differences in physical function, respiratory rehabilitation SE, and HRQoL based on the severity of the condition categorized by the degree of airflow limitation.

This study evaluated physical functioning, SE, and HRQoL and revealed differences in these variables depending on disease severity categorized by the degree of airflow limitation, a critical criterion in the GOLD guidelines. A noteworthy contribution of this finding lies in the application of these variables, which have been emphasized as crucial indicators for comprehensively assessing the conditions of COPD patients in previous international studies in the context of community-dwelling Korean patients.

The mean age of the study participants was 70.02 years, with many (68.2%) in the age range of 60s and 70s. The study included a higher proportion of male participants (87.2%), and 14.0% had a college degree or higher. These demographic characteristics are similar to those of a previous study that examined 1,092 patients with COPD from the sixth National Health and Nutrition Examination Survey [1], in which 75.6% of the participants were in their 60s and 70s, 76.1% were male, and 15.3% had a college degree or higher. These similarities indicate that the current study sample was representative of a larger COPD patient population. However, the distribution of the severity categories in this study differed from that reported by Mok and Jo [1]. In the current study, disease severity of 70.3% of participants were categorized as mild or moderate, whereas 29.7% were categorized as severe or very severe. However, Mok and Jo reported that 95.7% of their participants belonged to the mild or moderate category. Upon further analysis of the differences in demographic characteristics according to severity, we found that age was significantly lower in the moderate, severe, and very severe groups. This finding is consistent with that of a previous international study that analyzed demographic characteristics based on FEV₁ in patients with COPD [13], which also reported that patients in the very severe group tended to be younger. However, this observation differs from that of another study that examined the general characteristics of patients with COPD in the Korean National Health and Nutrition Examination Survey [1] and found that higher severity was associated with older age. The observed differences in the characteristics of the participants in this study compared with those in previous studies may be attributed to several factors. First, this and the aforementioned studies from outside Korea [13] were conducted on patients registered and treated at a tertiary general hospital. However, a Korean study analyzed data collected from a large sample of the Korean National Health and Nutrition Examination Survey, which included patients from various medical institutions of different sizes and service scopes in the community. Consequently, the Korean study may have included a more diverse range of patients with various characteristics, which may have influenced the outcomes. It is crucial to acknowledge the limitations of the current study, particularly the relatively small number of participants. This may have affected the generalizability of the findings. Therefore, future research should include a larger number of participants to enhance the exploration of this topic.

Among the disease-related characteristics of the participants, subjective dyspnea based on mMRC 2, "When I am walking with a group of my peers, I would stop and rest even if I fall behind or walk alone," showed a significant difference in the distribution of mMRC levels 2 and above versus levels 1 and below depending on severity. This contrasts with a study of Asian patients with COPD, in which the degree of dyspnea did not differ significantly as severity worsened [13]. This study analyzed the distribution of mMRC scores of 2 or more and 1 or less by severity, whereas a previous overseas study [13] analyzed the mean mMRC score by severity, leading to limitations in directly comparing the results. In this study, the proportion of patients reporting an mMRC ≥ 2 increased dramatically, from 28.8% in the mild group to 51.0% and 61.3% in the moderate and severe groups, respectively. Notably, 99.3% of the very severe group reported dyspnea with an mMRC ≥ 2. This finding underscores the importance of preventing exacerbations, as the proportion of patients with severe dyspnea increases sharply as COPD progresses from mild to moderate and then to severe. It has been consistently recognized that applying effective interventions to improve the physical and psychological conditions of patients with early stage COPD before their lung function declines is crucial, along with supporting long-term health-promoting behaviors [27]. However, the prevention of exacerbations in patients with advanced disease has received little attention. Interventions such as respiratory rehabilitation programs [27], which have proven effective in preventing exacerbations in patients with early-stage COPD, should also be implemented for patients with advanced disease. These interventions should include strategies to identify and tailor respiratory rehabilitation according to specific conditions and characteristics.

One variable emphasized for tailoring respiratory rehabilitation is SE [22]. Previous research [9] has shown that COPD patients with high SE report fewer dyspnea complaints and limitations in daily life, indicating a positive effect on their overall condition. In this study, although the difference was not significant, the severe group had higher respiratory rehabilitation SE scores and longer walking distances in the 6-minute walk test than the moderate group. This finding suggests that patients with severe disease and high respiratory rehabilitation SE may perform well in self-management, maintain physical function, and experience fewer daily life restrictions. This highlights that SE may play a significant role in influencing daily life restrictions and physical function decline, even in patients with advanced disease. This interpretation is also supported by the results of Kang et al. [28], who found that patients with COPD in the severe group had a higher degree of self-management than those in the moderate group. However, the study did not measure SE, which limits the interpretation. In addition, this study did not investigate the degree of self-management with other factors that may affect the participants’ walking ability, and the number of participants in some moderate groups was insufficient; therefore, the interpretation should be made with caution. Future studies with larger numbers of participants and more variables should be conducted to explore the relationship between SE and physical function by disease severity.

In this study, the participants' HRQoL tended to decrease with increasing severity, which is consistent with previous studies [1,13,28,29] and supports the interpretation that worsening airflow limitation interferes with daily activities and worsens quality of life [4,30]. In addition, the “activity” domain of HRQoL was the lowest in this study, which is consistent with the findings of previous studies that used the same instrument [13,29]. Furthermore, the significant difference in the total quality of life scores and scores in the activity and impact domains between the severe and very severe groups highlights the need to manage the severe group to prevent deterioration. As mentioned earlier, respiratory rehabilitation strategies tailored to the characteristics of the population are important to prevent deterioration in the severe group.

This study effectively categorized the severity of COPD based on FEV₁, an indicator of airflow limitation, and examined the patient characteristics, physical functioning, SE, and HRQoL. The findings indicated that subjective dyspnea, respiratory rehabilitation SE, and HRQoL tended to decline as the severity of COPD worsened, with notable differences between the severe and very severe COPD groups, particularly in terms of respiratory rehabilitation SE and quality of life. These results emphasize the importance of managing COPD not only in the early stages after diagnosis, but also in the severe group to prevent further exacerbations. To achieve this, disease severity must be assessed by evaluating the degree of airflow limitation, which reflects changes in disease status, and by comprehensively evaluating meaningful variables subjectively reported by patients during condition assessment and management. The patient-reported variables utilized in this study, including dyspnea, respiratory rehabilitation SE, and HRQoL, should be assessed during follow-up according to domestic and international guidelines [3,5]. These comprehensive assessment indicators hold great significance, as they empower nurses to independently assess COPD patients and develop individualized nursing interventions based on the severity and unique characteristics of each patient.

This study focused on patients diagnosed with COPD, a relatively under-recognized disease with a growing societal burden owing to delays in diagnosis and treatment caused by its complex characteristics. This study examined COPD severity, physical functional status, and patient-reported subjective characteristics. The results of this study have significant implications for nursing practice as they identify key variables that serve as indicators for the comprehensive assessment of COPD patients in Korea. This information is crucial for establishing nursing care tailored to the specific characteristics and needs of patients with COPD, thus ensuring effective follow-up and management. However, our results should be interpreted with caution primarily because of the cross-sectional collection of outpatient data at a local tertiary general hospital, which limits the representativeness of the sample. Additionally, the study did not investigate chronic diseases or psychological health problems such as depression, which may influence the severity of the disease. To address these limitations, future studies should adopt a prospective approach to monitor changes in the identified variables, and include a larger and more diverse participant pool.