The impact of smoking on respiratory rehabilitation efficacy and correlation analysis in patients with chronic obstructive pulmonary disease: a retrospective study

Introduction

Chronic obstructive pulmonary disease (COPD) was a prevalent and debilitating respiratory condition characterized by airflow limitation and persistent respiratory symptoms (1-3). COPD represents a significant public health burden, contributing to substantial morbidity, mortality, and healthcare costs worldwide (4,5). Smoking was the primary risk factor for the development and progression of COPD, with approximately 85–90% of cases attributed to tobacco smoke exposure (6-8). While the pathophysiological relationship between smoking and COPD was well-established, the impact of smoking on the efficacy of respiratory rehabilitation and its comprehensive implications for patients with COPD remain a topic of continuing interest and investigation.

Pulmonary rehabilitation plays a pivotal role in the management of COPD, encompassing a multidisciplinary approach designed to optimize respiratory function, alleviate symptoms, and enhance patients’ overall quality of life (9-11). The rehabilitation interventions typically include exercise training, disease education, nutritional guidance, psychological support, and behavioral interventions, tailored to individual patient needs and clinical characteristics (12,13). Despite the established benefits of pulmonary rehabilitation in improving exercise capacity, symptom control, and health-related quality of life, several challenges persist in optimizing its efficacy and accessibility for all COPD patients. Key among these are patient adherence, availability of resources, and individual patient characteristics that may affect rehabilitation outcomes (14,15). The pathophysiological relationship between smoking and COPD is well-established (16), but the nuanced impact of smoking on the efficacy of pulmonary rehabilitation remains underexplored. Smoking can exacerbate inflammation, oxidative stress, and tissue damage in the lungs, potentially hindering the beneficial effects of exercise training and other rehabilitation interventions (17). Moreover, smoking has been shown to interfere with patients’ motivation, adherence to rehabilitation programs, and their ability to achieve optimal outcomes (18). Given the high prevalence of smoking among COPD patients and the potential for smoking to modulate the physiological responses to pulmonary rehabilitation, there is a compelling need to investigate how smoking influences the effectiveness of these interventions.

This retrospective case-control study aimed to systematically evaluate the impact of smoking on the efficacy of pulmonary rehabilitation in patients with COPD. Through a comprehensive assessment of pulmonary function, exercise capacity, quality of life, and sleep patterns before and after rehabilitation, this study will contribute to a more nuanced understanding of the multifaceted relationship between smoking and COPD management. The findings from this study will be instrumental in tailoring rehabilitation strategies to better meet the needs of smoking COPD patients and in advocating for integrated smoking cessation programs as an essential component of pulmonary rehabilitation. We present this article in accordance with the STROBE reporting checklist (available at https://jtd.amegroups.com/article/view/10.21037/jtd-24-1267/rc).

Methods

Study design

This study was a retrospective case-control study. It involved patients with COPD who were treated at our hospital from October 2015 to October 2023. Patients were categorized into non-smoking and smoking groups based on their smoking status. The smoking group consisted of individuals with a smoking history of 40 pack-years or more, while the non-smoking group included COPD patients with no history of smoking.

The study was conducted in accordance with the Declaration of Helsinki (as revised in 2013). The study was approved by institutional Committee Ethics board of China Rehabilitation Research Center (No. 2024-091-3) and individual consent for this retrospective analysis was waived.

Inclusion and exclusion criteria

Inclusion criteria: (I) patients were 40 years of age or older; (II) with a diagnosis (19) of COPD with severe airflow limitation [confirmed by a post-bronchodilator forced expiratory volume in one second (FEV₁)/forced vital capacity (FVC) ratio <0.70 and a post-bronchodilator FEV₁, of ≤50% predicted]; (III) symptoms of chronic bronchitis; (IV) with normal cognitive functions; (V) stable vital signs; (VI) complete medical records; (VII) in the stable phase of COPD.

Exclusion criteria: (I) patients had a COPD exacerbation that was ongoing during the baseline period, or had a diagnosis of asthma or other major lung disease, such as lung cancer, bronchiectasis and interstitial lung disease; (II) existence of any pathology which makes it inadvisable to perform spirometry; (III) patients who, due to their intellectual disability or other psychiatric disorder, do not understand study procedures or cannot adequately perform spirometry; (IV) abnormal heart, liver, and kidney function; (V) malignant tumor or marked impairment of limb function; (VI) severe nutritional imbalance; (VII) previous participation in a pulmonary rehabilitation program.

The demographic characteristics of the two groups of patients strictly adhered to the aforementioned inclusion and exclusion criteria, and no other matching techniques were used.

Intervention

Rehabilitation protocol for both patient groups included standardized stable-phase medication, including correct use of inhalers and targeted medication for expectoration and cough relief. The comprehensive pulmonary rehabilitation program involved the following elements: (I) disease education encompassing smoking cessation, oxygen therapy, understanding and self-management of COPD, correct medication usage, and scientifically balanced diet, as well as pulmonary rehabilitation concepts; (II) respiratory training, primarily consisting of diaphragmatic and pursed-lip breathing, each lasting 15 minutes, three times a day; (III) exercise training, combining endurance and strength exercises with lower limb exercises as the core training method. Exercise cardiopulmonary function tests for both patient groups were conducted by utilizing German Jerger. Based on the results of the patients’ exercise cardiopulmonary function tests, as well as their age and gender, moderate- to high-intensity aerobic exercise was selected with a target training intensity of 65–85% of the target heart rate. A rational exercise prescription was devised, and patients were guided to engage in flat panel exercises (Jaeger, Germany) for 30 minutes, three times a week; (IV) nutritional assessment and guidance with a focus on calorie intake and appropriate dietary guidance to ensure the consumption of adequate levels of protein, vitamins, and trace elements; (V) psychological assessment and counseling. The pulmonary rehabilitation program lasted for 8 weeks.

General information

Patient general information was obtained through systematic retrieval of medical records, including age, gender, body mass index (BMI), smoking index, drinking history, hypertension, diabetes, hyperlipidemia, disease duration, educational status, employment status, living environment, respiratory symptoms and history of respiratory diseases.

Pulmonary function tests

The pulmonary function of both patient groups was assessed before and after rehabilitation using a spirometer. The baseline and post-rehabilitation measurements for FEV₁, FVC, the FEV₁/FVC ratio, maximal voluntary ventilation (MVV), and diffusing capacity of the lung for carbon monoxide (DLCO) were recorded.

Exercise capacity

Six-minute walk distance (6MWD) (20): a straight line was marked on a flat surface, and in strict accordance with the guidelines for the 6-minute walk test, the 6MWDs before rehabilitation (baseline) and at the end of the rehabilitation course were measured for both patient groups. A greater distance indicates greater exercise tolerance and better lung function. Additionally, dyspnea and leg fatigue were assessed using the Borg scale immediately after the 6MWD assessment. The Borg scores were ranging from 0 to 10. A higher score indicates a more severe level of breathing difficulty and fatigue.

VO₂ was measured for both patient groups before and after rehabilitation training. Peak VO₂ was defined as the maximum oxygen intake during exercise measured using a cardiopulmonary exercise testing system (MasterScreenCPX, Jaeger, Germany). The Ramp protocol was employed, and a maximal exercise test was chosen. The incremental load was selected based on the patient’s gender, age, height, weight, and physical activity level, increasing until the participant could no longer maintain the cycling cadence (55–65 revolutions per minute) or reached their age-predicted maximum heart rate (220 − age) or other criteria for test termination were met (21).

The Medical Research Council (MRC) dyspnea scale was calculated based on the modified MRC dyspnea scale criteria. A lower score indicates a lower level of breathlessness.

St. George’s Respiratory Questionnaire (SGRQ) score

The SGRQ was used to assess the quality of life for patients before and after rehabilitation treatment. The SGRQ comprises three domains: (I) symptoms, including cough, sputum production, and breathlessness; (II) activity, involving limitations in climbing stairs, dressing, domestic chores, and recreational activities; and (III) impacts, encompassing anxiety, distress, impact on social activities, and feelings of insecurity and despair. The scores for each domain as well as the total score were calculated, with a score range of 0 to 100. A lower score indicates better health status. The Cronbach’s α coefficient for internal consistency reliability was calculated as 0.905 (22).

Quality of life score

Statistical analysis was managed using the European Organization for Research and Treatment of Cancer Quality of Life Questionnaire (EORTC QLQ-C30), which comprises five functional dimensions (bodily, moving, character, social, and cognitive functions). Each item in the various dimensions was rated on a 1–7 Likert scale, with total scores ranging from 0 to 100, reflecting a positive correlation with quality of life. The Cronbach’s α coefficient was 0.927 (23).

Pittsburgh Sleep Quality Index (PSQI)

Participants completed the PSQI. The PSQI was a self-report assessment tool that evaluates sleep quality over a 1-month period. A global score and seven component scores can be derived from the scale. The component scores were the following: Subjective sleep quality, sleep latency, sleep duration, sleep efficiency, sleep disturbances, use of sleeping medications and daytime dysfunction. Each component was scored on a scale from 0–3, with the total score ranging from 0–21; where a higher score describes poorer sleep quality. A total PSQI score greater than 5 has been validated as being highly sensitive and specific in distinguishing good from poor sleepers across a number of populations. The Cronbach’s α coefficient of the PSQI was 0.78 (24).

Sample size

Using G*Power 3.1.9.7, based on the “t tests: means − difference between two independent means (two groups)” option, we selected an a priori analysis with the following settings: two-tailed mode, effect size d =0.92, α error probability =0.05, power (1 − β error probability) =0.8. The calculation resulted in a minimum sample size of 20 cases per group.

Statistical analysis

The data were analyzed using SPSS 29.0 statistical software (SPSS Inc., Chicago, IL, USA). For categorical data, [n (%)] was used for representation. The Chi-squared test was applied with the basic formula when the sample size was ≥40 and the theoretical frequency T was ≥5, with the test statistic represented by χ². When the sample size was ≥40 but the theoretical frequency 1≤T<5, the Chi-squared test was adjusted using the correction formula. In cases where the sample size was <40 or the theoretical frequency T<1, statistical analysis was conducted using Fisher’s exact probability method. Continuous variables were first tested for normal distribution using the Shapiro-Wilk method. For normally distributed continuous data, the format [mean ± standard deviation (SD)] was employed. Non-normally distributed data was analyzed using Wilcoxon rank-sum test, and the [median (25% quantile, 75% quantile)] was used for presentation. P<0.05 were considered as statistical significance.

Results

General information

Forty patients with COPD were included in this retrospective study and equally divided into the non-smoking group (n=20) and the smoking group (n=20) (Figure 1). There were no statistically significant differences between the two groups in terms of age (P=0.43), gender distribution (P=0.74), BMI (P=0.46), drinking history (P=0.69), comorbidities including hypertension (P=0.69), diabetes (P=0.45), hyperlipidemia (P=0.66), disease duration (P=0.37), educational status (P=0.74), employment status (P=0.45), living environment (P=0.74), respiratory symptoms (P=0.74), and history of respiratory diseases (P=0.74) (Table 1). The average smoking packs per year in the smoking group was 47.71±4.85 packs, showing a significant difference compared to the non-smoking group (P<0.001).

Figure 1 Patient selection flowchart.

Pulmonary function tests before and after rehabilitation

The pulmonary function tests before rehabilitation revealed no statistically significant differences between the non-smoking group and the smoking group in terms of FEV₁ (P=0.96), FVC (P=0.97), FEV₁/FVC ratio (P=0.73), MVV (P=0.69), and DLCO (P=0.63) (Table 2). The pulmonary function tests after rehabilitation demonstrated statistically significant differences between the non-smoking group and the smoking group in FEV₁ (2.14±0.48 vs. 1.83±0.25, t=2.598, P=0.02), FVC (3.48±0.55 vs. 3.05±0.43, t=2.741, P=0.009), FEV₁/FVC ratio (62.14±6.56 vs. 57.26±7.05, t=2.265, P=0.03), MVV (84.23±8.07 vs. 76.84±7.01, t=3.090, P=0.004), and DLCO (24.17±2.58 vs. 21.95±2.86, t=2.573, P=0.01).

Exercise capacity before and after rehabilitation

The exercise capacity assessments before rehabilitation revealed no statistically significant differences between the non-smoking group and the smoking group in terms of the 6MWD (P=0.91), VO₂peak (P=0.40), Borg scale rating (P=0.10), SGRQ score (P=0.27), and MRC dyspnea scale score (P=0.27) (Table 3). The exercise capacity assessments after rehabilitation demonstrated statistically significant differences between the non-smoking group and the smoking group in terms of the 6MWD (410.27±25.48 vs. 392.45±24.58, t=2.251, P=0.03), peak VO₂peak (17.83±2.56 vs. 15.83±2.26, t=2.625, P=0.01), Borg scale rating (2.96±0.89 vs. 3.56±0.64, t=2.445, P=0.02), SGRQ score (52.48±6.58 vs. 58.62±6.15, t=3.048, P=0.004), and MRC dyspnea scale score (1.89±0.26 vs. 2.39±0.67, t=3.110, P=0.005).

Quality of life scores and Sleep Quality Score

The comparison of quality of life scores between the non-smoking group and the smoking group revealed statistically significant differences in somatization (77.03±6.02 vs. 71.72±6.98, t=2.574, P=0.01), emotion management (78.14±7.78 vs. 71.14±8.26, t=2.759, P=0.009), role play (79.24±8.23 vs. 72.41±7.24, t=2.784, P=0.008), cognitive function (76.21±5.11 vs. 72.73±5.02, t=2.171, P=0.04), and return to social function (77.23±5.78 vs. 72.07±6.23, t=2.721, P=0.01) (Table 4). The comparison of PSQI scores between the non-smoking group and the smoking group revealed a statistically significant difference in sleep quality (7.89±2.21 vs. 10.03±3.34, t=2.397, P=0.02).

Discussion

COPD was a leading cause of morbidity and mortality globally, with smoking being a key risk factor for its development and progression (25-27). Our retrospective case-control study demonstrated significant disparities in pulmonary function, exercise capacity, quality of life, and sleep quality between non-smoking and smoking COPD patients following rehabilitation. Specifically, non-smokers exhibited significantly greater improvements in FEV₁, FVC, FEV₁/FVC ratio, MVV, and DLCO compared to smokers in post-rehabilitation. Non-smokers also achieved significantly greater improvements in exercise capacity, as evidenced by the 6MWD, peak VO₂peak, Borg scale ratings, and the MRC dyspnea scale score. These findings underscore the adverse impact of smoking on respiratory rehabilitation efficacy and comprehensive COPD management.

Pulmonary function tests play a crucial role in evaluating the severity and progression of COPD. Our study demonstrated that while there were no significant differences in baseline pulmonary function parameters between smokers and non-smokers, significant disparities emerged after rehabilitation. Non-smokers exhibited significantly greater improvements in FEV₁, FVC, FEV₁/FVC ratio, MVV, and DLCO compared to smoker’s post-rehabilitation. These findings were consistent with previous research indicating that smoking negatively impacts lung function and can impede the effectiveness of interventions aimed at improving respiratory outcomes (28,29). The underlying mechanisms likely involve the chronic inflammation and oxidative stress induced by smoking, which can lead to structural changes in the airways and alveoli, impairing lung function and reducing the responsiveness to rehabilitation efforts (30).

Exercise capacity was another critical aspect of COPD management and was closely linked to overall functional status. Our study revealed that non-smokers achieved significantly greater improvements in exercise capacity, as evidenced by the 6MWD, peak VO₂peak, Borg scale ratings, and the MRC dyspnea scale score after rehabilitation. These results are in line with previous studies that have reported similar findings. For instance, Soumagne et al. found that non-smokers with COPD showed greater improvements in exercise capacity following pulmonary rehabilitation compared to smokers (31). These results highlight the adverse impact of smoking on patients’ physical functional status and the efficacy of respiratory rehabilitation programs. Furthermore, our findings regarding the SGRQ scores provide additional support for the notion that smoking hinders the improvement of respiratory symptoms and overall quality of life in COPD patients undergoing rehabilitation. The diminished exercise capacity in smokers may be due to reduced lung function, increased airway resistance, and decreased oxygen transport efficiency, all of which are exacerbated by smoking. Furthermore, smoking-induced systemic inflammation and muscle wasting may contribute to the reduced exercise capacity and rehabilitation outcomes (17).

In addition to the detrimental effects of smoking on physiological parameters, our study also identified significant differences in quality of life scores between smokers and non-smokers. Non-smokers demonstrated better outcomes across various domains of the quality of life assessment, including somatization, emotion management, role play, cognitive function, and return to social function. These findings are consistent with previous research (32), which has reported that smoking is associated with poorer quality of life in COPD patients. For example, Papadopoulos et al. found that smoking was a significant predictor of lower quality of life scores in COPD patients, particularly in the domains of physical function and emotional well-being (33). Similarly, Postolache et al. observed that smoking cessation led to improvements in health-related quality of life, including reduced symptoms and improved daily functioning (34). These findings underscore the substantial impact of smoking on the overall well-being and functional capacity of COPD patients, further underscoring the importance of smoking cessation interventions as integral components of comprehensive COPD management programs.

Furthermore, our study revealed a notable difference in sleep quality between smokers and non-smokers, with non-smokers exhibiting better Sleep Quality Scores. Sleep disturbances were prevalent in COPD patients and can significantly impact disease management and overall health (35-37). The observed association between smoking and sleep quality in our study underscores the multifaceted nature of the impact of smoking on COPD and its implications for holistic patient care. Therefore, addressing sleep disturbances and promoting healthy sleep patterns should be acknowledged as essential components of comprehensive COPD rehabilitation and management programs.

This study had several strengths, including its rigorous retrospective case-control design, comprehensive assessment of physiological parameters, and robust statistical analysis. However, certain limitations should be considered when interpreting the results. The relatively small sample size and the single-center nature of the study may limit the generalizability of the findings. Additionally, the retrospective design inherently introduces the potential for selection bias and may not account for all confounding variables. Future studies with larger sample sizes and prospective, multi-center designs were warranted to further validate and expand upon our findings. Moreover, future studies will use two-way analysis of variance (ANOVA) to investigate the interaction between smoking and rehabilitation interventions before and after treatment, to gain a clearer understanding of the impact of smoking.

Conclusions

Our study provides compelling evidence of the adverse impact of smoking on respiratory rehabilitation efficacy and comprehensive COPD management. The observed associations between smoking status, pulmonary function, exercise capacity, quality of life, and sleep quality underscore the complex interplay between smoking and COPD outcomes. These findings emphasize the need for targeted smoking cessation interventions and comprehensive support for COPD patients to improve rehabilitation outcomes and overall well-being. By addressing the multifaceted impact of smoking on COPD management, healthcare providers can optimize interventions and enhance the effectiveness of respiratory rehabilitation programs, ultimately improving the quality of care for patients with COPD.

Acknowledgments

Funding: None.

Footnote

Reporting Checklist: The authors have completed the STROBE reporting checklist. Available at https://jtd.amegroups.com/article/view/10.21037/jtd-24-1267/rc

Data Sharing Statement: Available at https://jtd.amegroups.com/article/view/10.21037/jtd-24-1267/dss

Peer Review File: Available at https://jtd.amegroups.com/article/view/10.21037/jtd-24-1267/prf

Conflicts of Interest: Both authors have completed the ICMJE uniform disclosure form (available at https://jtd.amegroups.com/article/view/10.21037/jtd-24-1267/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. The study was conducted in accordance with the Declaration of Helsinki (as revised in 2013). The study was approved by institutional Committee Ethics board of China Rehabilitation Research Center (No. 2024-091-3) and individual consent for this retrospective analysis was waived.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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