The association between co-morbidities and physical performance in people with chronic obstructive pulmonary disease

Abstract

A systematic review was conducted to examine the association between co-morbidity and physical performance in people with chronic obstructive pulmonary disease (COPD). MEDLINE, EMBASE, CINAHL, SCOPUS and Cochrane Central Register of Controlled Trials were searched from inception to end-February 2013, using keywords ‘COPD’, ‘exercise’, ‘physical activity’, ‘rehabilitation’, ‘co-morbidity’ and individual co-morbid conditions. Studies reporting associations of co-morbidities in COPD with at least one objective measure of physical performance were included. Study quality was appraised using the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) checklist. Nine studies met inclusion criteria. Mean (standard deviation (SD)) STROBE score was 16 (3) (maximum score = 21). Four studies examined anxiety as a co-morbid condition; three examined depression; two examined obesity and two examined a range of conditions. Reduced physical performance was associated with higher Charlson score (odds ratio (OR) = 0.72, 95% confidence interval (CI) = 0.54–0.98), metabolic disease (OR = 0.58, 95% CI = 0.49–0.67), anxiety (OR = 0.37, 95% CI = 0.23–0.59) and osteoporosis (OR = 0.28, 95% CI = 0.11–0.70). Depression had minimal association with physical performance but was associated with higher dropout rates from pulmonary rehabilitation programmes. Obesity was negatively associated with baseline physical performance but not with change from an exercise intervention. The presence of co-morbid conditions in people with COPD may negatively affect physical performance and should be identified and accounted for analysis of interventions.

Keywords

COPD physical performance co-morbidity systematic review

Introduction

Chronic obstructive pulmonary disease (COPD) is a progressive condition characterised in the lung by airway inflammation and airflow obstruction which is not fully reversible.¹ COPD was ranked as the third leading global cause of death in 2010.² Clinical features include dyspnoea during physical activity, reduced exercise tolerance and symptom exacerbations that may require hospitalisation. As the condition progresses, dyspnoea and repeated exacerbations result in deterioration in quality of life.^3,4

Co-morbidity (the coexistence of one or more conditions in reference to an ‘index’ condition) is common in people with COPD.⁵ In a US-based study, Mapel et al.⁶ reported an average of 3.7 chronic conditions in cases with COPD compared with 1.8 chronic conditions in those without. Common co-morbid conditions in COPD include heart failure,⁷ osteoporosis,⁸ diabetes⁹ and mental illness.¹⁰ The presence of co-morbidity is a major determinant of functional capability, disease severity and patient outcomes for those with COPD.^5,11 Co-morbidity may also negatively influence the therapeutic management of COPD. For example, whilst corticosteroids are recommended in guidelines for the maintenance and treatment of acute exacerbations of COPD, approximately 20% of older patients with diabetes will also have COPD, thus corticosteroid use may interfere with blood glucose level and result in poor diabetes control.¹² People with multiple chronic conditions including COPD have been reported to prioritise the management of one condition over others,¹³ creating challenges for management of risk factors including lack of exercise, recognising signs and symptoms of illness and managing medications.

A key non-pharmacological intervention in the management of people with COPD is pulmonary rehabilitation (PR) that consists of exercise training, education and psychosocial support. Beneficial effects of PR on exercise tolerance, symptom reduction and quality of life have been demonstrated in people with COPD.¹⁴ Whilst depression has been associated with dropout from PR programmes,¹⁵ the influence of co-morbidity on degree of improvement gained from rehabilitation, or baseline physical performance in people with COPD, has not been reviewed. Aspects of physical performance of interest in people with COPD include measures of exercise tolerance, functional exercise capacity¹⁶ and physical activity levels.¹⁷

Narrative reviews of co-morbidity in COPD have examined possible common disease mechanisms¹⁸ and prompted consideration of the impact of co-morbidity on physical performance^19,20 but have not systematically reviewed existing literature regarding this association. Therefore, the aim of this study was to conduct a systematic review examining the association between co-morbidity and physical performance in people with COPD.

Methods

Search strategy and study selection

A primary systematic search of electronic databases, namely Cochrane Central Register of Controlled Trials (1996–February 2013), SCOPUS (1996–February 2013), CINAHL (1982–February 2013), MEDLINE (1946–February 2013) and EMBASE (1974–February 2013) was conducted. Search terms used in the major databases are listed in Table 1. Secondary searching examined the reference lists of all full text studies screened.

Table 1.

Systematic search strategy and terms.

Database	Search terms
MEDLINE EMBASE	1. COPD (including related terms) 2. Pulmonary rehab^a OR exercise OR physical activity (including related terms) 3. 1 or 2 4. Pulmonary 5. Rehabilitat^a 6. 4 and 5 7. 3 and 6 8. Co-morbid^a (including related terms) 9. 7 and 8 Replace (8) with congestive heart failure, myocardial or heart infarction, rheumatoid arthritis, peripheral vascular disease, diabetes or diabetes mellitus type2, cardiovascular diseases, depression, hypertension, hyperlipidaemia, osteoarthritis, obesity, kidney or renal disease (all including related terms except kidney or renal disease)
CINAHL SCOPUS	1. COPD OR chronic obstructive pulmonary disease – AND 2. Pulmonary rehab^a OR exercise OR physical activity – AND 3. Co-morbid^a Replace (3) with congestive heart failure, myocardial infarct^a, osteoarthritis, rheumatoid arthritis, arthritis, renal or kidney disease, peripheral vascular disease, diabetes, cardiovascular disease, depression, hypertension, hyperlipidaemia or cholesterol, obesity or overweight

Database

Search terms

MEDLINE EMBASE

1. COPD (including related terms) 2. Pulmonary rehab^a OR exercise OR physical activity (including related terms) 3. 1 or 2 4. Pulmonary 5. Rehabilitat^a 6. 4 and 5 7. 3 and 6 8. Co-morbid^a (including related terms) 9. 7 and 8 Replace (8) with congestive heart failure, myocardial or heart infarction, rheumatoid arthritis, peripheral vascular disease, diabetes or diabetes mellitus type2, cardiovascular diseases, depression, hypertension, hyperlipidaemia, osteoarthritis, obesity, kidney or renal disease (all including related terms except kidney or renal disease)

CINAHL SCOPUS

1. COPD OR chronic obstructive pulmonary disease – AND 2. Pulmonary rehab^a OR exercise OR physical activity – AND 3. Co-morbid^a Replace (3) with congestive heart failure, myocardial infarct^a, osteoarthritis, rheumatoid arthritis, arthritis, renal or kidney disease, peripheral vascular disease, diabetes, cardiovascular disease, depression, hypertension, hyperlipidaemia or cholesterol, obesity or overweight

COPD = chronic obstructive pulmonary disease.

^aTruncation, including related terms.

Studies were eligible for inclusion if (a) participants were diagnosed with COPD (or chronic bronchitis or emphysema), (b) participants had one or more co-morbid conditions documented, (c) the study investigated how co-morbidity in COPD patients affected physical performance, (d) the study reported at least one objective outcome measure related to physical performance and (e) they were written in English.

Quality assessment

Two independent reviewers (KJ and KL) critically appraised the included studies using the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) checklist.²¹ This tool consists of 22 items that relate to study description, bias, statistical analysis and external validity.²² One item (‘other analyses’) was combined with the ‘main results’ item, resulting in a maximum score of 21.

Data analysis

Associations between co-morbidities and measures of physical performance in people with COPD reported in the included articles were summarised. Where odds ratios (ORs) were not reported but significant associations were described using multiple regression, ORs and 95% confidence intervals (CIs) were calculated from the published data by exponentiating the unstandardised β regression coefficient (i.e. OR = exp(β)) to allow comparison of effect size between studies.

Results

The search strategy yielded 2430 records of which 2393 were excluded based on title or abstract and a further 10 were duplicates. A total of nine studies met the criteria for inclusion (Figure 1).

Figure 1.

Flow diagram of study selection.

Quality assessment

Table 2 presents the STROBE checklist score for each included study. Scores ranged between 13 and 20 with a mean (SD) score of 16 (3). A study by Eisner et al.¹⁰ reported how the study size was arrived at, whilst none justified sample size with power calculations. Strategies to address potential sources of bias were described in three studies.^10,25,29 Only three studies addressed the external validity or generalisability of their study results.^24,25,29

Table 2.

Characteristics of study participants, design and outcome measures.

Author	Crisafulli et al.²³	Crisafulli et al.²⁴	Eisner et al.¹⁰	Garrod et al.¹⁵	Giardino et al.²⁵	Ramachandran et al.²⁶	Sava et al.²⁷	Trappenburg et al.²⁸	Von Leupoldt et al.²⁹
Country	Italy	Italy	United States	United Kingdom	United States	United States	Canada	United States	Germany
Year of publication	2008	2010	2010	2006	2010	2008	2010	2005	2011
STROBE checklist score	13	14	19	15	20	13	17	15	18
Study design	Pre–post intervention retrospective	Pre–post intervention prospective	Cross-sectional study	Pre–post intervention	Cross-sectional study	Pre–post intervention retrospective	Pre–post intervention prospective	Pre–post intervention	Pre–post intervention prospective
Sample size (n)	2962	316	1202 (COPD); 302 (control)	74 (analysis on 51 who completed)	1828	114 (analysis on 98 patients as underweight excluded)	261	81 (analysis on 65 who completed)	238
Study population	Diagnosis of COPD with GOLD classification	Diagnosis of COPD with GOLD classification	Physician’s diagnosis of COPD	Known diagnosis of COPD	Moderate to severe emphysema	Diagnosis of COPD with GOLD classification	Diagnosis of COPD with GOLD classification	Diagnosis of COPD with GOLD classification	Diagnosis of COPD with GOLD classification
Age mean (SD) years	71.1 (8.0)	68.3 (7.6)	No anxiety: 59(6)’ anxiety: 55 (6.6)	68 (10)	66.7 (6.3)	70 (8)	65 (8)	63 (8)	62.0 (9.9)
Gender M/F	2150/812	235/81	696/808	41/33	1134/694	47/67	149/112	Not reported	135/103
FEV₁% predicted mean (SD)	49.3 (14.8)	49.6 (14)	FEV₁ in litres reported; COPD no anxiety: 1.78 (0.8); COPD anxiety: 1.88 (0.8); Control no anxiety: 2.73 (0.7); Control anxiety: 3.03 (0.8)	Mild: 65.7 (20); moderate: 43 (24.4); severe:38.1 (16.9)	27.1 (7.5)	46 (16)	46 (15)	42 (17)	53.9 (18.1)
Intervention	In/outpatient PR (cycle and resistance training); daily sessions up to a minimum of 15. (Not clear how much of programme people attended)	Outpatient PR (cycle and resistance training); 3-hour session × 3 per week (complete a minimum of 21 sessions)	Nil	Outpatient PR 1-hour exercise (walking and resistance training) + education session × 2 per week × 7 weeks + home exercises 20 minutes/day × 5 days/week	Nil	Outpatient PR Approximately 16 sessions, aerobic exercise and strength training + education	Outpatient PR 1.5-hour session (cycle and resistance training) × 3 per week × 6–12 weeks, but some of sample did home-based programme	Outpatient PR 2-hour session(cycle, walking and resistance training) × 3 per week × 12 weeks	Outpatient PR 6-hour session(cycle, walking and resistance training) × 5 per week × 3 weeks
Physical performance outcome measures	6MWD	6MWD	6MWD	6MWD; quadriceps torque; grip strength	6MWD; maximum exercise capacity (cycle ergometer)	6MWD; walk–work; arm lifts per minute; functional activities	6MWD; CET	6MWD	6MWD

GOLD: Global Initiative of Chronic Obstructive Lung Disease classification; 6MWD: 6-minute walk test distance; FEV₁: forced expiratory volume in 1 second, CET: constant work rate cycling exercise test; COPD: chronic obstructive pulmonary disease; STROBE: the Strengthening the Reporting of Observational Studies in Epidemiology.

Participant characteristics, study design and outcome measures

Table 2 presents the participant characteristics, study design and outcome measures of the included studies. The total number of patients included in 9 studies was 7076, ranging from74 to 2962 subjects per study. The mean age of participants in individual studies ranged from 55 to 71 years. The 6-minute walk test distance (6MWD) was used as an outcome measure of physical performance in all studies which met the inclusion criteria.

Two types of study design were used in the nine studies. Eisner et al.¹⁰ and Giardino et al.²⁵ used cross-sectional study designs and found anxiety in COPD patients to be related to poorer health outcomes. The other seven studies^{15,23,24,26

–29} measured participant’s physical performance before and after they participated in a PR programme.

Co-morbidities examined

Four studies^10,25,28,29 examined anxiety as a co-morbid condition; depression was examined in three studies^15,28,29 and obesity was examined in two studies.^26,27 A range of co-morbid conditions were examined in two studies by Crisafulli et al.^23,24 including chronic heart failure, coronary disease, diabetes, osteoporosis, dyslipidaemia and hypertension; however, mental health or obesity were not identified or examined in these studies. Details of co-morbidities examined in each study and how they were identified are included in Table 3.

Table 3.

Co-morbidities examined in included studies.

Author	Co-morbidities examined	How co-morbidities were identified
Crisafulli et al.²³	Hypertension Congestive heart failure Coronary disease Diabetes Dislipidaemia Osteoporosis/arthrosis Others	Medical record extraction Charlson co-morbidity index
Crisafulli et al.²⁴	Chronic heart failure Diabetes Osteoporosis Dyslipidaemia Coronary disease Hypertension	Medical record extraction Charlson co-morbidity index
Eisner et al.¹⁰	Anxiety	Hospital anxiety and depression scale^30,a
Garrod et al.¹⁵	Depression	Brief assessment schedule depression card^31,a
Giardino et al.²⁵	Anxiety Depression	Beck depression inventory^32,a Spielberger state trait anxiety inventory^33,a
Ramachandran et al.²⁶	Obesity	BMI (measured) (normal BMI = 21–24.9 kg/m², overweight = 25–29.9 kg/m², obesity = BMI > 30 kg/m²)
Sava et al.²⁷	Overweight and obesity	BMI (measured) (normal BMI = 18.5–24.99 kg/m², overweight = 25–29.99 kg/m², obese = BMI > 30 kg/m²)
Trappenburg et al.²⁸	Anxiety Depression	Hospital Anxiety and Depression Scale^30,a
Von Leupoldt et al.²⁹	Anxiety Depression	Hospital Anxiety and Depression Scale^30,a

BMI: body mass index.

^aValidated instrument.

Associations between co-morbid conditions and physical performance in COPD

Table 4 summarises associations reported between co-morbidities and physical performance in people with COPD. Inhomogeneity of statistical analyses for associations between physical performance and individual co-morbidities in the different studies prevented meta-analysis of findings. In the studies where ORs were reported or calculated, a higher Charlson co-morbidity index score had the largest negative association with physical performance (OR = 0.72, 95% CI = 0.54–0.98)²³ followed by the presence of metabolic disease (OR = 0.58, 95% CI = 0.49–0.67),²³ anxiety (OR = 0.37, 95% CI = 0.23–0.59)²⁵ and osteoporosis (OR = 0.28, 95% CI = 0.11–0.70)²⁴ in descending order. Depression only had a minimal effect on physical performance (OR = 0.003, 95% CI = 0.00–0.25).²⁹ The OR of the two studies on the effect of obesity on performance could not be calculated, however both authors concluded that obesity had a negative association with physical performance in COPD patients.^26,27 On the other hand, heart disease had a positive association with physical performance (OR = 2.86, 95% CI = 1.85–3.01).²³

Table 4.

Associations between co-morbidities and physical performance in people with COPD.

Authors	Co-morbidity examined	Associated physical performance measure	Statistical analysis of association	OR 95% CI or effect size	Direction of association with physical performance
Crisafulli et al.²³	Higher Charlson index	Achievement of MCID improvement in 6MWD after PRP	Multiple logistic regression	OR = 0.72 (95% CI = 0.54–0.98) p = 0.024	Negative
Crisafulli et al.²³	Metabolic disease	Achievement of MCID improvement in 6MWD after PRP	Multiple logistic regression	OR = 0.57 (95% CI = 0.49–0.67) p = 0.001	Negative
	Heart disease	Achievement of MCID improvement in 6MWD after PRP	Multiple logistic regression	OR = 2.36 (95% CI = 1.85–3.01) p = 0.001	Positive
	also examined self-reported heart failure, diabetes, hypertension, coronary disease, dyslipidaemia, osteoporosis/arthrosis ‘other diseases’, and heart disease, skeletal disease as groupings: no associations reported
Crisafulli et al.²⁴	Osteoporosis	Achievement of MCID improvement in 6MWD after PRP	Multiple logistic regression	OR = 0.28 (95% CI = 0.11–0.7) p < 0.01	Negative
	also examined heart failure, diabetes, hypertension, coronary disease, dyslipidaemia, ‘other diseases’, and heart disease, metabolic disease as groupings: no associations reported
Eisner et al.¹⁰	Anxiety	Cross-sectional 6MWD	Multiple linear regression associated with distance of −66.3 feet for anxious versus non-anxious group	95%CI = −127.3 to −5.36 feet), p = 0.033	Negative
Garrod et al.¹⁵	Depression	Achievement of clinically significant improvement in either 6MWD or SGRQ after PRP ‘responder’ change in 6MWD after PRP	Univariate and multiple linear regression		None
Giardino et al.²⁵	Anxiety	Cross-sectional 6MWD	Multiple linear regression	OR = 0.37 (95% CI = 0.23–0.59)^a p < 0.001	Negative
Giardino et al.²⁵	Depression	Cross-sectional 6MWD	Multiple linear regression	OR = 0.07 (95% CI = 0.01–0.38)^a p = 0.009	Negative
Ramachandran et al.²⁶	Obesity (BMI > 30 kg/m²)	Baseline 6MWD	Independent t test, less in obese (203 ± 13 m) than non-obese (269 ± 11 m, p = 0.0002)		Negative
Ramachandran et al.²⁶	Obesity (BMI > 30 kg/m²)	Change in 6MWD after PRP	Independent t test of change in 6MWD outcome obese versus non-obese.		None
Sava et al.²⁷	Obesity (BMI > 30 kg/m²)	Baseline 6MWD	ANOVA: significantly different in obese group (342 ± 79 m) from overweight (391 ± 78 m) and normal weight groups (407 ± 75 m)		Negative
		Change in 6MWD after PRP	Multivariate analysis with age, sex, lung function (data not shown)		None
		Proportion of group who reached MCID in 6MWD after PRP	ANOVA: significantly less in obese versus normal weight group (15% vs. 24%, p < 0.01)		Negative
Trappenburg et al.²⁸	Depression (HADS depression)	Change in 6MWD after PRP			None
	Depression (HADS depression)	Change in peak workload (cycle) after PRP	Weak univariate correlation −0.34, p = 0.008		Slight negative
	Also examined anxiety (HADS anxiety), no associations with 6MWD or peak workload changes.
von Leupoldt et al.²⁹	Anxiety (HADS anxiety)	Baseline 6MWD 6MWD after PRP	Multivariate analysis β = −2.13; SE = 1.43, not significant		None
von Leupoldt et al.²⁹	Depression (HADS depression)	Baseline 6MWD	Multivariate analysis β = −5.55; SE = 2.11	OR = 0.003 (95% CI = 0.00–0.25)^a	Slight negative
		6MWD after PRP	Very similar result to that at baseline.		Slight negative

6MWD: 6-minute walk distance; MCID: minimal clinically important difference; PRP: pulmonary rehabilitation program; β: unstandardised regression coefficient; SE: standard error, OR: odds ratio; CI: confidence interval; HADS: Hospital Anxiety and Depression Scale.

^aOR and 95% CI calculated from published data (regression coefficient β). All multivariate analyses accounted for effects of age, sex and lung function.

These associations are complicated by two issues. First, associations of Charlson index, metabolic disease and heart disease with achievement of minimal clinically important difference in 6MWD reported in a retrospective study by Crisafulli et al.,²³ were not replicated in their subsequent prospective study.²⁴ Second, associations of co-morbidity are reported both with baseline measures of 6MWD (for anxiety,^10,25,29 depression^25,29 and obesity^26,27) and with improvements in physical performance after a PR program (for Charlson index,²³ metabolic disease,²³ heart disease,²³ osteoporosis,²⁴ depression^15,28 and obesity^26,27; Table 4).

Discussion

The findings of this systematic review indicate that the presence of co-morbid anxiety (in two cross-sectional studies^10,25), osteoporosis (one intervention study²⁴) or obesity (in two cross-sectional studies^26,27, one retrospective²⁶) were associated with reduced physical performance in people with COPD. Associations with reduced physical performance were reported for metabolic disease,²³ higher Charlson index²³ and depression^25,28,29 but not found in other included studies of the same co-morbidities.^15,24 Obesity did not have an effect on the outcome of PR but was associated with lower baseline 6MWD.^26,27

The presence of co-morbid conditions is increasingly recognised as a major clinical consideration in the management of people with COPD, influencing both therapeutic management and clinical outcomes.³⁴ Recommendations made in clinical guidelines for COPD care are based on findings from clinical trials, which often exclude older patients and those with co-morbidity. For example, Herland et al.³⁵ found that a ‘typical’ clinical study would only include 17% of the ‘real-life’ asthma or COPD patient population, potentially limiting the generalisation of study findings to ‘real-life’ settings. Changes in physical performance are a common endpoint in COPD studies evaluating pharmacological³⁶ and non-pharmacological³⁷ interventions, and this review demonstrates existing evidence for the potentially confounding influence of co-morbidity on such results.

In order to control for the effects of co-morbidity on outcomes of clinical research, indices such as the Charlson co-morbidity index have been used.³⁸ Based on diagnosis of 17 specific conditions, the Charlson co-morbidity index is an established predictor of mortality.^38,39 However, the index does not include all conditions (e.g. depression, anxiety and obesity) and is not strongly predictive of other important health outcomes including quality of life⁴⁰ or health-care utilisation.⁴¹ Other co-morbidity scores such as the medication based Rx-Risk score^42,43 may complement the Charlson co-morbidity index, as this includes co-morbid conditions not included in the latter, such as hypertension, anxiety, depression and osteoporosis, which are common in COPD and could negatively impact physical performance.

Two cross-sectional studies in this review^10,25 reported a negative effect of anxiety and/or depression on COPD patients’ physical performance, two reported no or negligible association^28,29 whilst one reported no significant association between baseline depression and change in 6MWD after pulmonary rehabilitation.¹⁵ Other studies that examined obesity and multiple co-morbidities did not examine mental health issues.^23,24,26,27 If anxiety or depression were present in these patient populations, it may have had a confounding effect on physical performance outcomes. Anxiety and depression are common co-morbidities in people with COPD, with prevalence rates as high as 75% for anxiety and 71% for depression.⁴⁴ Depression has been associated with sedentary lifestyle or lack of adherence to exercise programmes in the general population.⁴⁵ Consistent with this finding, Garrod et al.¹⁵ in an included study from this review reported greater drop out from a PR programme in participants with depression. Future studies in people with COPD that use a measure of physical performance as an outcome should document anxiety and depression and account for their presence in analysis, as these conditions may confound results and more information on the influence of anxiety and depression is required.

In one retrospective study, heart disease was associated with improved 6MWD after PR.²³ Cardiac risk factors have been shown to be reduced with a rehabilitation program.⁴⁶ PR may provide a common exercise pathway to address breathlessness during exercise, skeletal muscle dysfunction and lifestyle changes in chronic heart and lung conditions,⁴⁷ which may explain this observed association with greater benefit in 6MWD.

The primary measure of physical performance in the included studies was distance walked in the 6MWD. Whilst the 6MWD is a validated measure of exercise tolerance in people with COPD,⁴⁸ it only measures a narrow range of the physical performance spectrum. No studies were identified that examined the relationship of co-morbidities in COPD with other measures of physical performance. Future studies could examine other aspects of physical performance that might be affected by co-morbidities, for example, objective measures of physical activity or functional capacity.

Quality assessment of included studies used the STROBE checklist, which was developed to encourage authors to reported adequate information on research methods and data analysis for readers to interpret the results of observational studies with confidence.²² Studies by Giardino et al.,²⁵ Eisner et al.¹⁰ (both reporting negative association of anxiety with cross-sectional 6MWD) and von Leupoldt et al.²⁹ (reporting no association of anxiety with cross-sectional 6MWD) had the highest scores thus greatest levels of completeness in reporting.

Co-morbidities were not identified in a uniform manner across studies included in this systematic review, except for obesity which was defined consistently with the body mass index.^26,27 Heterogeneous study designs and statistical analyses in the included studies prevented meta-analysis of associations with physical performance even for the same co-morbid condition. Associations with co-morbidity were reported in some studies with baseline measures of physical performance, and in other studies with improvement gained after a PR program. These inconsistencies in collecting and analysing co-morbidity data make it difficult to compare patient characteristics and results between studies and limit the strength of conclusions able to be drawn from this review. The inclusion of only English-language studies may have limited the completeness of this review.

Conclusion

An association between reduced physical performance in people with COPD who also have co-morbid anxiety, obesity and osteoporosis and possibly in those with metabolic disease or depression was observed in this study. This highlights the need to identify all co-morbid conditions present in those with COPD, to facilitate the development of specific strategies to ensure optimal participation in physical activity programmes and to improve exercise tolerance and health outcomes.

Footnotes

Conflict of interest

The authors declared no conflicts of interest.

Funding

This project received no specific grant from any funding agency in the public, commercial or not for profit sectors.

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