This is the first epidemiological study assessing the levels of physical activity practiced by COPD patients. Seventy-eight percent of patients walked daily whereas 17% did not practice any physical activity. Overall, median energy expenditure in physical activity was 109 kcal·d−1. Several clinical studies have reported levels of physical activities in small convenience samples of COPD patients (7,16,27–29,32). Three of these studies (16,27,29) reported that the level of physical activity in COPD patients was lower than in healthy age-matched controls, although their small sample size and methods of data presentation do not allow for a meaningful comparison with our data. In fact, the comparison of our data with levels of physical activity in other samples is quite difficult due to differences in methods and in populations studied. Physical activity was measured using a validated questionnaire in a representative sample from the general population of the region of Murcia, Spain (34). In that study, median levels of energy expenditure in physical activity for the men aged 61–65 yr were 150 kcal·d−1 higher than the 109 kcal·d−1 of our sample. Among the 236,386 participants in the European Prospective Investigation into Cancer and Nutrition (EPIC study) aged 50–64 yr, over 80% of subjects engaged daily in walking (18), similar to our 78% of patients. In the United States, data from the National Health Interview Survey of 7801 subjects aged 65 yr or older showed that about 69% of men and 75% of women used to walk at least once a week for ≥30 min (36). These data suggest that severe COPD patients have a level of physical activity that is similar to other general populations and probably not far from the general recommendation to “accumulate 30 min or more of moderate-intensity physical activity on most days of the week” (25) (equivalent to an energy expenditure of 120–150 kcal·d−1). Nevertheless, the differences in the physical activity measurement methods, age ranges, and geographic settings (18) as well as the lack in our study of a direct measure of metabolic expenditure due to usual physical activity do not allow for more precise comparisons.
We have also assessed the association between individual characteristics and physical activity. LTOT use was related to a lower level of physical activity. This association could not be explained by hypoxemia, as PO2 was not related to physical activity, although could be probably due to the mobility limitation imposed by domiciliary LTOT. Use of liquid oxygen could avoid this limitation, but we did not collect data on types of oxygen, and it is likely that only few patients (less than 5%) were using liquid oxygen at the time of the study, according to previous reports in our area (17).
We found a cross-sectional association between both physical and mental summaries of HRQL and physical activity, as has already been reported by studies in older adults. Prospective studies have demonstrated a longitudinal association between physical activity and better health status (30), but no information is yet available regarding the inverse association, that is, a longitudinal association between HRQL and physical activity. Dyspnea was associated with lower physical activity practice, according to previous clinical studies (27,32). Our finding confirms that dyspnea not only limits exercise capacity (21) in experimental conditions (a measure of the performance or physical fitness) but also limits physical activity under real conditions (the behavior). The fact that dyspnea only achieved statistical significance in the final multivariate model if mental HRQL was excluded may reflect factors more related to behavior than to physiology, as suggested by a previous experimental study that claimed that more importance needs to be given to symptom tolerance and motivation when interpreting findings involving dyspnea (21).
Women and older subjects exhibited lower levels of energy expenditure in physical activity, according to a large number of previous studies in the general population (25), and also in our geographic area (9). The independent association between high socioeconomic status and lower physical activity level that we have found is in contradiction with most previous reports (25), even in Barcelona (9). We cannot exclude that high SES patients were less likely to be included in our sample, being selection bias a possible explanation for the present results. Self-reported diabetes was also found to be associated with a lower level of physical activity, according to previous reports (5). Studies in diabetic patients (Types 1 and 2) have demonstrated that this association may be explained by several alterations of the disease, including hypoglycemia, peripheral and autonomic neuropathy, peripheral artery disease, nephropathy, and retinopathy (5).
We did not find an association between BMI and physical activity in our COPD patients, despite the fact that many studies in the general population have demonstrated an increased risk of inactivity among obese persons (25) and an increased risk of overweight in subjects with lower physical activity level (22). We found a normal range of BMI values, in accordance with a previous study that reported a lower prevalence of low weight syndrome in COPD subjects from the Mediterranean area compared with northern countries (10). Probably, differences in the BMI of COPD patients in different geographical areas, as well as differences in the role of nutritional status between COPD and healthy subjects, could explain the lack of association between BMI and physical activity in our study.
The main limitation of the current study is that its cross-sectional design makes it impossible to demonstrate cause-effect relationships. An additional problem arises in our data because the outcome (physical activity level) could have influenced some of the exposures, both through a direct effect on exposure or through an effect on reported or measured exposure. To help with causality assessment, we considered dose-response or checkmark patterns (12) (results not shown; data available from the authors). Unfortunately, we found inconsistent patterns for all continuous exposure variables (previous COPD admissions, dyspnea, FEV1, PO2, BMI, and physical and mental summaries of HRQL), which did not allow inferring about direction of the association and which, otherwise, may be the result of several methodological limitations (an inappropriate comparison group, nonlinearity of associations, or nonnormality of distributions (12)).
The validity of the questionnaire in quantifying physical activity could be argued. It has been reported that it underestimates daily physical activity compared with direct techniques (31). Despite that, our results regarding the proportion of subjects practicing physical activities (e.g., walking), or its comparison with other populations, would be still valid. In addition, a possible underestimation of physical activity quantification would be equal for all individuals, which does not influence the relation between physical activity group and the several factors tested.
In conclusion, one third of COPD patients in our study reported a level of physical activity lower than the equivalent to walking less than 15 min·d−1. Apart from sociodemographic variables, comorbidity, health-related quality of life, and LTOT were the only factors independently associated with low level of physical activity. These results can be useful in the design of rehabilitation programs and should stimulate further interest in investigating and improving the physical activity practice in people with COPD.
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