The authors have declared that no competing interests exist.
Conceived and designed the experiments: MM EG. Performed the experiments: NW. Analyzed the data: MM MT DG EG. Wrote the paper: MM MT NW DG EG.
COPD is characterized by variability in exercise capacity and physical activity (PA), and acute exacerbations (AEs). Little is known about the relationship between daily step count, a direct measure of PA, and the risk of AEs, including hospitalizations.
In an observational cohort study of 169 persons with COPD, we directly assessed PA with the StepWatch Activity Monitor, an ankle-worn accelerometer that measures daily step count. We also assessed exercise capacity with the 6-minute walk test (6MWT) and patient-reported PA with the St. George's Respiratory Questionnaire Activity Score (SGRQ-AS). AEs and COPD-related hospitalizations were assessed and validated prospectively over a median of 16 months.
Mean daily step count was 5804±3141 steps. Over 209 person-years of observation, there were 263 AEs (incidence rate 1.3±1.6 per person-year) and 116 COPD-related hospitalizations (incidence rate 0.56±1.09 per person-year). Adjusting for FEV1 % predicted and prednisone use for AE in previous year, for each 1000 fewer steps per day walked at baseline, there was an increased rate of AEs (rate ratio 1.07; 95%CI = 1.003–1.15) and COPD-related hospitalizations (rate ratio 1.24; 95%CI = 1.08–1.42). There was a significant linear trend of decreasing daily step count by quartiles and increasing rate ratios for AEs (
Lower daily step count, lower 6MWT distance, and worse SGRQ-AS predict future AEs and COPD–related hospitalizations, independent of pulmonary function and previous AE history. These results support the importance of assessing PA in patients with COPD, and provide the rationale to promote PA as part of exacerbation-prevention strategies.
COPD is the fourth most common cause of death in the US, affects 5% of US adults, and accounts for a large number of hospitalizations
COPD is also characterized by acute exacerbations (AEs) which result in poorer health-related quality of life (HRQL), a faster decline in lung function, and increased mortality
Persons with COPD have a wide range of PA levels which may be potentially modifiable. The relationship between PA and risk of AEs and COPD-related hospitalizations is unclear
The protocol was approved by the VA Boston Healthcare System Committee on Human Research, and written informed consent obtained from each participant.
This work arises from a study which has been previously published, and the subjects reported here include the 127 subjects previously reported
At baseline, information about demographics, medical history, and medications was obtained. Subjects reported being at their usual clinical status at the time of enrollment. Participants underwent measurement of FEV1, using an Eaglet spirometer (nSpire Health, Inc.)
The StepWatch Activity Monitor (SAM) (Orthocare Innovations, Seattle, WA, USA), an ankle-worn accelerometer, measures step counts from all walking–as part of PA and exercise in persons with COPD. We have shown the SAM to be accurate and valid in persons with COPD
The primary outcomes were AEs and COPD-related hospitalizations. AE was defined as “a complex of respiratory symptoms (increased or new onset) of at least two of the following: cough, sputum, wheezing, dyspnea, or chest tightness lasting 3 or more days, requiring a course of treatment with antibiotics or systemic steroids
Descriptive results are reported as means ± SD or percentages, as appropriate. Comparisons of descriptive characteristics were performed with the use of unpaired T tests or Fisher's Exact Test, as appropriate. The incidence rates of AEs and COPD-related hospitalizations were determined by dividing the numbers of AEs and COPD-related hospitalizations by the person-years of follow-up. Predictors of AEs and COPD-related hospitalizations were assessed using negative binomial models with the logarithm of observation time as an offset variable (PROC GENMOD, SAS 9.2, SAS Institute; Cary, NC)
A total of 188 persons with COPD were enrolled. The analysis excluded 12 subjects who did not have baseline step-count data because 5 were noncompliant with step-count monitoring (had ≥8 no-wear days), 5 had no baseline step-count data due to an AE during the monitoring period, 1 lost the SAM, and 1 had SAM accuracy <90%. An additional 7 subjects did not participate in follow-up telephone calls. There were no differences in FEV1 % predicted, 6MWT distance, SGRQ Total Score (SGRQ-TS), SGRQ-AS, or MMRC dyspnea score among the 19 subjects excluded and the 169 subjects included in the analysis. In 169 persons with baseline and follow-up data, 167 were males, mean age was 71±8 years, mean FEV1 was 1.55±0.57 L (54±20% predicted)
Total | Mean Daily Step<5232 |
Mean Daily Step≥5232 |
|
n = 169 | n = 85 | n = 84 | |
Age |
71±8 | 73±8 | 69±8 |
Body-mass index |
29±6 | 30±7 | 28±5 |
Marital status |
|||
Married | 76 (45) | 46 (54) | 30 (36) |
Not married | 93 (55) | 39 (46) | 54 (64) |
Race | |||
White | 156 (92) | 79 (93) | 77 (92) |
Non-White | 13 (8) | 6 (7) | 7 (8) |
Employment status |
|||
Full or part-time | 20 (12) | 5 (6) | 15 (18) |
Not working | 42 (25) | 18 (21) | 24 (28) |
Retired | 107 (63) | 62 (73) | 45 (54) |
Education | |||
Some/Completed high school | 76 (45) | 40 (47) | 36 (43) |
Some/Completed college or higher | 93 (55) | 45 (53) | 48 (57) |
Alcohol use | |||
≥1 day/week | 52 (31) | 30 (35) | 22 (26) |
<1 day/week | 117 (69) | 55 (65) | 62 (74) |
Prior participation in pulmonary rehabilitation |
20 (12) | 16 (19) | 4 (5) |
Supplemental oxygen use |
43 (25) | 32 (38) | 11 (13) |
Prednisone for AE in previous year | 51 (30) | 31 (36) | 20 (24) |
Coronary artery disease |
63 (37) | 39 (46) | 24 (29) |
Congestive heart failure | 23 (14) | 15 (18) | 8 (10) |
Diabetes mellitus | 48 (28) | 24 (28) | 24 (29) |
Pack-years | 68±37 | 70±33 | 66±40 |
FEV1 (liters) |
1.55±0.57 |
1.42±0.57 | 1.68±0.55 |
FEV1, % predicted |
54±20 |
51±20 | 58±20 |
GOLD stage |
|||
I | 16 (10) |
6 (7) | 10 (12) |
II | 77 (46) | 35 (41) | 42 (51) |
III | 56 (33) | 28 (33) | 28 (34) |
IV | 19 (11) | 16 (19) | 3 (4) |
6MWT distance (meters) |
371±100 | 319±90 | 423±81 |
MMRC dyspnea score |
|||
0–1 | 68 (40) | 20 (24) | 48 (57) |
2–4 | 101 (60) | 65 (76) | 36 (43) |
SGRQ-TS |
45±20 | 49±18 | 42±20 |
SGRQ-AS |
63±23 | 69±19 | 57±25 |
Beck depression index | 12±11 | 11±11 | 12±11 |
Medication for COPD |
|||
Any short-acting β2 agonist | 150 (89) | 78 (92) | 72 (86) |
Any short-acting muscarinic antagonist | 29 (17) | 18 (21) | 11 (13) |
Any long-acting β2 agonist | 108 (64) | 56 (66) | 52 (62) |
Any long-acting muscarinic antagonist | 125 (74) | 63 (74) | 62 (74) |
Any inhaled corticosteroid | 115 (68) | 61 (72) | 54 (64) |
AE denotes acute exacerbation; FEV1 forced expiratory volume in 1 second; GOLD Global Initiative for Chronic Obstructive Lung Disease; 6MWT 6-minute walk test; MMRC Modified Medical Research Council; SGRQ-TS St. George's Respiratory Questionnaire Total Score; and SGRQ-AS St. George's Respiratory Questionnaire Activity Score.
Mean ± standard deviation for continuous variables and N (%) for categorical variables.
The median average daily step count is 5,232.
Information on medication was self-reported; subjects may have been taking more than one medication.
N = 168.
N = 83.
Median is 5,232 steps per day, N = 169.
Over 209 person-years of follow-up, there were 263 AEs (incidence rate 1.3±1.6 per person-year) in 99 of 169 subjects (59%). Of these, 167 AEs were experienced by 54 of the 85 persons with daily step count < the median, and 96 AEs were experienced by 45 of the 84 persons with daily step count ≥ the median. There were 116 COPD-related hospitalizations (incidence rate 0.56±1.09 per person-year) in 54 of 169 subjects (32%). Of these, 79 hospitalizations were experienced by 37 persons with daily step count < the median, and 37 hospitalizations were experienced by 17 persons with daily step count ≥ the median. 224 AEs (85%) and 108 COPD-related hospitalizations (93%) were verified with medical records.
In univariate models, lower daily step count was a significant predictor of higher rates of future AEs and COPD-related hospitalizations (
Characteristics | Acute Exacerbations | COPD-Related Hospitalizations | ||||
Rate Ratio | 95% CI | Rate Ratio | 95% CI | |||
Age (per year increase) | 1.02 | 0.99–1.04 | 0.13 | 1.03 | 0.995–1.07 | 0.08 |
Body-mass index (per kg/m2 increase) | 0.996 | 0.96–1.03 | 0.81 | 0.96 | 0.92–1.01 | 0.16 |
Mean Daily Step Count (per 1000 step decrease) | 1.11 | 1.04–1.19 | 0.003 | 1.29 | 1.13–1.49 | 0.0003 |
Mean daily step Quartiles | ||||||
(ref ≥6956) | ||||||
<3667 | 3.00 | 1.68–5.36 | 0.0002 | 8.69 | 2.92–25.8 | <0.0001 |
3667≤×<5232 | 2.62 | 1.46–4.71 | 0.001 | 6.94 | 2.31–20.9 | 0.0006 |
5232≤×<6956 | 2.36 | 1.30–4.27 | 0.005 | 6.80 | 2.25–20.6 | 0.0007 |
0.0003 | 0.0003 | |||||
6MWT distance | 1.10 | 1.03–1.17 | 0.003 | 1.21 | 1.10–1.34 | 0.0002 |
(per 30-meter decrease |
||||||
SGRQ-AS (per 4-point worsening |
1.07 | 1.03–1.12 | 0.0005 | 1.12 | 1.04–1.19 | 0.002 |
FEV1, % predicted |
1.13 | 1.02–1.25 | 0.01 | 1.22 | 1.05–1.42 | 0.008 |
(per 10% decrease in % of predicted value) | ||||||
Prednisone for AE in previous year (ref = no) | 2.44 | 1.66–3.58 | <0.0001 | 2.16 | 1.17–4.00 | 0.01 |
SGRQ-TS | 1.07 | 1.03–1.12 | 0.002 | 1.09 | 1.01–1.17 | 0.02 |
(per 4-point worsening) | ||||||
MMRC dyspnea score 2–4 (ref = 0–1) | 1.47 | 0.97–2.22 | 0.07 | 1.67 | 0.89–3.14 | 0.11 |
Supplemental oxygen use (ref = no) | 1.56 | 1.01–2.40 | 0.04 | 1.50 | 0.77–2.91 | 0.23 |
Pack-years | 1.003 | 0.998–1.01 | 0.27 | 1.001 | 0.99–1.01 | 0.87 |
Diabetes mellitus | ||||||
(ref = no) | 1.14 | 0.73–1.79 | 0.55 | 1.23 | 0.63–2.41 | 0.54 |
Coronary artery disease | 1.08 | 0.71–1.65 | 0.72 | 0.90 | 0.47–1.73 | 0.75 |
(ref = no) | ||||||
Beck depression index | 1.01 | 0.99–1.02 | 0.50 | 1.005 | 0.98–1.03 | 0.74 |
Season of step count monitoring | ||||||
(ref = Summer) | ||||||
Fall | 0.85 | 0.51–1.40 | 0.52 | 0.98 | 0.45–2.13 | 0.97 |
Winter | 0.80 | 0.40–1.61 | 0.54 | 0.61 | 0.20–1.88 | 0.39 |
Spring | 0.79 | 0.44–1.41 | 0.43 | 1.14 | 0.48–2.73 | 0.76 |
6MWT denotes 6-minute walk test; SGRQ-AS St. George's Respiratory Questionnaire Activity Score; FEV1 forced expiratory volume in 1 second; AE acute exacerbation; SGRQ-TS St. George's Respiratory Questionnaire Total Score; MMRC Modified Medical Research Council; and ref reference group.
Rate ratios calculated for a MCID of 30 m
N = 168.
In multivariate models adjusting for FEV1 % predicted and prednisone for AE in previous year, for each 1000 fewer steps per day walked at baseline, there was a significantly increased rate of AEs (rate ratio 1.07; 95%CI = 1.003–1.15) and COPD-related hospitalizations (rate ratio 1.24; 95%CI = 1.08–1.42) (
Model 1 |
Acute Exacerbations | COPD-Related Hospitalizations | ||||||
Rate Ratio | 95% CI | Rate Ratio | 95% CI | |||||
FEV1, % predicted (per 10% increase in % of predicted value) | 1.05 | 0.95 | 1.16 | 0.33 | 1.14 | 0.98 | 1.32 | 0.09 |
Prednisone for AE in previous year (ref = no) | 2.17 | 1.48 | 3.18 | <0.0001 | 1.72 | 0.94 | 3.13 | 0.08 |
Mean Daily Step Count (per 1000 step decrease) | 1.07 | 1.003 | 1.15 | 0.04 | 1.24 | 1.08 | 1.42 | 0.003 |
Model 2 |
Acute Exacerbations | COPD-Related Hospitalizations | ||||||
Rate Ratio | 95% CI | Rate Ratio | 95% CI | |||||
FEV1, % predicted (per 10% increase in % of predicted value) | 1.04 | 0.94 | 1.15 | 0.41 | 1.15 | 0.99 | 1.34 | 0.07 |
Prednisone for AE in previous year (ref = no) | 2.12 | 1.45 | 3.1 | 0.0001 | 1.65 | 0.92 | 2.99 | 0.1 |
Mean Daily Step Count in Quartiles | ||||||||
(ref ≥6956) | ||||||||
<3667 | 2.26 | 1.25 | 4.08 | 0.007 | 6.01 | 1.99 | 18.2 | 0.002 |
3667≤×<5232 | 2.11 | 1.19 | 3.74 | 0.01 | 6.04 | 2 | 18.19 | 0.001 |
5232≤×<6956 | 1.96 | 1.09 | 3.51 | 0.02 | 5.04 | 1.65 | 15.41 | 0.004 |
0.008 | 0.003 |
FEV1 denotes forced expiratory volume in 1 second; AE acute exacerbation; and ref reference group.
N = 168.
Two separate multivariate models. Model 1 examines daily step count as a continuous variable. Model 2 examines daily step count in quartiles.
Similarly, in multivariate models, lower 6MWT distance and worse SGRQ-AS were significant predictors of AEs and COPD-related hospitalizations, independent of FEV1 % predicted and prednisone for AE in previous year (
Acute Exacerbations | COPD-Related Hospitalizations | |||||||
Rate Ratio | 95% CI | Rate Ratio | 95% CI | |||||
FEV1, % predicted (per 10% increase in % of predicted value) | 1.06 | 0.96 | 1.17 | 0.23 | 1.15 | 0.99 | 1.33 | 0.06 |
Prednisone for AE in previous year (ref = no) | 2.14 | 1.46 | 3.14 | 0.0001 | 1.71 | 0.95 | 3.07 | 0.08 |
6MWT distance (per 30-meter decrease |
1.07 | 1.01 | 1.14 | 0.03 | 1.18 | 1.07 | 1.30 | 0.001 |
FEV1 denotes forced expiratory volume in 1 second; AE acute exacerbation; ref reference group; and 6MWT denotes 6-minute walk test.
N = 168.
Rate ratios calculated for a MCID of 30 m for 6MWT. The regression coefficients (SE) in natural log risk per 30-m decrease in 6MWT distance predicting AEs and COPD-related hospitalizations are 0.0674 (0.0308) and 0.1624 (0.0502), respectively.
Acute Exacerbations | COPD-Related Hospitalizations | |||||||
Rate Ratio | 95% CI | Rate Ratio | 95% CI | |||||
FEV1, % predicted (per 10% increase in % of predicted value) | 1.07 | 0.97 | 1.17 | 0.17 | 1.17 | 1.02 | 1.36 | 0.03 |
Prednisone for AE in previous year (ref = no) | 1.99 | 1.34 | 2.95 | 0.0006 | 1.60 | 0.87 | 2.94 | 0.13 |
SGRQ-AS (per 4-point worsening |
1.05 | 1.01 | 1.09 | 0.02 | 1.10 | 1.02 | 1.17 | 0.008 |
FEV1 denotes forced expiratory volume in 1 second; AE acute exacerbation; ref reference group; and SGRQ-AS St. George's Respiratory Questionnaire Activity Score.
N = 168.
Rate ratios calculated for a MCID of 4 units for SGRQ-AS. The regression coefficients (SE) in natural log risk per 4-unit decrease in SGRQ-AS predicting AEs and COPD-related hospitalizations are 0.0484 (0.0202) and 0.0923 (0.0347), respectively.
Our results demonstrate that persons with COPD with lower daily step count have significantly higher rate ratios for AEs and COPD-related hospitalizations, independent of FEV1 % predicted and previous exacerbation history. These novel findings are further supported by the significant linear associations over the entire range of daily step counts with rate ratios for AEs and COPD-related hospitalizations. Our results strongly support the rationale to study PA promotion as part of future exacerbation-prevention interventions in COPD.
A strength of our study is the use of 3 complementary measures of functional status to assess exacerbation risk prospectively in the same cohort. We examined daily step count as a direct measure of PA in the community, 6MWT as a clinic-based test of exercise capacity, and the SGRQ-AS as a patient-reported assessment of PA. Our data demonstrate that the relationship between PA and exacerbation risk is robust since daily step count predicts AE and COPD-related hospitalizations in a similar fashion as 6MWT distance and SGRQ-AS. Our results add to the evidence that daily step count is an important clinical characteristic of persons with COPD that can complement the 6MWT and questionnaire assessment of PA
We focus on daily step count because it can be easily and directly translatable from the research to the clinical setting. Daily step count is a meaningful and relevant metric that, from the public health standpoint, can help define PA recommendations and promote PA in persons with COPD
In addition, directly measured daily step count overcomes limitations of questionnaire assessments of PA. First, it is well-known that persons overestimate self-reported physical activity. Second, the SGRQ-AS is used primarily in research settings and has no obvious meaning to most clinicians and all patients. Finally, prior studies using self-reported PA to examine risk for AEs/hospitalizations crudely characterized PA as ≥2 hours per week versus <2 hours per week
To date, history of previous AEs has emerged as the strongest predictor of future AEs and hospitalizations, and FEV1 % predicted has been consistently found to be a significant predictor of future AEs and hospitalizations
6MWT Distance MCIDs | Acute Exacerbations | COPD-Related Hospitalizations | ||
Rate Ratio | 95% CI | Rate Ratio | 95% CI | |
Per 25 meter decrease |
1.058 | 1.006–1.112 | 1.145 | 1.055–1.243 |
Per 35 meter decrease |
1.082 | 1.008–1.161 | 1.209 | 1.078–1.356 |
Per 54 meter decrease |
1.129 | 1.013–1.258 | 1.340 | 1.122–1.599 |
MCID denotes minimum clinically important difference; and 6MWT 6-minute walk test.
A host of variables including chronic hypercapnia, pulmonary hypertension, hypoxemia, current smoking, older age, lower BMI, higher MMRC dyspnea score, and season have been inconsistently associated with AEs or hospitalizations in previous studies
Compared to other studies, our higher mean % predicted FEV1 is due to the fact that we included persons with all stages of COPD, including GOLD I or mild COPD. Our study was designed to be as inclusive as possible to increase generalizability, and thus included persons with all GOLD stages of COPD severity. Our cohort has similar frequencies of moderate COPD (GOLD II) and very severe (GOLD IV) as previously published clinical trials in COPD such as Evaluation of COPD Longitudinally to Identify Predictive Surrogate Endpoints (ECLIPSE)
The main strengths of our study include our use of 3 complementary measures of functional status, our validated method of measuring daily step count, our structured approach to obtaining a prospective history of AEs and COPD-related hospitalizations, and the high percentage of events confirmed with medical records. We used an
Some limitations need to be considered. We did not capture upper extremity activities. However, total daily PA has been shown to be closely related to leg activity in persons with COPD
We did not track daily step count during follow-up, but we have previously shown that daily step count does not change significantly over time in stable COPD
We considered a history of previous AEs requiring therapy with corticosteroids as a potential confounder since AEs tend to recur in the same person. To assess AEs in the year prior to study enrollment, we asked, ‘Have you used prednisone for breathing problems in the past year?’ This approach of adjustment for previous exacerbations has been used in previously published studies
We did not collect information on the time period between participation in a pulmonary rehabilitation program and participation in this study. In case the results are biased because the 20 subjects who had ever participated in pulmonary rehabilitation had an increased daily step count, we performed a sensitivity analysis excluding the 20 subjects. We found similar results, and thus, included the 20 subjects in the final results. Finally, these results need to be confirmed in larger studies, and intervention studies are needed to assess whether increases in daily step count reduce AE and COPD-related hospitalization risk in persons with COPD.
In conclusion, these results provide evidence for the importance of daily step count as a determinant of health status and exacerbation risk in persons with COPD. Our results suggest that there is a subgroup of COPD patients with low daily step count who have significantly increased risk of AEs and COPD-related hospitalizations, regardless of their % predicted FEV1. We speculate that the “low walker” may be a novel COPD phenotype. In contrast to other proposed phenotypes defined by frequent exacerbations, radiologic differences, or persistent systemic inflammation, the PA phenotype is potentially amenable to behavioral modification