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Results
Cohort Description
In our cohort of patients with clinically manifest CHD, 1043 of 1188 subjects still alive at 1 year after rehabilitation (89%) participated in the year 1 follow-up. Among these, 1038 subjects reported on the frequency of physical activity, constituting the study sample. The majority of subjects were older than 60 years (median age 61), male, overweight, current or former smoker, and had a history of myocardial infarction and hypertension (Table 1). Around 40% of subjects were physically active 2 to 4 times per week, around 30% of subjects reported higher, and another 30% reported lower frequencies of physical activity, with around 10% reporting that they rarely or never engaged in physical activity. Median duration of strenuous activity was 5 h per week (IQR 7).
Physical Activity Level Over Time
We observed substantial changes of physical activity level over follow-up (figure 1). While the share of patients engaging in physical activity 5 to 6 times per week or daily decreased continuously over time, the share of those who were never physically active or for only 1 to 4 times per month increased. The portion of subjects who engaged in physical activity 2 to 4 times a week however remained comparably stable over time at around 40%.
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Figure 1.
Distribution of physical activity level over follow-up.
Associations of Physical Activity Level With Prognosis
Mean follow-up time (±SD) was 8.1 years (±3.1).
The crude rates of incidence of major cardiovascular events and mortality rates were consistently highest in the least physically active group ('rarely/never') and lowest in those who engaged in physical activity 2 to 4 times per week (Table 2). Increased rates were also found in daily active subjects. For the mortality outcomes, the crude relationships with physical activity levels revealed a reverse J-shaped association with decreasing levels of physical activity, that is, with increased mortality rates in both the most and the least frequently active, but with considerably higher mortality rates in the least physically active.
HRs for different levels of physical activity and outcomes of prognosis are reported in Table 3. After adjustment for potential lifestyle and health related confounders, a roughly twofold increased risk for major cardiovascular events was observed in those who at the first measurement indicated rare or no physical activity (models 2 and 3), and persisted when treating physical activity and also other covariates as time-dependent variables (models 4 and 5). In the models including time-dependent variables, a tendency towards increased risk was also observed for the group engaging in physical activity 1 to 4 times per month, and in those who engaged in daily physical activity.
For non-fatal cardiovascular events, no clear dose–response association was observed with different levels of physical activity.
The HRs for cardiovascular mortality were substantially and significantly increased in the least physically active group compared to the reference group of moderately active subjects in all models, with a roughly fourfold risk. The relationship tended to be reverse J-shaped, although a significantly increased HR in the most frequently active group was only observed in those models that included time-dependent covariates.
Comparable patterns were seen for physical activity and risk of all-cause mortality, with the HR being significantly increased and roughly four times higher in the least active group than in the reference group, and with a nearly twofold risk in the more frequently active groups. Again, the overall relationship tended to take a reverse J-shaped form.
Dose–response relationships of hours of physical activity per week with outcomes are shown in figure 2. The dose–response curves reveal reverse J-shaped associations for cardiovascular and all-cause mortality.
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Figure 2.
Relationship between overall duration of strenuous activity in hours per week and different outcomes of prognosis. Solid black line: point estimates; grey dashed lines: 95% CIs. The black circles in the point estimates curve represent the knots, which were set at the 5th, 50th and 95th percentiles according to the distributions of the continuous exposure variable. The median was set as the reference value. The models were adjusted for season, sex, age, education, employment status, study site, cotinine-validated smoking status, BMI, self-reported poor health, history of myocardial infarction, diabetes mellitus, hypertension and number of affected vessels.
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