There were 268 (0.4%) cases of incident idiopathic pulmonary embolism among the 69 950 women in our primary analysis of physical inactivity. Incident pulmonary embolism was diagnosed in the following years of the questionnaire cycle: 1990-2 (n=15, 6%), 1992-4 (n=19, 7%), 1994-6 (n=30, 11%), 1996-8 (n=37, 14%), 1998-2000 (n=29, 11%), 2000-2 (n=41, 15%), 2002-4 (n=43, 16%), 2004-6 (n=52, 19%), and 2006-8 (n=2, 1%). There were 267 (0.4%) cases of incident idiopathic pulmonary embolism among the 69 647 women in our secondary analysis of physical activity. Table 1 shows baseline characteristics of women in the primary analysis, adjusted for age and stratified according to inactivity time. In 1988, 20 475 (29%) women sat <10 hours a week, 34 331 (49%) sat between 11 and 40 hours a week, and 15 144 (22%) sat 41 or more hours a week. In 1990, when the question focused on sitting at home, 29 875 (43%) of women sat <10 hours a week, 36 428 (52%) sat between 11 and 40 hours a week, and 3647 (5%) sat 41 or more hours a week. The distribution of time sitting in 1988 was more similar to time sitting at home in 1990 than time sitting at work, with most nurses (43 530, 64%) reporting <10 hours a week sitting at work in 1990. Most women were white and had given birth to at least two children. There were no major differences in the baseline characteristics across categories of inactivity, though women with more physical inactivity tended to have a higher energy intake, higher BMI, and a lower level of physical activity.
Age standardised baseline characteristics (1990) according to physical inactivity, Nurses’ Health Study (n=69 950). Figures are percentages of participants unless stated otherwise
After adjustments for age, coronary heart disease, hypertension, menopausal status, multivitamin use, non-aspirin non-steroidal anti-inflammatory drug use, parity, race, rheumatological disease, spouse’s highest educational attainment, smoking status and pack years of smoking, and warfarin use, physical inactivity was associated with an increase in the risk of idiopathic pulmonary embolism (hazard ratio for a 1 unit increased score 1.29, 95% confidence interval 1.15 to 1.45; P<0.001) (table 2). For women with the most physical inactivity, the risk of pulmonary embolism was more than twice that of women with the least physical inactivity (multivariable hazard ratio 2.68, 1.50 to 4.79). Further adjustments for BMI, total energy intake, physical activity, and dietary pattern led to similar results. Subanalysis excluding women taking warfarin also yielded similar results (data not shown).
Association between physical inactivity and incident idiopathic pulmonary embolism, Nurses’ Health Study (n=69 950), with hazard ratios (HR) for trend
We did not find a significant association between increased overall physical activity (metabolic equivalents a week) and incident idiopathic pulmonary embolism. Across fifths of physical activity, the multivariable hazard ratios were 1.00 (reference), 0.91 (0.62 to 1.33), 1.17 (0.82 to 1.67), 0.85 (0.58 to 1.26), and 0.89 (0.60 to 1.32) (P=0.53 for trend). Further adjustment for physical inactivity led to similar result (P=0.42 for trend).
We performed several subanalyses of the association between physical inactivity and pulmonary embolism. To assess the effect of brief periods of physical activity, we stratified our analysis of physical inactivity according to overall level of physical activity (table 3). Women below the cohort median of 8.4 metabolic equivalents of physical activity a week had a significantly increased risk of idiopathic pulmonary embolism with time sitting (multivariable hazard ratio 1.30, 1.00 to 1.69; P=0.05 for trend), whereas this association was not significant among women at or above the cohort median of physical activity (1.25, 0.89 to 1.76; P=0.20 for trend). The formal test for interaction was not significant (P=0.59 for interaction).
Association between physical inactivity and incident pulmonary embolism according to level of physical activity, Nurses’ Health Study (n=69 950), with hazard ratios (HR) and 95% confidence intervals
We stratified our analysis according to age (<65 and ≥65). We found a similarly strong association between physical inactivity and risk of pulmonary embolism in both groups (multivariable hazard ratio 1.54 (1.11 to 2.12; P=0.009 for trend) and 1.32 (0.99 to 1.75; P=0.06 for trend), respectively; P=0.65 for interaction).
Lastly, to assess whether the association between physical inactivity and pulmonary embolism varied with obesity, we stratified our analysis according to BMI. We reported a significant positive association between physical inactivity and pulmonary embolism for women with BMI <30: the risk of pulmonary embolism by time sitting was 1.39 (0.86 to 2.25) and 2.33 (1.37 to 3.99) for medium and high levels of inactivity (multivariable hazard ratio 1.42, 1.23 to 1.66; P<0.001 for trend). For women with BMI ≥30.0, the association was not significant; the risk for pulmonary embolism was 0.69 (0.41 to 1.17) and 0.97 (0.53 to 1.77), for similar categories (multivariable hazard ratio 1.01, 0.84 to 1.23; P=0.90 for trend). The formal test for interaction was not significant (P=0.22 for interaction).