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Long Working Hours and the Prevalence of Masked and Sustained Hypertension

Originally publishedhttps://doi.org/10.1161/HYPERTENSIONAHA.119.12926Hypertension. 2020;75:532–538

Abstract

Previous studies on the effect of long working hours on blood pressure have shown inconsistent results. Mixed findings could be attributable to limitations related to blood pressure measurement and the lack of consideration of masked hypertension. The objective was to determine whether individuals who work long hours have a higher prevalence of masked and sustained hypertension. Data were collected at 3-time points over 5 years from 3547 white-collar workers. Long working hours were self-reported, and blood pressure was measured using Spacelabs 90207. Workplace clinic blood pressure was defined as the mean of the first 3readings taken at rest at the workplace. Ambulatory blood pressure was defined as the mean of the next readings recorded every 15 minutes during daytime working hours. Masked hypertension was defined as clinic blood pressure < 140/90 mm Hg and ambulatory blood pressure ≥135/85 mm Hg. Sustained hypertension was defined as clinic blood pressure ≥140/90 mm Hg and ambulatory blood pressure ≥135/85 mm Hg or being treated hypertension. Long working hours were associated with the prevalence of masked hypertension (prevalence ratio 49+=1.70 [95% CI, 1.09–2.64]), after adjustment for sociodemographics, lifestyle-related risk factors, diabetes mellitus, family history of cardiovascular disease, and job strain. The association with sustained hypertension was of a comparable magnitude (prevalence ratio 49+=1.66 [95% CI, 1.15–2.50]). Results suggest that long working hours are an independent risk factor for masked and sustained hypertension. Workplace strategies targeting long working hours could be effective in reducing the clinical and public health burden of hypertension.

Introduction

Long working hours are frequent. According to the International Labour Office, approximatively one out of 5 workers around the world were working more than 48 hours per week at the beginning of the 2000s, representing over 614 million workers.1 According to a United States population-based survey and the sixth European Working Conditions Survey, the prevalence of working 48+ weekly hours remains within a similarly elevated range: 19% of Americans and 15% of Europeans regularly worked 48+ weekly hours in 2010 and 2015, respectively.2,3 Studies documenting the effect of long working hours on blood pressure (BP) have reported mixed results.4–12 These inconsistencies could be explained by the use of different instruments to measure BP; some studies relied on clinic BP, measured by a sphygmomanometer or an automated device, and others relied on self-reported hypertension. Few studies have used ambulatory BP (ABP), defined as BP that is assessed repeatedly over the day.10,11 The superiority of ABP is now well-acknowledged, given its higher precision and validity.13–15

One important novelty introduced by ABP measurements is the capacity to screen for masked and sustained hypertension.16 Masked hypertension is defined by high ABP and normal clinic BP while sustained hypertension is defined by high BP according to both types of BP measurements. Both hypertension phenotypes are associated with an increased cardiovascular risk.17,18 There is scarce evidence to date on the relation between long working hours and masked/sustained hypertension.10 The objective of the present study was to examine the association between long working hours and the prevalence of masked and sustained hypertension in a large sample (N=3547) of white-collar men and women in Quebec City, Canada.

Methods

Data and study materials are available from the corresponding author on reasonable request.

Study Design and Population

The study population has been described previously.19 The study relied on an open cohort with 3 waves of data collection over 5 years (Year 1, 3, and 5). The study population was composed of all white-collar workers from 3 public institutions. Their main professional activities involved planning and providing insurance services to the general population of Quebec. Their jobs covered the full range of white-collar positions, including senior and middle managers, professionals, technicians, and office workers. At each wave and at their workplace, workers completed a self-administered questionnaire on the work environment and BP risk factors. All employees were invited to participate. The participation rate was 80.1% at baseline and 85% at years 3 and 5. Across all 3 measurement times, we excluded 862 observations due to insufficient or unavailable BP measurements, and 80 observations reported by pregnant women. We also excluded 33 observations because participants were working less than 21 hours and 457 observations due to missing data on exposure and covariates. The final study sample consisted of 3547 workers and 6733 observations. The unit of analysis for the present study was the number of observations.

Long Working Hours

Self-reported weekly worked hours were used as the exposure. This information was collected in the self-administered questionnaire. Work hours were grouped into the following categories: 21 to 34 hours, 35 to 40 hours, 41 to 48 hours, and 49 or more hours per week.20

Blood Pressure

BP was measured using the Spacelabs 90207 device (Spacelabs Produits Médicaux Ltée, St-Laurent, Quebec, Canada).21,22 In a research office at the participant’s workplace, the device was installed on the participant’s nondominant arm if the BP difference measured on both arms was inferior to 10 mm Hg. Otherwise, it was installed on the arm with the higher BP values. After the participant had been sitting for 5 minutes, BP was measured 3× in the presence of a trained research assistant using the office check mode of the Spacelabs monitor. Afterwards, BP was measured every 15 minutes by the same monitor for the rest of the working day. All devices were fitted to participants at the same time of day (8:00), and monitoring was conducted from 8:00 to 16:00 in all cases. Participants had at least 20 ABP measurements, in accordance with criteria recently recommended by expert committees.23

Participants were white-collar workers whose tasks were mainly performed from a seated position. To minimize motion artifacts, clear instructions were given to participants in the case that the monitor performed a reading while they were in a standing position (ie, participants were instructed to stop walking and rest their arm at their side).

Workplace clinic BP (CBP) was defined as the mean of the first 3 readings taken at rest in the presence of trained personnel. ABP was defined as the mean of the next readings taken every 15 minutes and recorded by the same monitor (daytime ABP).

Participants were classified into 4 categories according to the European Society of Hypertension guidelines24: (1) normotension, defined as systolic/diastolic CBP <140/90 mm Hg and systolic/diastolic ABP <135/85 mm Hg; (2) white-coat hypertension, defined as CBP ≥140/90 mm Hg and ABP <135/85 mm Hg; (3) masked hypertension, defined as CBP <140/90 mm Hg and ABP ≥135/85 mm Hg; and (4) sustained hypertension, defined as CBP ≥140/90 mm Hg and ABP ≥135/85 mm Hg. By definition, participants treated for hypertension were also categorized as sustained hypertensives. When workers’ ABP values met the diagnostic criteria for hypertension, they were informed of their hypertensive status, and a letter was sent inviting them to consult their physician.

Covariates

Sociodemographics included sex, age, education (less than college, college completed), and occupation (clerical/other, technician, professional/executive). Lifestyle-related risk factors included current cigarette smoking status, body mass index, sedentary behaviors, and alcohol intake. Smoking status was defined as the daily consumption of at least one cigarette per day. Body weight and height were measured by a trained nurse to calculate body mass index (kg/m2). Alcohol intake was measured using the following 3 categories of weekly intake frequency during the past 12 months: less than one drink per week, one to 5 drinks per week, and 6 or more drinks per week. Sedentary behaviors were defined according to weekly leisure physical activity frequency. The items used to measure the level of physical activity varied between measurement time. Therefore, the variable was harmonized using shared categories at each time, resulting in a binary variable (<1/wk/≥1/wk). Self-reported diabetes mellitus and family history of cardiovascular disease were also considered. The latter was defined as a cardiovascular event (such as angina, myocardial infarction, coronary revascularization, or stroke) suffered by the participant’s father, mother, brother, or sister before the age of 60 years. The risk factors listed above were evaluated using validated protocols.25,26 In addition, psychosocial stressors at work, as defined by Karasek’s job strain model, were also used as covariates. Eighteen items from the Job Content Questionnaire27 were used to assess these stressors: 9 items to assess psychological demands (which reflect quantity of work, time constraints, and level of intellectual effort required) and 9 items to assess decision latitude (which reflects opportunities for learning, autonomy and participation in the decision-making process). The psychometric properties of the original 18 itemed Job Content Questionnaire28,29 and its French version29–32 have previously been demonstrated. Workers with a psychological demands score of 24 or higher (the median for the general working population of Quebec) were classified as having high psychological demands.26 Workers with a decision latitude score of 72 or lower (median for the general working population of Quebec) were classified as having low decision latitude.26 Job strain exposure was defined as high psychological demands combined with low decision latitude.

Analyses

Student t test and χ2 analyses were used to compare participant’s characteristics across working hours categories. Mean BP values for each hypertension subtype were computed using an ANOVA. Generalized estimating equations were used to examine the association between long working hours, masked hypertension, and sustained hypertension.33 Generalized estimating equations take into account within-subject correlation due to repeated measurements as well as the evolution of covariates over time. All participants at each measurement time were pooled together in the analyses. The prevalences of masked and sustained hypertension were estimated according to contemporaneous exposure to long working hours, that is, exposure assessed at the same measurement time. The reference hypertension category was normotension. The prevalence of white-coat hypertension was very low, resulting in unreliable point estimates. Therefore, estimates for white-coat hypertension are not presented. Prevalence ratios (PR) and their 95% CI were computed using robust Poisson regressions. Models were sequentially adjusted for sociodemographic factors, lifestyle-related risk factors, diabetes mellitus, family history of cardiovascular disease, and job strain. The potential modifying effect of sex was examined using a multiplicative interaction term and was not statistically significant. Stratified analyses for men and women are available in the online-only Data Supplement. Participants’ characteristics and cross-sectional analyses at each time are also presented in the online-only Data Supplement. This study was approved by the ethical review board of the CHU de Québec-Université Laval research center. All participants signed a consent form describing the study and were free to withdraw at any time.

Results

The overall prevalence of masked and sustained hypertension was 13.5% and 18.7%, respectively. Table 1 presents study participants’ characteristics according to work hours categories. Women were overrepresented in the lowest category, that is, among those working <35 h/wk (77.3%). However, there was no difference in the proportion of men and women in the highest category of working hours, that is, among those working 49 hours or more per week. Mean age was higher among those with long working hours (49+ h/wk). Participants working long hours had a higher formal education, were employed in greater proportions as professional/executives and consumed more alcohol. Job strain exposure was less frequent in those working 49 hours or more per week. Table 2 presents mean BP according to hypertension subgroups. Participants with white-coat, masked hypertension, and sustained hypertension consistently had higher CBP and ABP levels when compared with normotensive participants.

Table 1. Description of the Study Population According to Work Hours

CharacteristicsWork Hours, h/wkP Value
<3535–4041–48≥49
Sex
 Men154 (22.7)2424 (41.7)98 (54.1)32 (50.8)<0.01
 Women524 (77.3)3387 (58.3)83 (45.9)31 (49.2)
 Age (Mean, SD)46.7 (8.3)44.2 (8.7)46.4 (7.1)48.2 (6.0)<0.01
Education
 Less than college149 (22.0)1326 (22.8)15 (8.3)6 (9.5)<0.01
 College206 (30.4)1936 (33.3)37 (20.4)7 (11.1)
 University323 (47.6)2549 (43.9)129 (71.3)50 (79.4)
Occupation
 Clerical/other159 (23.5)1545 (26.6)8 (4.4)3 (4.8)<0.01
 Technician150 (22.1)1690 (29.1)21 (11.6)7 (11.1)
 Professional/executive369 (54.4)2576 (44.3)152 (84.0)53 (84.1)
Smoking status
 No631 (93.1)5130 (88.3)169 (93.4)57 (90.5)<0.01
 Yes47 (6.9)681 (11.7)12 (6.6)6 (9.5)
Body mass index (Mean, SD)25.9 (4.3)26.3 (5.9)26.6 (4.5)27.2 (4.7)0.20
Sedentary behaviors
 No548 (80.8)4654 (80.1)140 (77.4)44 (69.8)0.16
 Yes130 (19.2)1157 (19.9)41 (22.6)19 (30.2)
Alcohol intake
 <1/wk222 (32.7)1948 (33.5)38 (21.0)17 (26.9)<0.01
 1–5/wk279 (41.2)2400 (41.3)82 (45.3)19 (30.2)
 ≥6/wk177 (26.1)1463 (25.2)61 (33.7)27 (42.9)
Diabetes mellitus
 No652 (96.2)5621 (96.7)172 (95.0)63 (100.0)0.23
 Yes26 (3.8)190 (3.3)9 (5.0)0 (0.0)
Family history of CVD
 No449 (66.2)3783 (65.1)119 (65.8)40 (63.5)0.93
 Yes229 (33.8)2028 (34.9)62 (34.2)23 (36.5)
Job strain
 No544 (80.2)4646 (80.0)151 (83.4)57 (90.5)0.13
 Yes134 (19.8)1165 (20.0)30 (16.6)6 (9.5)

Table 2. Mean Blood Pressure According to Hypertension Subgroups

NormotensionWhite-Coat HypertensionMasked HypertensionSustained Hypertension
N (%)N (%)N (%)N (%)
4451 (66.1)113 (1.7)907 (13.5)1262 (18.7)
Blood pressure, mean (SD)
Workplace clinic blood pressure
 Systolic118.4 (9.0)142.1 (5.8)128.8 (6.7)137.0 (13.3)
 Diastolic73.1 (6.7)76.6 (5.3)84.9 (4.9)85.9 (9.8)
 Pulse pressure45.2 (6.8)55.5 (8.8)46.8 (6.7)51.1 (9.5)
Ambulatory blood pressure
 Systolic119.6 (7.4)128.6 (4.5)133.8 (6.1)135.2 (12.2)
 Diastolic76.0 (5.3)80.1 (3.4)87.5 (3.9)86.4 (9.1)
 Pulse pressure43.6 (5.6)48.5 (4.8)46.4 (6.7)48.8 (8.3)
% hypertensive treatment0 (0)0 (0)0 (0)690 (54.7)

All differences with the normotensive group are statistically significant at P<0.0001.

Table 3 presents the PRs of masked hypertension according to total weekly working hours. After adjustment for age, sex, education and occupation, participants working 41 to 48 h/wk (PR=1.54 [95% CI, 1.09–2.19]) and those working 49+ h/wk (PR=1.76 [95% CI, 1.12–2.77]) had a higher prevalence of masked hypertension. The association between long working hours and masked hypertension was slightly attenuated but remained statistically significant in the fully adjusted model (PR41–48=1.51 [95% CI, 1.06–2.14]; PR49+=1.70 [95% CI, 1.09–2.64]). Table 4 presents the PRs of sustained hypertension. In the fully adjusted model, the prevalence of sustained hypertension was higher among those in the highest categories of working hours (PR41–48=1.33 [95% CI, 0.99–1.76]; PR49+=1.66 [95% CI, 1.15–2.50]). This association was slightly stronger among participants not being treated for hypertension (in the online-only Data Supplement). The association between long working hours and the prevalence of masked hypertension was mainly observed at year 3 and 5. Estimates for sustained hypertension were similar to those reported in the main analysis. These results should, however, be interpreted with caution given the reduced power and wider CIs (in the online-only Data Supplement).

Table 3. Prevalence Ratio for Long Working Hours and Masked Hypertension

Work Hours, h/wkPrevalence (%)Model 1Model 2Model 3
PR95% CIPR95% CIPR95% CI
<3514.71.001.001.00
35–4016.71.140.91–1.431.080.86–1.351.050.84–1.33
41–4828.21.931.36–2.731.541.09–2.191.511.06–2.14
≥4936.42.481.56–3.961.761.12–2.771.701.09–2.64

PR indicates prevalence ratio. Reference category: normotension; Model 1: crude; Model 2: adjusted for age, sex, education and occupation; and Model 3: Model 2+body mass index, alcohol intake, smoking, sedentary behaviors, diabetes mellitus, family history of CVD and job strain.

Table 4. Prevalence Ratio for Long Working Hours and Sustained Hypertension

Work Hours, h/wkPrevalence, (%)Model 1Model 2Model 3
PR95% CIPR95% CIPR95% CI
<3521.01.001.001.00
35–4021.71.040.86–1.261.070.89–1.271.040.87–1.23
41–4832.91.571.14–2.151.421.07–1.881.330.99–1.76
≥4940.41.941.28–2.931.761.17–2.651.661.15–2.50

PR indicates prevalence ratio. Reference category: normotension; Model 1: crude; Model 2: adjusted for age, sex, education and occupation; and Model 3: Model 2+body mass index, alcohol intake, smoking, sedentary behaviors, diabetes mellitus, family history of CVD and job strain.

Discussion

In the present study, long working hours was associated with the prevalence of masked and sustained hypertension, after controlling for socio-demographics, lifestyle-related risk factors, diabetes mellitus, family history of cardiovascular disease, and job strain. The effect of long working hours on sustained hypertension was of a comparable magnitude.

To our knowledge, only one previous study has examined the association between long-working hours and masked hypertension.10 That study was conducted among 110 hospital and home care employees in the United States with CBP of <140/90 mm Hg and that were not being treated for hypertension. In that study, weekly worked hours were not associated with masked hypertension. The small sample size could partly explain this null finding. In the present study, a positive association between long-working hours and the prevalence of masked hypertension was observed in a sizable sample of workers and while controlling for many potential confounders.

Previous studies on the association between long-working hours and high BP have provided mixed results. The measurement methods used to assess BP could likely explain these discrepancies. Indeed, the majority of previous studies relied on office BP measurements or used self-reported hypertension. Some of these previous studies reported no effect4,7 or even a protective effect of long working hours on the incidence of hypertension.5,6,8 On the contrary, some previous studies conducted in large samples or using ABP are consistent with ours and reported a deleterious effect of long working hours on BP. One such study was conducted in the United States among 24 205 workers and showed a positive association between work hours and the prevalence of self-reported hypertension.9 Another study conducted in Japan used ABP and reported higher 24-hour BP and heart rate among workers exposed to long working hours.11 This previous study was conducted among a small sample (N=46) composed of only men. Our study, conducted in a sizable sample of men and women, therefore adds to the current knowledge and provides evidence in support of the deleterious effect of long working hours on both masked and sustained hypertension.

Different mechanisms have been put forward to explain the relationship between long working hours and cardiovascular health. First, long working hours could be associated with sleep deprivation, which has been shown to increase cardiovascular risk.34,35 It has also been hypothesized that overtime work could imply prolonged exposure to and less time to recover from psychosocial stressors from the work environment.36 However, our results do not support this hypothesis since the association between long working hours and hypertension was robust to statistical adjustment for job strain exposure. Finally, lifestyle-related risk factors such as smoking and alcohol consumption were also hypothesized to play a role.9 Again, this potential explanation was not supported by the current study since the effect of these lifestyle risk factors was accounted for in the analyses. Therefore, results from the present study do not support the mediating role of either psychosocial stressors at work or lifestyle-related risk factors in the association between long working hours and masked/sustained hypertension. Future studies are needed to clarify the causal pathways linking long working hours and hypertension.

The methodology used to measure BP in the current study has been discussed elsewhere.37 Briefly, CBP and ABP were assessed using the same semi-automatic device at the workplace. In contrast, previous studies on masked hypertension used different devices (auscultatory versus oscillometric) and modalities (ambulatory versus home) to obtain office and out of office BP measurements, which could influence the prevalence of hypertension subtypes. In the current study, the prevalence of both masked hypertension (13%) and sustained hypertension (18.7%) remained in the range of previous studies.18,38 However, the low prevalence of white-coat hypertension could be attributable to factors related to measurement procedures, including the use of a trained technician instead of a physician and the fact that CBP measurements were collected at work rather than in an hospital setting.39,40 It could also be attributable to the middle-age composition of study participants since white-coat hypertension is more often detected in older patients.41

Potential limitations of the present study should be mentioned. First, the measurement of work hours involved total weekly hours in the participant’s main occupation, possibly leading to nondifferential misclassification and bias toward the null value. However, since workers where all occupying white-collar jobs, the potential for an important amount of unmeasured working hours is not likely. In addition, masked hypertension was assessed using daytime BP. Therefore, nocturnal hypertension could not be examined. While previous studies have suggested a stronger cardiovascular disease risk prediction for night-time BP when compared with daytime BP,42,43 others have shown a similar association of 24-hour, daytime and nighttime systolic BP on mortality.18 Nevertheless, in addition to elevated daytime BP, higher nocturnal BP could also contribute to explain the harmful effect of long working hours on cardiovascular health. In the present study, the use of daytime ABP favors comparability with most previous studies and is in accordance with current clinical guidelines for the definition of masked hypertension (2017 American College of Cardiology/American Heart Association Task Force algorithm on the detection of white-coat hypertension or masked hypertension in patients not on drug therapy).44 Daytime measurements were also preferred for logistical, practical, and financial reasons, since the present study was conducted among a large number of participants. Finally, since the study population is composed exclusively of white-collar workers, generalization of our findings should be restricted to workers sharing the same type of occupations. For example, blue-collar workers could be more frequently exposed to shift and night work where the association could differ.45 Occupations with higher physical demands (repetitive movements, frequent lifting, long walking distance) might also yield different results. However, the selection of our study population minimized confounding by these unmeasured occupational factors.

This study also has important strengths. First, both CBP and ABP were assessed in a sizable cohort composed of both men and women. Second, a number of potential confounders were assessed and were adjusted for in the analyses, including psychosocial stressors at work from the job strain model (the most frequently used model to define these stressors). Finally, the use of a repeated measurement strategy increased the study’s power and allowed us to examine the differential effect of long working hours on masked and sustained hypertension.

Perspectives

Masked and sustained hypertension are associated with increased cardiovascular mortality.18 Moreover, masked hypertension could persist for several years, delaying diagnosis and appropriate management.46 There is, therefore, a need to identify novel and modifiable risk factors to reduce the clinical and public health burden of masked and sustained hypertension. Findings from the present study suggest that long working hours are associated with both hypertension subtypes. This is the first study to examine these effects in a sizable sample of working men and women. From a clinical perspective, assessing long working hours might be useful for the early identification of at-risk workers who could benefit from ABP monitoring. Increased clinical awareness on the adverse effect of long working hours could, therefore, contribute to improve hypertension prevention and management at both individual and population levels. From a public health perspective, health policies and workplace initiatives should consider reducing the prevalence of long working hours for the primary prevention of both types of hypertension.

Footnotes

This work was performed at the CHU de Québec-Université Laval Research Center, Quebec City, Canada.

The online-only Data Supplement is available with this article at https://www.ahajournals.org/doi/suppl/10.1161/HYPERTENSIONAHA.119.12926.

Correspondence to Xavier Trudel, PhD, Axe Santé des populations et pratiques optimales en santé, Centre de recherche du CHU de Québec 1050, Chemin Ste-Foy Québec, Qué, Canada G1S 4L8. Email

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Novelty and Significance

What Is New?

  • Long-working hours is associated with a higher prevalence of masked and sustained hypertension in this cohort of 3547 white-collar workers.

  • These associatons were robust to adjustment for sociodemographic factors, body mass index, alcohol intake, smoking, sedentary behaviors, diabetes mellitus, family history of cardiovascular disease, and job strain.

What Is Relevant

  • Previous studies on the effect of long working hours on blood pressure observed mixed results, which is likely attributable to the use of different measurement methods including the fact that very few studies used ambulatory blood pressure measurements.

  • Our findings provide evidence from a large sample of women and men using ambulatory measurement of blood pressure in support of the adverse effect of long working hours on the prevalence of masked and sustained hypertension.

  • Preventive workplace strategies should consider the reduction of working hours for the primary prevention of masked and sustained hypertension.

Summary

In this open cohort of 3547 white-collar workers, those working longer hours had a higher prevalence of masked and sustained hypertension.

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