Occupational Standing and Change in the Ankle-Brachial Index: The Jackson Heart Study

Abstract

Background:

A growing interest in reducing occupational sitting has resulted in public health efforts to encourage intermittent standing in workplaces. However, concerns have been raised that standing for prolonged periods may expose individuals to new health hazards, including lower limb atherosclerosis. These concerns have yet to be corroborated or refuted. The purpose of this study was to investigate the association between occupational standing and adverse changes in the ankle-brachial index (ABI).

Methods:

We studied 2,121 participants from the Jackson Heart Study, a single-site community-based study of African Americans residing in Jackson, MS. Occupational standing (“never/seldom”, “sometimes”, “often/always”) was self-reported at baseline (2000–2004). ABI was measured at baseline and again at follow-up (2009–2013).

Results:

Over a median follow-up of 8 years, 247 participants (11.6%) exhibited a significant decline in ABI (e.g. ABI decline >0.15). In multivariable-adjusted models, higher occupational standing was not significantly associated with ABI decline (occupational standing sometimes vs. never/seldom: OR=1.05; 95% CI: 0.67, 1.66; occupational standing often/always vs. never/seldom: OR=1.22; 95% CI: 0.77, 1.94). Similarly, higher occupational standing was not associated with low ABI at follow-up reflective of peripheral artery disease (ABI <0.90) or high ABI at follow-up reflective of incompressible vessels (ABI >1.40).

Conclusions:

In this community-based study of African Americans, we found no evidence that occupational standing is deleteriously associated with adverse changes in ABI over a median follow-up of 8.0 years. These findings do not provide evidence implicating occupational standing as a risk factor for lower limb atherosclerosis.

INTRODUCTION

Extended periods of standing while at work is common across many occupations with 66% of US adults estimated to often or always stand/walk during their workday.¹ Evidence has implicated sedentary behavior as a risk factor for cardiovascular disease-related morbidity and mortality.² Consequently, a growing interest in reducing occupational sitting has resulted in recommendations for employers to encourage intermittent standing/walking in the workplace that totals ≥4 h/day.³ However, concerns have been raised that standing for prolonged periods may expose individuals to new health hazards. Given the large proportion of US adults employed in standing-based occupations and considering the shift towards standing-based work in many office workplaces, there is an urgent need to elucidate the possible health complications of occupational standing.

Mechanistically, prolonged static standing increases arterial and venous pressure and subsequently causes lower limb blood pooling.⁴ For this reason, it is thought that prolonged static standing exposes the vasculature to a pro-atherogenic milieu that facilitates vascular dysfunction.⁵ These concerns have yet to be corroborated or refuted. Epidemiological evidence is needed to determine if standing is associated with pathogenesis of atherosclerosis in the lower limbs. To address this evidence gap, the purpose of this study was to investigate the association between occupational standing and adverse changes in the ankle-brachial index (ABI) among African Americans enrolled in the Jackson Heart Study (JHS).

METHODS

The JHS is a community‐based study of cardiovascular disease (CVD) among African Americans. Briefly, the JHS enrolled 5,306 African Americans, aged ≥21 years, from the Jackson, Mississippi metropolitan area between 2000 and 2004.⁶ The baseline examination included an in‐home interview and clinical examination. A follow‐up examination was conducted from 2009 to 2013 (Exam 3) to repeat collection of key variables. For the current analysis, JHS participants who were employed and had an ABI >0.90 in both legs, no evidence of non-compressible vessels (ABI >1.40), and no self-reported symptoms of intermittent claudication at Exam 1 were included (n=2,121).

Occupational standing was assessed using the item, “When you are at work, how often do you stand?” Response options included: “Never,” “Seldom,” “Sometimes,” “Often,” and “Always.” This item has been validated against accelerometry (rho=0.52).⁷ ABI was measured using a sphygmomanometer along with an Ultrasonic Doppler Flow Detector as described elsewhere.⁸ Systolic blood pressure (SBP) was measured in each leg with ultrasound measurement of the posterior tibial artery while the participant was supine and following 5 minutes of rest. Measurements of brachial SBP were taken before and after leg measurements. An ABI for each leg was calculated as the average of 2 ankle measurements divided by the average of 2 brachial measurements. Our primary outcome was a decline in ABI from Exam 1 to Exam 3 of >0.15 in either leg, consistent with an American Heart Association definition of significant PAD progression.⁹ Secondary outcomes included ABI <0.90 (indicative of Peripheral Artery Disease) or ≥1.40 (reflective of incompressible arteries purported to manifest from vascular calcification) in either leg at Exam 3.

Odds ratios (OR) and 95% confidence intervals (CI) for adverse changes in ABI (decline >0.15, ABI <0.90, or ABI >1.40) were estimated from logistic regression models that included occupational standing category (expressed as never/seldom [referent], sometimes, or often/always) as the primary predictor. Occupational standing Never/Seldom and Often/Always were grouped together due to insufficient sample size or number of cases. Age, sex, education, income, full-time employment, heavy alcohol drinking, current smoking, healthy diet score, body mass index, hypertension, diabetes, total cholesterol, leisure time moderate-vigorous physical activity, and occupational walking were included as covariates in progressively adjusted models. Interactions for occupational walking (never/seldom/sometimes vs. often/always) were also tested.

RESULTS

Among 2,121 participants included in the current analysis, mean (SD) age was 51.1 (10.8) years, 840 (39.6%) were male, and 334 (15.8%), 724 (34.1%), and 1063 (50.1%) reported standing at work never/seldom, sometimes, and often/always, respectively. Over a median follow-up of 8.0 years (range: 6.4–12.2 years), 247 participants exhibited a significant decline in ABI. In unadjusted and multivariable-adjusted models, higher occupational standing was not significantly associated with ABI decline (Table 1). Similarly, higher occupational standing was not associated with low ABI (<0.90) or high ABI (>1.40) at follow-up. Results did not vary by level of occupational walking (interaction p-values >0.10).

Table 1.

Odds Ratios and 95% Confidence Intervals of Adverse ABI Changes across Occupational Standing Categories.

	Occupational Standing
	Never or Seldom (n=334)	Sometimes (n=724)	Often or Always (n=1063)	P-Trend
ABI Decline ≥0.15
N cases	38	80	129
Unadjusted	1.00 (ref)	0.97 ( 0.64, 1.46)	1.08 ( 0.73, 1.58)	0.574
Model 1a	1.00 (ref)	0.97 ( 0.64, 1.47)	1.05 ( 0.71, 1.55)	0.699
Model 2b	1.00 (ref)	1.00 ( 0.66, 1.52)	1.07 ( 0.72, 1.59)	0.656
Model 3c	1.00 (ref)	1.05 ( 0.67, 1.66)	1.22 ( 0.77, 1.94)	0.341
Low ABI (<0.90)
N cases	8	15	30
Unadjusted	1.00 (ref)	0.86 ( 0.36, 2.05)	1.18 ( 0.54, 2.61)	0.475
Model 1a	1.00 (ref)	0.90 ( 0.37, 2.16)	1.16 ( 0.52, 2.57)	0.555
Model 2b	1.00 (ref)	1.01 ( 0.41, 2.48)	1.14 ( 0.50, 2.60)	0.684
Model 3c	1.00 (ref)	1.13 ( 0.43, 2.94)	1.29 ( 0.50, 3.33)	0.585
High ABI (>1.40)
N cases	51	102	125
Unadjusted	1.00 (ref)	0.91 ( 0.63, 1.31)	0.74 ( 0.52, 1.05)	0.060
Model 1a	1.00 (ref)	0.91 ( 0.63, 1.32)	0.75 ( 0.53, 1.07)	0.075
Model 2b	1.00 (ref)	0.91 ( 0.63, 1.32)	0.79 ( 0.55, 1.13)	0.165
Model 3c	1.00 (ref)	0.84 ( 0.56, 1.25)	0.69 ( 0.46, 1.06)	0.081

Data presented as odds ratio (95% confidence interval).

^aModel 1: Adjusted for age and gender.

^bModel 2: Adjusted for covariates in model 1 plus education <high school, income ≥$50,000, full-time employment, heavy alcohol drinking, current smoker, healthy diet score, body mass index, hypertension, diabetes, total cholesterol, and leisure time moderate-vigorous physical activity.

^cModel 3: Adjusted for covariates in model 2 plus occupational walking.

DISCUSSION

In this community-based study of African Americans, we found no evidence that occupational standing is deleteriously associated with adverse changes in ABI over a median follow-up of 8.0 years. Relatively little epidemiologic research has examined the health impacts of prolonged standing. To our knowledge, this is the first study to evaluate the association between occupational standing and ABI progression. In a previous study, Krause et al. reported that occupational standing was associated with progression of carotid atherosclerosis, indicated by carotid intima media thickness, in 584 men followed-up after 4 years.⁵ These findings gave rise to the ‘hemodynamic hypothesis’ that prolonged standing is a CVD risk factor in that the hemodynamic changes incurred from prolonged standing (e.g. blood pooling) elicits a physiological cascade that induces atherosclerosis.⁵ As such, it would be expected that atherosclerosis would be particularly manifested in the lower limbs at the site of the hemodynamic disturbances. Prolonged standing has been associated with hospitalization for varicose veins in previous studies,¹⁰ which has been viewed as indirect evidence for the ‘hemodynamic hypothesis’.⁵ However, in the present study, wherein we utilize a more direct measure of lower limb atherosclerosis, we were unable to corroborate this hypothesis.

Limitations of the present findings should be noted. First, occupational standing was assessed by self-report and is thus subject to reporting bias and classification error. A systematic bias towards under or over estimation of occupational standing with the self-report item used in the JHS has not been observed.⁷ Thus, non-differential misclassification of occupational standing would bias toward the null. Second, there may be limited generalizability as the study was conducted in a single metropolitan area and was restricted to African Americans. Finally, standing with and without ambulation was not distinguished, which may confound results as muscle contraction during ambulation offsets lower limb blood pooling elicited by static standing.^{11, 12} Nonetheless, tests for effect modification showed that results did not vary by level of occupational walking. These limitations notwithstanding, our findings suggest occupational standing is not associated with worsening of ABI in African Americans. As public health guidelines and workplace policies/designs are adapted to reduce workplace sitting and promote intermittent standing/walking, the health hazards of standing warrants consideration. Our results may be helpful to inform such considerations and do not provide evidence implicating occupational standing as a risk factor for lower limb atherosclerosis.

What is already known about this subject?

• Prolonged static standing increases arterial and venous pressure and subsequently causes lower limb blood pooling; raising concerns that standing for long periods at work exposes the vasculature to a pro-atherogenic milieu that facilitates vascular dysfunction.

• Few studies have evaluated whether occupational standing is associated with the pathogenesis of atherosclerosis in the lower limbs.

What are the findings?

• In this prospective observational study, we found no evidence that occupational standing is deleteriously associated with adverse changes in the ankle brachial index over a median follow-up of 8.0 years.

How might it impact on clinical practice in the future?

• As public health guidelines and workplace policies/designs are adapted to reduce/interrupt workplace sitting, these results may be helpful to inform such considerations and do not provide evidence implicating occupational standing as a risk factor for lower limb atherosclerosis.