Heavy Lifting at Work and Risk of Ischemic Heart Disease: Protocol for a Register-Based Prospective Cohort Study

Abstract

BackgroundThere are theoretical grounds to suspect that heavy lifting at work is an important risk factor for ischemic heart disease (IHD). However the relationship has not been sufficiently acknowledged by empirical studies. Positive and statistically significant associations have been found in studies that utilize self-reported exposure data. Such studies are, however, prone to reporting bias. All else equal, people with a poor cardiovascular fitness/health may have a higher propensity to perceive their work environment as heavy.ObjectiveThe study described in the present protocol aims to investigate the relationship between heavy lifting at work and IHD by use of material and methods that are free from reporting bias.MethodsThis is a register-based prospective cohort study. Male blue-collar workers in Denmark will be identified and followed through national registers, from 2001-2010, for hospital treatment or death due to IHD. Relative rates of IHD between “workers in occupations likely to involve heavy lifting” and “other blue-collar workers” will be estimated through Poisson regression.ResultsResults are expected to be ready in mid-2015.ConclusionsSince this is not a randomized study, it cannot confirm etiological hypotheses. It may, however, confirm that employment in occupations that involve heavy lifting is a predictor for IHD and thereby lend support to the hypothesis of a causal relationship.

Ischaemic heart disease among workers in occupations associated with heavy lifting

“…Therefore, we cannot rule out the possibility that a significant effect from heavy lifting has been missed (offset) by the factors which we could not control for. We know, however, from collateral data [16] that the nullfinding is unlikely to be explained by differences in BMI and smoking habits. We can also rule out confounding from socio-economic status, since in the study we only included blue-collar workers.…”

“…By including the total target population in the analyses, we eliminated volunteer bias, and by using job codes as a proxy for occupational exposure, we eliminated reporting bias. We also eliminated hindsight bias through the publication of our study protocol [16], in which all hypotheses and statistical models were specified carefully before we looked at any relation between the exposure and response variables in our data material. The large number of participants gave us an extraordinary high statistical power, and the prospective design ascertained that the exposure took place before the outcome.…”

“…The present study concerns workers from the groups 6-9 which we, for the sake of brevity, will call blue-collar workers in thereof text. An expert opinion and a subsequent validation [16] allowed us to select the following DIS-CO-88 categories to represent blue-collar occupations in which heavy lifting is likely to occur: -712 -building frame and related trades workers, -921 -agricultural, forestry and fishery labourers, -931 -construction labourers, -933 -transport and storage labourers. for single arm curl exercises [11].…”

“…We hypothesized that the rate of hospital treatments or deaths due to IHD and the rate of death due to IHD were higher in the occupations where frequent heavy lifting is likely to occur in comparison with other blue-collar occupations. In keeping with good epidemiological practice, the hypotheses and statistical methods were completely defined, peer-reviewed and published [16] before the statistical analyses were commenced.…”

The Effect of Occupational Lifting on Hypertension Risk: Protocol for a Project Using Data From the Copenhagen City Heart Study (Preprint)

“…Also the collection of BP only in consultation during rest is a limitation due to the lower prognostic value than obtained by monitoring of 24 hours BP or BP during sleep 33,34 . Furthermore, a previous study has shown occupational lifting to reduce the odds for having prolonged working hours 35 , however this is not possible to adjust for in the present analysis due to the lacking information of amount of weekly working hours.…”