Influence of Shift Work on Coronary High Risk Status among Natural Gas Field Workers

Background: The association between shift work and coronary high risk status (CHRS) is still unclear. The present work was designed to study the influence of shift work in a gas field petroleum company in Dakahlia on CHRS; as well as, to investigate its possible mechanisms. Methods: A cross-sectional approach was used and a systematic random technique was applied to select 200 shift workers and 200 non-shift workers. Both study groups were subjected to interview questionnaire about personal, social, smoking, occupational and medical histories. Clinical examination, including general and cardiovascular examinations, was done. Height, weight and waist circumference were measured and body mass index (BMI Kg/m) was calculated. Laboratory investigations included measurements of fasting blood glucose, lipid profile and uric acid; as well as, electrocardiographic examination (ECG). Coexistence of more than 2 conventional risk factors or diabetes mellitus (DM) or new and old coronary artery disease (CAD) defined CHRS. Vanillyl mandelic acid (VMA) was measured in 24 hours urine samples as a stress indicator for a randomly selected sub-sample from those with CHRS (25 shift and 25 non-shift workers). The R-R deep breathing variability test on the resting ECG as a measure for autonomic function was done for the same group. Results: The study revealed that 42.5% of shift workers were complaining of deterioration in performance and concentration during work. In addition, 35.5% of shift workers had the desire to change shift work. Among shift workers, 36% were smokers in comparison to 26.5% of non-shift workers with a statistically significant difference (p = 0.04). Rates of hypertension, hypercholesterolemia, high LDLcholesterol, low HDL-cholesterol, hypertriglyceridemia, obesity and DM showed a statistically significant increase among shift workers than non-shift workers (p < 0.05). The overall rate of CHRS was significantly higher among shift workers (p = 0.001). Multiple logistic regression analysis of coronary high risk status as the dependant variable revealed that shift work kept a significant effect on CHRS even after adjustment for other confounders (p= 0.002). Mean level of urinary VMA was significantly higher for shift workers than non-shift workers (p= 0.0001); meanwhile, significantly lower R-R variability indices were recorded among shift workers than non-shift workers (p < 0.05). Conclusion and Recommendations: The present study revealed that shift work has a significant impact on CHRS that might be related to sympathetic over-activity. It is recommended to screen for coronary risk factors in pre-placement and periodic medical examinations of shift workers with implementation of special preventive programs. Urinary VMA and R-R variability testing in ECG could be used as stress indicators among CHRS shift workers. Cases of CAD should be excluded from shift work.


INTRODUCTION
Shift work is an employment practice designed to make use of the 24 hours of the clock, rather than a standard working day.The term shift work includes both long-term night shifts and work schedules in which employees change or rotate shifts to optimize technical and economic values. (1)Many types of shift work are described worldwide. (2)In Egypt, shift work in petroleum industry (oil and gas) generally goes monthly like that; a 12-hour night shift for one week followed by a vacation week, then a 12-hour day time shift for another week followed again by a vacation week.
An association between shift work and coronary artery disease (CAD) has been postulated since many years. (3)In a prospective cohort study among industrial workers in Finland, the relative risk for CAD was 1.4 for shift workers in comparison to day time workers.This relation kept significant after adjustment for life style factors, blood pressure and serum lipid levels. (4)In another follow up study; Furlan et al. 2000, (5) reported higher levels of blood pressure and sympathetic overactivity as measured by heart rate variability among shift workers than regular workers.These changes might partly explain the high incidence of CAD in shift workers.
The exact mechanism by which shift work may impose a coronary high risk status (CHRS), as defined by existence of more than 2 of the conventional coronary risk factors, diabetes mellitus (DM) and/or established CAD is still unclear. (11)Also, it is of particular importance to investigate this problem among workers in the strategic industry of natural gas production.

Objective:
The present work was designed to study the influence of shift work in a gas field petroleum company in Dakahlia on CHRS; as well as, to investigate its possible mechanism.

Study design and setting:
A cross-sectional approach was used.
This study was carried out at a natural gas field belonging to a petroleum company located in Dakahlia governorate.

Study population:
Using an arbitrary prevalence of CAD from previous studies ( 4,12,13 )  Each individual in the two studied groups (shift and non-shift) was subjected to the following: • Interview questionnaire, about personal, social and medical histories was applied according to the guidelines of WHO. (14)moking history was obtained and smoking index was calculated as the number of cigarettes smoked / day multiplied by years of smoking. (15) and diastolic (DBP) blood pressure were done in sitting position according to standard methods. (10)Height (Ht), weight (Wt) and waist circumference were measured and body mass index (BMI Kg/m 2 ) was calculated and classified where obesity was considered at ≥ 30 Kg/m 2 . (17)Laboratory investigations: A fasting venous blood sample was obtained by vein-puncture and was left to clot.Serum was separated to measure levels of blood glucose, lipid profile (total cholesterol (TC), LDL-cholesterol, HDL-cholesterol and triglycerides TG) and uric acid according to the standard methods. (18)Electro-cardio-graphic examination was done by the standard resting 12 leads techniques and signs of ischemia were defined according to standard criteria. (19)• A sub-sample was randomly selected from those with CHRS free from diabetes mellitus, new or old CAD and other diseases that may affect autonomic function (21) (25 shift and 25 non-shift workers).Vanillyl mandelic acid (VMA) was measured in 24 hours urine samples as a stress indicator and the R-R deep breathing variability test on resting ECG as a measure for autonomic function (22) were done for each worker in the sub-group.The indices of R-R variability were calculated including the minimum (R-R min), maximum (R-R max), mean

Statistical Analysis:
Data were analyzed using SPSS  with statistically significant differences.
There was no statistically significant difference for family history of premature CAD between both groups of the study.5)(26) Smoking was reported as a common unhealthy behavior response among shift workers. (25)Moreover, Su et al. 2008, (26) observed a consistent elevation of SBP  (27) and Suwazona et al.
2008, (28) (6,7) They explained it by a possible interaction between changes in dietary habits with tendency for overeating among shift workers and a stress mediated mechanism among them.
Other metabolic changes that were reported in the current study included higher prevalence rate of diabetes mellitus and greater mean level of serum uric acid in shift workers than non-shift workers.This is going with the findings of Joseph 2001, (29) who reported that shift work is associated with increased risk of diabetes and other metabolic changes.
On the other hand, Mina and Jungsun 2005, (30) and Hublin et al. 2010, (31) denied the association between coronary risk and shift work.This discrepancy may be attributed to differences in methodological design and poor control for the known healthy workers' effect.However, the only

*
Difference is significant at 0.05 level Figure 1: Coronary risk status among the studied population observed positive risk reducing factor in the present study is the higher rate of physical activity among shift workers than non-shift workers; possibly as related to their specific occupational duties.The most important finding of the current study is the higher rate of coexistence of coronary risk factors in shift workers than non-shift workers.Clustering of more than 2 coronary risk factors, diabetes mellitus and/or CAD resulted in a CHRS which maintained a consistent significant association with shift work even after adjustment for other confounders.These results are going with the finding ofKnutsson et al. 2000,(13) who highlighted the association between increased coronary risk profile and shift work independent from other risk factors.In a trial to understand the exact risk mechanism of shift work in the present study, a subsample of CHRS shift workers and a similar subgroup of CHRS non-shift workers were tested for urinary VMA and resting heart rate (R-R) variability.The outcome indicated a possible status of increased sympathetic activity as measured by high urinary VMA and reduced R-R variability.Similar findings were reported by Furlan et al. 2000. (5)They demonstrated reduced heart rate (R-R) variability indices during 24-hour Holter ECG monitoring of shift workers.In another study, Fujiwara et al. 1992, (32) stated that the increased rates of excretion of urinary catecholamines are possible markers not only for increased sympathetic drive; but Bull High Inst Public Health Vol.41 No.1 [2011] also, disruption of circadian rhythm among shift workers.Therefore, a multi-factorial mechanism characterized by autonomic dysfunction with predominating sympathetic over-activity is possibly responsible for the currently observed CHRS in relation to shift work.CONCLUSION AND RECOMMENDATIONS Shift work was significantly related to CHRS even after control for other confounders.Sympathetic over-activity might be a possible mechanism for CHRS among shift workers.Therefore, it is recommended, in pre-placement and periodic medical examinations of shift workers, to screen for coronary risk factors including smoking, obesity, (general and abdominal), blood pressure, heart rate, lipid profile, diabetes mellitus and uric acid.Simple markers of sympathetic overactivity such as 24 hour urinary VMA and R-R variability testing in ECG could be used as stress indicators among CHRS shift workers.Cases of CAD should be prevented from shift work.Special control programs for coronary risk factors promoting healthy life-style should be designed and implemented among shift workers.Further studies for shift work system modification are needed to reduce the stress mediated sympathetic overactivity.