Shift working is related to increased cardiovascular morbidity. Peripheral endothelial dysfunction, an inherent feature of early atherosclerosis, has been suggested as a surrogate marker of cardiovascular risk. Whether shift working is associated with peripheral endothelial dysfunction has not been investigated to date. A total of 48 male shift workers (SWs) and 47 male nonshift workers (NSWs) (mean age 43 ± 5 years) were recruited from a glass manufactory. The SWs and NSWs were matched according to age, body mass index, smoking habits, family history of premature coronary artery disease, prevalence of hypercholesterolemia and hypertension, and work place. Their sport habits were also documented. Peripheral endothelial function was assessed using the EndoPAT technique to determine the peripheral arterial tone (PAT) index. According to the study design, no difference was found in the risk factor profiles between the SWs and NSWs. Despite a greater percentage of regular physical activity among the SWs (16.7 vs 4.3%, p = 0.05), shift working was associated with a reduced PAT index compared to working only on the day shift (PAT index 1.73 ± 0.4 vs 1.94 ± 0.5, p = 0.03). In the NSW group, the participants with regular physical training (n = 16) had a greater PAT index than those without regular physical activity (n = 12; PAT index 2.28 ± 0.45 vs 1.86 ± 0.51, p = 0.03). No such difference was found in the SWs. In conclusion, SWs had a reduced PAT index compared with NSWs, suggesting endothelial dysfunction. Therefore, the known increased cardiovascular risk in those shift working might be related to endothelial dysfunction.
Several epidemiologic studies have reported an increased cardiovascular risk in shift workers that was partially due to a greater incidence of arterial hypertension, hyperlipidemia, or the metabolic syndrome. Peripheral endothelial function has been suggested to be a surrogate marker of cardiovascular risk. Hence, reduced peripheral endothelial function was shown to be associated with cardiovascular events. In medical doctors, working a 24-hour shift, including night duty, resulted in a deterioration of peripheral endothelial function. Furthermore, the greatest decrease in endothelial function was found in physicians with a longer history of night shift duty and in those reporting fewer sleeping hours during the shift. Whether endothelial dysfunction can also be found in nonmedical shift working subjects has not yet been investigated. The aim of our study was to compare the peripheral endothelial function in shift workers (SWs) compared with matched nonshift workers (NSWs).
Methods
Male employees (aged 35 to 55 years) working in a glass factory were invited to participate in the present study, if they have been working there for 5 to 15 years. Two groups of probands were recruited: NSWs—working in the factory only during the day (from 8:00 a.m. to 6:00 p.m. ); and SW—who changed among 3 shifts.
The exclusion criteria were the presence of any cardiovascular disease, diabetes mellitus, or angina pectoris. The probands in the NSW and SW groups were matched according to age, body mass index, cardiovascular risk factors (i.e., smoking, hyperlipidemia, arterial hypertension, positive family history for premature cardiovascular diseases) and work place. Their physical activity was also documented and was grouped as no sports, regular physical activity (e.g., walking), occasional sports (≤1/week), regular sports (2 to 4 times/week), and frequent sports (≥4 times/week).
An institutional review committee approved the study, all patients provided written informed consent, and the study complied with the Declaration of Helsinki.
The total cholesterol, triglycerides, high-density lipoprotein, and low-density lipoprotein cholesterol levels were measured with the subjects in the fasting state. Study participants with a plasma low-density lipoprotein cholesterol level >130 mg/dl or who were receiving cholesterol lowering therapy were classified as having hypercholesterolemia. The probands who refused to allow a blood sample to be taken were included if they had no history of hypercholesterolemia. Smokers were defined as those who had smoked regularly during the previous 12 months. Arterial hypertension was defined as taking antihypertensive medication or presenting with an elevated blood pressure (≥140/90 mm Hg) before endothelial function measurement. A positive family history for cardiovascular disease was assumed if a first-degree male relative ≤55 years old or female relative ≤65 years old with any cardiovascular disease was reported by the study participant.
All measurements were performed with the subjects in the fasting state during the morning (8:00 a.m. to 12:00 p.m. ) and not having worked the night before. All participants were asked not to smoke for 2 hours before the examination. The peripheral endothelial function was assessed using the commercially available EndoPAT device (Itamar Medical, Caesarea, Israel). With this device, a beat-to-beat plethysmographic record of the finger arterial pulse wave amplitude can be captured with pneumatic probes. For this purpose, a probe was placed on the index finger of each hand, and the peripheral arterial tone (PAT) was recorded from both hands throughout the study. To assess endothelial function, the reactive hyperemia technique was used. The measurements were obtained for 5 minutes at baseline (at rest), followed by 5-minute occlusion of the right upper arm with a sphygmomanometer cuff inflated to suprasystolic levels (50 mm Hg greater than the initially measured systolic blood pressure). Next, the cuff was deflated to induce reactive (flow-mediated) hyperemia, which was measured for 5 minutes. The left hand remained unoccluded as a reference to correct for potential systemic changes.
The endothelial function was calculated using the PAT index as the ratio between the magnitude of the average postocclusive pulse wave amplitude (1.5 to 2.5 minutes after release of the arterial occlusion) and an average of 5 minutes of the preocclusion pulse wave amplitude, corrected for systemic changes. The measurements were analyzed using a computerized automated algorithm (Itamar Medical), resulting in no intra- or interobserver variability. The EndoPAT system has been validated and shown to be accurate compared to other methods used to assess the peripheral endothelial function.
The results are expressed as mean ± SD. For the statistical calculations, the Statistical Package for Social Sciences, version 15.0 for Windows (SPSS, Chicago, Illinois) was used. Because all variables were normally distributed, Student’s t test or the chi-square test was used to compare the continuous and categorical variables, respectively.
Results
The demographic and clinical characteristics are listed in Table 1 . A lower PAT index was found in the SWs than in the NSWs (1.73 ± 0.4 vs 1.94 ± 0.5, respectively, p = 0.03; Figure 1 ).
| Variable | SW (n = 48) | NSW (n = 47) | p Value |
|---|---|---|---|
| Age (years) | 43 ± 4 | 43 ± 6 | 0.53 |
| Body mass index (kg/m 2 ) | 26.1 ± 3.0 | 25.9 ± 2.8 | 0.78 |
| Total cholesterol (mg/dl) | 223 ± 24 | 235 ± 74 | 0.50 |
| High-density lipoprotein (mg/dl) | 51 ± 13 | 55 ± 10 | 0.33 |
| Low-density lipoprotein (mg/dl) | 147 ± 23 | 136 ± 23 | 0.33 |
| Triglycerides (mg/dl) | 150 ± 115 | 145 ± 122 | 0.93 |
| Arterial hypertension | 23% | 15% | 0.32 |
| Smoking | 65% | 57% | 0.48 |
| Positive family history | 2% | 2% | 0.99 |
| Working duration (years) | 9.9 ± 5.3 | 14.5 ± 8.9 | 0.02 |
Although the rate of physical activity was not different between the SWs and NSWs (81% of SWs vs 74% of NSWs reported any physical activity, p = 0.43), the 16 NSWs performing regular sports had a greater PAT index than the 12 NSWs not performing sports (2.28 ± 0.46 vs 1.86 ± 0.51, respectively, p = 0.03). In the SWs, no influence of sports activity on endothelial function was found ( Figure 2 ).
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