Risk of Severe Cardiac Arrhythmia in Male Utility Workers: A Nationwide Danish Cohort Study
Christoffer Johansen1,
Maria Feychting2,
Mogens Møller3,
Per Arnsbo3,
Anders Ahlbom2 and
Jørgen H. Olsen1
1 Institute of Cancer Epidemiology, Danish Cancer Society, DK-2100 Copenhagen, Denmark.
2 Institute of Environmental Medicine, Karolinska Institutet, S-171 77 Stockholm, Sweden.
3 Danish Pacemaker and ICD Register, Department of Cardiology, Odense University Hospital, 5000 Odense C, Denmark.
Received for publication March 18, 2002; accepted for publication July 16, 2002.
 |
ABSTRACT
|
---|
To address concern about the potential cardiovascular effects of occupational exposure to electromagnetic fields in the 50- to 60-Hz frequency band, the authors launched an epidemiologic study of the incidence of severe cardiac arrhythmia, as indicated by the need for a pacemaker, in a nationwide cohort of Danish utility workers. The cohort of 24,056 men employed at utility companies between 1900 and 1993 was linked to the nationwide, population-based Danish Pacemaker Register, and the numbers of persons who had undergone pacemaker implantation between 1982 and 2000 were compared with corresponding numbers in the general population. In addition, the data on the utility workers were fitted to a multiplicative Poisson regression model in relation to estimated levels of exposure to 50-Hz electromagnetic fields. Overall, based on 135 men with pacemakers (140 expected), there was no increased risk of severe cardiac arrhythmia among the utility employees; the risk estimate was 0.96 (95% confidence interval: 0.81, 1.14). No clear dose-response pattern emerged with increasing levels of exposure to electromagnetic fields or with duration of employment. These results are largely reassuring, since they do not support the hypothesis of a link between occupational exposure to electromagnetic fields and an excess risk of severe cardiovascular arrhythmia leading to permanent implantation of a pacemaker.
arrhythmia; electricity; electromagnetic fields; occupational exposure; pacemaker, artificial
 |
INTRODUCTION
|
---|
In a cohort study of 138,903 workers at five large electric utility companies in the United States, Savitz et al. (1) found an increased rate of mortality from arrhythmia-related conditions and acute myocardial infarction among workers with a long duration of work (rate ratios were 1.41.5 for the longest employment intervals) and workers with high exposure to magnetic fields (rate ratios were 1.62.4 in the highest exposure category). The study hypothesis was based on observations made in a double-blind laboratory investigation in which exposure to 20 µT of intermittent 60-Hz magnetic fields was found to reduce the normal variation of the heart rate reversibly (2). Several prospective cohort studies have indicated that reductions in some components of the variation in heart rate increase the risk of heart disease (36), increase the overall mortality rate among survivors of myocardial infarction (710), and increase the risk of sudden cardiovascular death (11). Contrary to the results of Savitz et al., in a nationwide Danish cohort study of male utility employees, Johansen and Olsen (12) observed no increase in the rate of mortality from cardiovascular disorders, including acute myocardial infarction and cardiac arteriosclerosis, by duration of employment or by estimated average exposure to 50-Hz magnetic fields at work.
A permanent pacemaker is implanted for the treatment of severe symptomatic bradycardias such as atrioventricular block, sinus node dysfunction, and atrial fibrillation with bradycardia. Using data from the Danish Pacemaker Register, we determined the frequency of pacemaker implantation in a large cohort of male electric utility employees to investigate their incidence of severe cardiac arrhythmia and to compare it with rates of this procedure in the general population. The frequency of implantation was examined with regard to duration of employment and estimated average level of exposure to electromagnetic fields at work.
 |
MATERIALS AND METHODS
|
---|
Study population
The study population comprised all male employees of the 99 utility companies that supply Denmark with electricity. The construction of the cohort has been described in detail elsewhere (13) and is summarized here. The utility companies were established between 1900 and 1979, and data on the study subjects were retrieved from 1) employment records at each company, 2) the files of the Danish Supplementary Pension Fund, which has retained employment information on all wage-earners (aged 1866 years) in Denmark since April 1, 1964, and 3) the files of the public payroll administration of all Danish counties. With these procedures, we identified a total of 32,006 employees (26,135 men and 5,871 women) who had been employed for at least 3 months at a Danish electric utility company between 1900 and 1993. Because there were few women in the data set, the analyses were restricted to men. The information available on each employee included name, date of birth or personal identification number (a unique 10-digit number introduced in Denmark on April 1, 1968, that incorporates sex and date of birth), job task(s), date of first employment, and subsequent period(s) of employment.
A job-exposure matrix specific to electromagnetic fields was designed that distinguished among 25 job titles held by workers in utility companies and 19 work areas within the utility industry. The job-exposure matrix has been described in detail previously (13). We assigned to each of the 475 combinations of job title and work area an average level of exposure to 50-Hz electromagnetic fields during a working day, which in turn was grouped into three categories of exposure to extremely low frequency electromagnetic fieldsbackground exposure (
0.09 µT), medium exposure (0.10.99 µT), and high exposure (
1.0 µT)and a category with unknown exposure (14). We assigned exposure to individuals on the basis of the characteristics of the first employment held; according to the company files, only 323 men changed their job title during their employment. We assumed that category-specific levels of exposure were unchanged over the study period. Table 1 gives the estimated average levels of exposure to extremely low frequency electromagnetic fields over a full working day for various combinations of job titles and work areas.
View this table:
[in this window]
[in a new window]
|
TABLE 1. Examples of combinations of job titles and work areas from a job-exposure matrix for men employed at Danish utility companies for more than 3 months between 1900 and 1993, according to category of average estimated level of exposure to 50-Hz electromagnetic fields at work
|
|
The Danish personal identification number permits accurate linkage of information between registries. Linkage of these data with the Central Population Register allowed us to verify the personal identification numbers of 26,135 male workers and provided us with information on their vital status and migration through the year 2000. With this procedure, we excluded 2,075 workers (8 percent) who had died before January 1, 1982, and four workers (<1 percent) for whom personal data could not be verified; this left 24,056 male employees (92 percent) for inclusion in the study.
The Danish Pacemaker Register
The population-based Danish Pacemaker Register was established in 1982. Since its inception, all of the data in the register have been validated and are considered 100 percent complete (15). All 12 medical centers that implant pacemakers in Denmark report new implantations and renewal of pacemakers or pacemaker electrodes to the Pacemaker Register on a continuous basis. Each Pacemaker Register record contains the date of implantation, the name of the hospital and cardiology department in which the implantation was performed, the type of pacemaker and lead implanted, and the patients Central Population Register number.
Analyses
We calculated age- and calendar year-specific national rates of pacemaker implantation by dividing the number of men in each 5-year age group and calendar year in Denmark who had undergone this procedure (first known pacemaker implantation) by the corresponding mean population. Pacemaker implantation rates provide a measure of the frequency of pacemaker implantation in Denmark. We measured the association between exposure to magnetic fields and rate of pacemaker implantation as a standardized incidence ratio and assessed random variation through 95 percent confidence intervals based on the Poisson distribution (16). We calculated expected numbers of pacemaker implantations in the cohort by multiplying the number of person-years of follow-up during the period January 1, 1982December 31, 2000 by the age- and calendar-year specific national pacemaker implantation rates.
To evaluate the possible influence of a "healthy worker" effect, we conducted an internal comparison within the cohort with a multiplicative Poisson regression model. The model included age, duration of employment, calendar year, and level of exposure to electromagnetic fields.
 |
RESULTS
|
---|
The 24,056 men in the study accrued 379,486 person-years of follow-up, with an average of 15.8 years (range, 019 years). Overall, there was no significantly increased frequency of pacemaker implantation among employees: 135 received implants and 140 were expected, yielding a risk estimate of 0.96 (95 percent confidence interval: 0.81, 1.14) (data not shown). The highest frequency of pacemaker implantation was in the group of employees with unknown exposure and a short duration of employment (standardized incidence ratio = 2.60, 95 percent confidence interval: 0.3, 9.4); however, the standardized incidence ratio was based on only two cases. No clear dose-response pattern emerged with increasing exposure to electromagnetic fields or with duration of employment (table 2). The Poisson regression analysis showed a risk of unity (95 percent confidence interval: 0.6, 1.5) in the group with high exposure as compared with the group with background exposure, and there was no trend in the risk estimate when workers were compared according to their level of occupational exposure to electromagnetic fields (p for trend = 0.7; table 3).
View this table:
[in this window]
[in a new window]
|
TABLE 2. Standardized incidence ratios for pacemaker implantation during the period 19822000 among 24,056 men employed for at least 3 months at a utility company in Denmark between 1900 and 1993, by average estimated level of exposure to electromagnetic fields at work and duration of employment
|
|
View this table:
[in this window]
[in a new window]
|
TABLE 3. Adjusted relative risk of pacemaker implantation among 24,056 men employed for at least 3 months at a utility company in Denmark between 1900 and 1993, by average estimated level of exposure to electromagnetic fields at work*
|
|
 |
DISCUSSION
|
---|
The results of this nationwide cohort study do not suggest an association between occupational exposure to electromagnetic fields and risk of arrhythmia-related heart disease that requires implantation of a permanent artificial pacemaker. This result confirms the findings of the previous study on cause-specific mortality in the same cohort (12). In that study, two of the current authors (C. J. and J. H. O.) observed risk estimates close to unity for acute myocardial infarction, cardiac arteriosclerosis, other heart diseases, and cerebrovascular diseases by duration and level of exposure to electromagnetic fields. Other previous epidemiologic studies of utility workers found a reduced rate of mortality from cardiovascular diseases in comparison with the general population (1719). Health examinations of employees occupationally exposed to above-average levels of electromagnetic fields did not reveal cardiovascular symptoms or diseases (20, 21). These results may be explained by a "healthy worker" effect due to selection of healthy persons into employment in the utility industry. In addition, continued employment in the cohort may be differential according to health status, including cardiovascular disease status. Kelsh and Sahl (19) addressed this possibility and observed significantly greater rates of mortality from major cardiovascular diseases (rate ratios were 1.31.8) in all groups likely to have been exposed to electromagnetic fields than among administrative employees (19). Likewise, Savitz et al. (1) observed an increased rate of mortality from arrhythmia-related conditions and acute myocardial infarction among utility workers that was associated with duration of employment and exposure to magnetic fields.
We did not observe an increased frequency of pacemaker implantation in analyses in which risks were compared among groups with different exposures to electromagnetic fields. However, the majority of the workers in the cohort had worked at a utility company for more than 20 years, and the analyses of pacemaker implantation in workers with shorter durations of employment had limited power. A possible explanation for the results of Savitz et al. (1) is that reporting of cardiovascular diseases such as myocardial infarction as causes of death on death certificates is an imperfect reflection of the occurrence of those diseases, and errors might be introduced by the assignment of cause of death. Furthermore, it is well known that cardiovascular diseases are among the most commonly overreported causes of death on death certificates (22, 23). Finkelstein (24) questioned the use of death certificates as a source of information on the diagnosis of loss of autonomic cardiovascular control by Savitz et al. (1) and pointed out that etiologic conclusions could not be drawn on the basis of death certificate codes.
Our study had certain limitations, including the assessment of exposure. The ideal approach for estimating occupational exposure to magnetic fields is to obtain individual data from long-term measurements with personal dosimeters. This is not possible in large or retrospective cohort studies. One of the main problems in studies of the health effects of magnetic fields is uncertainty about which aspect of magnetic fields should be used to define exposure. In the absence of measurements of past exposure and of a clear biologic rationale, the usual approach is to use a job-exposure matrix to estimate individual time-weighted average exposure. This consists of a list of job titles and a list of exposures to magnetic fields corresponding to those job titles. A validation of the job-exposure matrix used in the present study showed that reducing the four exposure categories to three categories allowed detection of a true association between exposure to 50-Hz magnetic fields and disease (14).
We had no information about other exposures or lifestyle factors associated with cardiovascular disease, such as cigarette smoking, diet, or physical activity. Although we cannot exclude the possibility of negative confounding, this would mean that these utility workers had a more healthy lifestyle than the general population, including less smoking and frequent physical activity. On the contrary, we observed an increased risk of lung cancer (standardized incidence ratio = 1.14, 95 percent confidence interval: 1.1, 1.2) among men in this cohort in a previous study, which does not support this objection (13). Sinus node dysfunction and atrioventricular conduction disturbances are most often found in the elderly as an isolated phenomenon, but one may speculate as to whether cardiovascular diseases caused by smoking may cause these disorders, leading to pacemaker implantation. However, the correlationthat is, the correlation between obstruction of the sinus node artery and evidence of clinical sinus node dysfunctionis poor (25). To the best of our knowledge, no studies have identified tobacco smoking as a major risk factor for disorders leading to pacemaker implantation. In addition, the previous studys failure to identify a significantly decreased rate of mortality from cardiovascular diseases (acute myocardial infarction, cardiac arteriosclerosis, and cerebrovascular diseases) associated with such lifestyle factors indicates that negative confounding does not explain our results (12). Furthermore, these findings, of a mortality rate from cardiovascular diseases close to that expected, do not indicate that a healthy worker effect explains the present results.
This study addressed only those heart diseases that require implantation of a pacemaker. This limits generalization of our findings to all types of arrhythmias, but it ensures that the patients had a certain degree of illness, including a well-defined surgical event.
Because the Danish Pacemaker Register contains data for the entire population of Denmark, practically all pacemaker implantations occurring among utility workers after 1982 were identified. The workers files were established years before pacemaker implantation was reported to the Register, and the completeness of the employment files and of the Register reduces the likelihood of selection or information bias.
In this nationwide Danish study of a large cohort of employees in the utility industry, we did not identify an increased frequency of pacemaker implantation associated with occupational exposure to electromagnetic fields. We conclude that the results of our study do not support the hypothesis of a link between occupational exposure to 50-Hz electromagnetic fields and excess risk of severe arrhythmia-related heart diseases.
 |
ACKNOWLEDGMENTS
|
---|
This study was funded by the Association of Danish Energy Companies and the Danish Cancer Society.
The authors thank Andrea Meehrsohn and Lars H. Thomassen for skillful computer assistance and statistical support.
 |
NOTES
|
---|
Reprint requests to Dr. Christoffer Johansen, Institute of Cancer Epidemiology, Danish Cancer Society, Strandboulevarden 49, DK-2100 Copenhagen, Denmark (e-mail: christof{at}cancer.dk). 
 |
REFERENCES
|
---|
- Savitz DA, Liao D, Sastre A, et al. Magnetic field exposure and cardiovascular disease mortality among electric utility workers. Am J Epidemiol 1999;149:13542.[Abstract]
- Sastre A, Cook MR, Graham C. Nocturnal exposure to intermittent 60-Hz magnetic fields alters human cardiac rhythm. Bioelectromagnetics 1998;19:98106.[ISI][Medline]
- Martin GJ, Magid NM, Myers G, et al. Heart rate variability and sudden death secondary to coronary artery disease during ambulatory electrocardiographic monitoring. Am J Cardiol 1987;60:869.[ISI][Medline]
- Tsuji H, Larson MG, Venditti FJ Jr, et al. Impact of reduced heart rate variability on risk of cardiac events. The Framingham Heart Study. Circulation 1996;94:28505.[Abstract/Free Full Text]
- Dekker JM, Schouten EG, Klootwijk P, et al. Heart rate variability from short electrocardiographic recordings predicts mortality from all causes in middle-aged elderly men. The Zutphen Study. Am J Epidemiol 1997;145:899908.[Abstract]
- Liao D, Cai J, Rosamond WD, et al. Cardiac autonomic function and incident coronary heart disease: a population-based case-cohort study. The ARIC Study. Am J Epidemiol 1997;145:696706.[Abstract]
- Kleiger RE, Miller JP, Bigger JT Jr. Decreased heart rate variability and its association with increased mortality after acute myocardial infarction. Am J Cardiol 1987;59:25662.[ISI][Medline]
- Lombardi F, Sandrone G, Pernpruner S, et al. Heart rate variability as an index of sympathovagal interaction after acute myocardial infarction. Am J Cardiol 1987;60:23945.
- Ori Z, Monir G, Weiss J, et al. Heart rate variability: frequency domain analysis. Cardiol Clin 1992;10:499537.[Medline]
- Vaishnav S, Stevenson R, Marchant B, et al. Relation between heart rate variability early after acute myocardial infarction and long-term mortality. Am J Cardiol 1994;73:6537.[ISI][Medline]
- Malik M, Farell T, Camm AJ. Circadian rhythm of heart rate variability after acute myocardial infarction and its influence on the prognostic value of heart rate variability. Am J Cardiol 1990;66:104954.[ISI][Medline]
- Johansen C, Olsen JH. Mortality from amyotrophic lateral sclerosis, other chronic disorders, and electric shocks among utility workers. Am J Epidemiol 1998;148:3628.[Abstract]
- Johansen C, Olsen JH. Risk of cancer among Danish utility workersa nationwide cohort study. Am J Epidemiol 1998;147:54855.[Abstract]
- Johansen C, Raaschou-Nielsen O, Skotte J, et al. Validation of a job-exposure matrix for assessment of utility worker exposure to magnetic fields. Appl Occup Environ Hyg 2002;17:30410.[Medline]
- Arnsbo P, Moller M. Danish Pacemaker and ICD Register. 1999. Pacing Clin Electrophysiol 2000;23:S1S93.[Medline]
- Rothman KJ, Boice JD Jr. Epidemiologic analysis with a programmable calculator. Washington, DC: US GPO, 1979.
- Savitz DA, Loomis DP. Magnetic field exposure in relation to leukemia and brain cancer mortality among electric utility workers. Am J Epidemiol 1995;141:12334.[Abstract]
- Baris D, Armstrong BG, Deadman J, et al. A mortality study of electrical utility workers in Québec. Occup Environ Med 1996;53:2531.[Abstract]
- Kelsh MA, Sahl JD. Mortality among a cohort of electric utility workers, 19601991. Am J Ind Med 1997;31:53444.[ISI][Medline]
- Knave B, Gamberale F, Bergström E, et al. Long-term exposure to electric fields. Scand J Work Environ Health 1979;5:11525.[ISI][Medline]
- Baroncelli P, Battisti S, Checcucci A, et al. A health examination of railway high-voltage substation workers exposed to ELF electromagnetic fields. Am J Ind Med 1986;10:4555.[ISI][Medline]
- Kircher T, Nelson J, Burdo H. The autopsy as a measure of accuracy of the death certificate. N Engl J Med 1985;313:12639.[Abstract]
- Smith CJ, Scott SM, Wagner BM. The necessary role of the autopsy in cardiovascular epidemiology. Hum Pathol 1998;29:146979.[ISI][Medline]
- Finkelstein MM. Re: "Magnetic field exposure and cardiovascular disease mortality among electric utility workers." (Letter). Am J Epidemiol 1999;150:1258.[ISI][Medline]
- Fauci AS, Resnick WR, Wintrobe MM, et al. Harrisons principles of internal medicine. 14th ed. New York, NY: McGraw-Hill Company, Inc, 1998.