In a pilot study, in which we recruited 125 cases with acute myocardial infarction and 125 controls individually matched for age and occupation, we found a significant relation between working hours and risk of myocardial infarction.
In the present study, we recruited 199 male patients who had been admitted to three university hospitals and one general hospital between November 1990 and November 1993 for a first attack of acute myocardial infarction. The diagnosis was confirmed and was defined as typical chest pain lasting at least 20 minutes, an electrocardiogram showing ST elevation of at least 2
mm in two or more contiguous leads with subsequent evolution of the typical electrocardiographic changes, and diagnostic enzyme changes. We had to remove two patients, classified as agricultural workers, from the study because we found no control subjects for that occupation. In addition, we removed one patient because we could find no control with matching age. Another subject was found to have a history of cerebrovascular stroke and was therefore also removed from the study. Consequently, 195 patients (98%) were included in the study.
For controls, we recruited 339 male workers free of coronary heart disease who were matched to the cases for age (within three years) and occupation. These men were selected at routine medical examinations conducted at their workplace. In Japan, under the Medical Service Law, each worker receives an annual medical examination, including an electrocardiogram and measurement of blood pressure, serum total cholesterol concentration, urinary glucose concentration, body weight, and height. Therefore, these men were well informed of their health status, including risk factors for cardiovascular diseases. We used their occupational categories, defined in the Japanese census of 1990 (table ), for matching.15
We sent these men questionnaires (see below) and obtained 332 complete responses. One of these 332 subjects was found to have a history of angina pectoris and was therefore removed from the study. Consequently, 331 subjects (98%) were included in the control group.
Background characteristics of 195 Japanese men with acute myocardial infarction and 331 controls matched for age and occupation, and odds ratios for infarction in relation to established risk factors and psychosocial conditions
All the subjects completed a self administered questionnaire about their working hours and psychosocial working conditions, cases being given the questionnaire before their discharge from hospital. The cases were asked for their mean working hours per day, excluding holidays and days of rest, for each of the two months preceding their acute myocardial infarction and for each of the months with the shortest and longest mean daily working hours for the year before their infarction. The controls were asked about working hours for the months before their recruitment to the study. In Japan each worker is given a table of salary details every month and is informed of the number of days he or she is going to work, including the overtime hours. It was therefore easy for the subjects to recall the number of hours they had worked in earlier months. From these results we calculated the degree to which mean working hours had changed.
The subjects were asked about psychosocial conditions in the month before their infarction (or recruitment to the study). We asked for the proportion of their working hours that were spent in sedentary work.16,17
To assess the subjects’ emotional state,18
we used the “burnout measure” developed by Pines (presented at the National Conference on Burnout in the Human Services, Philadelphia, November 1981). This measure, which has been well validated in Japan, is composed of 21 questions about the frequency of positive and negative feelings. We used the score derived from the responses to define burnout and borderline states.
We recorded established risk factors for acute myocardial infarction: history of hypertension, hypercholesterolaemia, diabetes mellitus (including impaired glucose tolerance), and smoking habits. These were assessed during the cases’ hospitalisation or during the controls’ medical check up in their workplace.14
To calculate body mass index,19
we also recorded body weight and height, which were measured at the hospital or at the most recent medical check up.
We assessed associations between premorbid variables and the risk of acute myocardial infarction by conditional logistic regression,20
which was performed by the PHREG procedure in the SAS statistical package.21
For this analysis, we used occupational categories (see table ) to separate the subjects into matched sets. Age was used as a continuous variable for adjustment because it was matched within three years between cases and controls. Established risk factors for myocardial infarction and psychosocial working conditions were assessed as categorical indicator variables (table ). We used analyses of variance and covariance to compare the mean working hours (adjusted for age and occupation), and the changes in working hours during the months before myocardial infarction, of the case and control groups. Conditional logistic regression analysis was used to assess crude associations between mean working hours and the risk of acute myocardial infarction.
We then grouped the data on mean working hours, and changes in hours, into four categories (see table ). Further conditional logistic regression analysis was used to examine whether crude associations were confounded by established risk factors for myocardial infarction (hypertension, hypercholesterolaemia, diabetes, body mass index, and smoking habits) or psychosocial aspects of work (amount of sedentary work and burnout index). Variables were selected for inclusion in the models on the basis of biological and psychosocial plausibility and evidence of confounding. The SAS statistical package was used for the analysis.
Categorisation of working hours of 195 Japanese men with acute myocardial infarction and 331 controls matched for age and occupation, and odds ratios for infarction in relation to working hours