CHICAGO (Reuters) - Middle-aged heart attack victims who go back to chronically stressful jobs are twice as likely to suffer a second heart attack or a related problem than those in less taxing jobs, Canadian researchers reported on Tuesday.

The risk is there even after accounting for other heart disease-related factors such as high blood pressure, diabetes, obesity and smoking, the report from Quebec's Laval University concluded.

The study looked at 972 men and women younger than 60 who had suffered a heart attack between 1995 and 1997 and returned to work. During a follow-up that lasted about six years, 124 suffered a second heart attack, 13 of them fatal, and 82 had unstable angina.

Chronic job strain, defined as high psychological stress and little control over decisions that have to be made, was linked to a two-fold increase in the risk of a second problem, said the study published in this week's Journal of the American Medical Association.

The types of jobs involved were not detailed in the study.

The authors said several, though not all, previous studies have linked job strain to initial heart attacks but only two earlier limited studies had tried to explore the risk of returning to a high-stress job.

The authors, led by Dr. Corine Aboa-Eboule, said the risk involved in stressful jobs may be related to the body's response that can lead to inflammation of the artery walls and subsequent formation of blood clots.

It might be thought that failing to develop a healthier post-heart attack lifestyle or not sticking to drug therapy also would be a factor, the study said. But none of the data supported that hypothesis.

"Chronic job strain significantly increased the risk of recurrent (heart problems) among middle-aged patients who returned to work ... ," the study said.

"These results suggest that preventive interventions aimed at reducing job strain might have a significant impact on recurrent ... events," it added.

The study "should be disseminated in cardiac practice and in occupational health services with the aim of reducing job strain for workers returning to work," it concluded.