Men whose heart rates increase the most during mental stress before exercise seem to have a higher risk of dying of a sudden heart attack later in life, French researchers say.

Doctors have long sought a simple and cheap way to predict people who may be at increased risk of a dying from a sudden heart attack.

In Wednesday's issue of the European Heart Journal, researchers say men whose heart rate increased the most during mild mental stress just before an exercise test had twice the risk of dying of a sudden heart attack in later life, compared with men whose heart rate did not increase as much.

"People who showed a higher heart rate increase with mild mental stress could be considered for additional investigations and for tailored preventive strategies, aimed in the first place at reducing the probability of heart disease," Prof. Xavier Jouven of the Hopital Européen Georges Pompidou in Paris said in a release.

The study looked at data from 7, 746 French men aged 42 and 53 who worked as police officers.

The men had physical exams, including electrocardiograms, between 1967 and 1972.

Their heart rates were measured several times:

• At rest.
• A few minutes before they did an exercise bicycle test.
• Sitting on the bike, which the researchers considered to be mild mental stress in preparation for exercise.
• During exercise.
• During the recovery period.

Over an average of 23 years of follow-up, there were 516 deaths — including 81 sudden deaths — as a result of heart attacks.

The doubling in risk of death was found in men whose heart rate increased by more than 12 beats a minute during mild mental stress, compared with men whose heart rate increased by less than four beats per minute.

The increased risk was found after taking into account age, smoking, weight, physical exercise, cholesterol levels and diabetes.

Accelerator, brake balance

The largest proportion of sudden deaths occurred among men whose heart rate increased the most during mental stress, 14 out of 471 men, compared with no sudden deaths among men with the lowest heart rate.

An interaction between the vagus nerves that control the body's unconscious functions such as heart beat and activation by the sympathetic nervous system could be behind the effect, the researchers said.

"There is a balance between the accelerator [sympathetic activation] and the brake [vagal activation], Jouven said.

During an ischemic episode such as a heart attack, if there is no protection from the vagal "brake" then the activation can become uncontrolled and dangerous.

"Our underlying assumption, which this study appears to have proved correct, is that the faster the vagal withdrawal in response to mental stress, the greater will be — during an ischemic episode — the damaging effect of sympathetic activation unopposed by vagal activity," Jouven explained.

There was also a genetic predisposition, with the risk of sudden death increased nearly 10-fold among men whose parents had died suddenly, compared with men whose parents did not die that way.