HEARTBEAT DANGERS HIGHEST ON MONDAYS, FRIDAYS, STUDY SAYS.

Byline: Denise Grady The New York Times

Dangerous abnormalities in heart rhythm are more likely to occur on Mondays and Fridays than on other days of the week, scientists have concluded from a study of patients being treated for heart disease.

The study did not explain the pattern, but because the abnormal heartbeats peaked at the beginning and end of the workweek, researchers suggested that stress and fatigue might have played a role.

The findings mirrored those already reported for heart attacks and heart-related deaths, which are also more common on Mondays, and in the morning. The study, the first to examine the timing of rhythm abnormalities, appears in the current issue of the journal Circulation.

``Our hypothesis, which really has not been proved or tested, is that somehow, getting up on a Monday morning triggers this kind of thing,'' said the director of the study, Dr. Robert Peters, a professor of medicine at the University of Maryland in Baltimore and chief of cardiology at the Baltimore Veterans Affairs Medical Center.

Although many of the patients in the study were thought to have retired, he said that habits developed during their working lives might have persisted, including tensing up on Monday morning. ``Or it may be just seeing a spouse go back to work or a child go back to school,'' he said.

Changes in heart rate might be brought on by increased levels of adrenaline, a hormone released during periods of stress, Peters said.

He and his colleagues studied 683 people who had histories of life-threatening arrhythmias, or disturbances in heartbeat. The patients were in their 50s, 60s and 70s, and 82 percent were men. All had been given implantable defibrillators - miniature, surgically installed versions of the paddles used in emergency rooms to shock a person's heart back to life when it stops beating. The implantable devices monitor heart rhythm and, if it goes awry, fire electric shocks to return it to normal.

The implants also record the arrhythmias that set them off, and it was this information which Peters used to track their timing. His team studied the records of patients who had received powerful, lifesaving shocks when their hearts suddenly began racing at 214 beats a minute.

At that rate, the heart cannot pump effectively, and both the brain and the heart itself become starved for oxygen. If normal pumping is not quickly restored, a patient can pass out and die within minutes. More than 250,000 such ``sudden cardiac deaths'' occur in the nation each year.

Peters' team found that more jolts were fired on Monday than on any other day; 21 percent occurred on Monday, nearly double the number for Saturday and Sunday. A second, smaller peak occurred on Friday. Numbers were lower on the other weekdays, though none so low as on Saturday and Sunday.

The researchers also found that a type of heart medication called a beta blocker appeared to protect patients from the Monday surge in arrhythmias. Beta blockers can counter the effects of adrenaline on the cardiovascular system.

In the 123 patients taking beta blockers, only 9 percent of the serious arrhythmias occurred on Mondays. That probably meant that their overall risk of sudden death on any day was lowered as well, Peters said.

``This might encourage people to take beta blockers,'' Peters said. ``We've known for a while that they're good after a heart attack, because they prolong life and prevent the morning peak in acute heart attacks.''

But he also cautioned that the number of patients taking beta blockers in the current study was small, and that further research would be needed to determine whether the apparent effect was real.