Most everyone knows that major trauma can set off a heart attack, but a new study shows that even mental stress can cause this result when the person has heart disease already.

Observational studies on large groups of people have linked the acute mental stress caused by earthquakes or major sporting events with increased heart attack risk but, in contrast, stress on patients with known coronary heart disease that is stable is thought to be safe.

Coronary heart disease, or atherosclerosis, is the largest killer disease in the U.S. It occurs when plaque and other substances form on the walls of the heart’s coronary arteries, which results in an inadequate flow of blood to the heart, and it can set the stage for a heart attack.

A study funded by the British Heart Foundation looked at 15 people with significant coronary heart disease and 15 people without it. The study volunteers underwent tests to trigger mental stress while the blood pressure and speed of blood flow inside the coronary arteries supplying blood to the heart were measured.

The researchers saw an increase in heart rate and blood pressure during mental stress testing reflecting an increase in oxygen demand by the heart muscle. Surprisingly in people with coronary heart disease, the blood flow did not increase to meet that demand. The researchers also saw an increased inability of the blood to flow through the small blood vessels in the heart.

These studies show that stress is not a harmless factor for people with coronary heart disease and that doctors should advise their patients accordingly, the researchers say of the study, which was presented at the British Cardiovascular Conference.