Researchers at Harvard Medical School have found that key factors involved in blood clotting, known as platelets, become stickier in the morning in people with coronary artery disease.
The findings may shed more light on why more heart attacks and sudden death occur in the early morning hours.The study reported in the March issue of the American Heart Association journal "Circulation," is the latest work from scientists who earlier had shown the incidence of heart attack and sudden death has "circadian" peaks between 6 a.m. and 9 a.m.
The study included 10 patients with stable coronary artery disease who are more likely than the general population to have unstable angina (chest pain), heart attacks and sudden cardiac death.
"Our study is the first to look at the changes in platelet stickiness in patients with heart disease," said Dr. Peter H. Stone, noting that previous studies have looked only at healthy test subjects.
The Boston group's study also is the first to find a correlation between morning increases in platelet stickiness and periods of silent ischemia, Stone said.
Silent ischemia is a decreased blood flow to part of the heart muscle due to constriction or obstruction of the coronary artery that occurs without chest pain or other symptoms. It is detected by measuring the electrical output of a person's heart during daily activities.
The study found that periods of silent ischemia and increases in blood platelet stickiness both tended to occur in the morning as the patients began their daily activities, but neither seemed to be causing the other to occur, he said.
The Harvard researchers said that although platelet stickiness may contribute to the onset of most heart attacks, their results indicate that platelets are not very important to the development of periods of ischemia.
Instead, increased heart rate and blood pressure seemed to be the major factors contributing to ischemia.
"Our results suggest that one goal of therapy in patients with coronary artery disease should be to try to make the heart work less hard," he said.
When the body's systems start up in the morning, the sympathetic nervous system is stimulated and the heart rate and blood pressure increase, Stone said.
During the study, the 10 patients were given a drug known as a beta blocker, to block the body's responses to the chemical cues from the sympathetic nervous system that cause the heart to speed up and blood pressure to increase.
By slowing the heart and lowering the blood pressure, the beta blocker was able to almost entirely suppress the episodes of ischemia, but it had no effect on platelet stickiness, Stone said.