"The whole is larger than the sum of the parts when it comes to the combined risk of heart attack from high blood pressure and high LDL-cholesterol," said Duanping Liao, M.D., Ph.D., associate professor of health evaluation sciences, Penn State College of Medicine, Penn State Milton S. Hershey Medical Center, and lead investigator of the study. "Our study showed that about 21 percent of middle-aged Americans might have both risk factors, implying that these findings could have significant public health implications with regard to controlling both hypertension and high cholesterol in reducing cardiovascular disease."
Liao's study is one of the few to identify people with these joint risk factors and evaluate on a long-term basis the relationship between the clustering of the risk factors and the incidence of myocardial infarction, or heart attack. The study was presented today (Aug. 30, 2004) at the European Society of Cardiology Congress in Munich, Germany.
Cardiovascular disease remains the No. 1 killer in the United States with about 1 million Americans dying of the disease each year. High blood pressure, or hypertension, and high LDL (low-density lipoprotein) cholesterol are highly prevalent and well-established risk factors for cardiovascular disease.
The study team used data from 14,500 people in the Atherosclerosis Risk in Communities (ARIC) study who were free of coronary disease at initial examination and were followed for an average of nine years. Started in 1987, the ARIC is an ongoing, long-term, observational study of cardiovascular and pulmonary diseases sponsored by the National Heart, Lung, and Blood Institute of the National Institutes of Health. The ARIC study initially included about 16,000 men and women between the ages of 45 and 64 from four U.S. communities (Washington County, Md., Forsyth County, N.C., selected suburbs of Minneapolis, Minn., and Jackson, Miss.). Participants were given questionnaires and were examined every three years. Follow-up occurs yearly by telephone to maintain contact with participants and to assess participants' health status.
The study showed that those without high blood pressure or high cholesterol showed an incidence for heart attack of 23 per 10,000 person-years, which is roughly 23 cases per 10,000 persons per year. Because these people lacked any risk factors, the incidence of 23 is considered background risk and is subtracted from the total number of heart attacks for those with risk factors present. After deducting background risk, those with high blood pressure had an incidence of 16 and those with elevated LDL-cholesterol had an incidence of 12. The expected incidence for those with both high blood pressure and elevated LDL-cholesterol would be the sum of those two risks, or 28. In fact, the actual incidence of heart attack for those with both risk factors was 51.
"The observed disease rate due to the combination of these two risk factors was significantly higher than what was expected," Liao said. "In terms of excess risk, 31 percent of the risk of developing heart attack among people with both high blood pressure and elevated LDL-cholesterol was attributable to the synergistic interaction of these two risk factors."
In terms of relative risk, the findings showed that those with high blood pressure, elevated LDL-cholesterol or both have a 1.95, 1.70 and 3.31 times higher risk of having a heart attack than those without either of the two conditions.
"This study highlighted the need for physicians to consider treating both high blood pressure and high cholesterol in persons with both risk factors," Liao said, "because the patient's risk of developing a heart attack is beyond simple addition of each risk factor acting alone."
In addition to Liao, the study team included: Jingping Mo, M.D., Ph.D., Global Epidemiology, Pfizer Inc.; and Yinkang Duan, M.D., M.S., Megan Darnell, M.S., Hung-Mo Lin, Ph.D., Zhengmin Qian, M.D., Ph.D., Department of Health Evaluation Sciences, Penn State College of Medicine, Penn State Hershey Medical Center.
The study was partially supported by a research grant from Pfizer Inc., New York.