July 8, 2008 The ankle brachial index, a ratio of blood pressure measurements used to indicate the risk of peripheral artery disease and atherosclerosis, may be useful to improve the accuracy of cardiovascular risk prediction, according to a meta-analysis of previous studies.
Major cardiovascular and cerebrovascular events including heart attack and stroke often occur in individuals without known pre-existing cardiovascular disease. The prevention of such events, including the accurate identification of those at risk, remains a serious public health challenge. Scoring equations to predict those at increased risk have been developed using cardiovascular risk factors, including cigarette smoking, blood pressure, total and high-density lipoprotein cholesterol, and diabetes mellitus, according to background information in the article. The Framingham risk score (FRS) is often considered the reference standard but has limited accuracy, tending to overestimate risk in low-risk populations and underestimate risk in high-risk populations.
Attention has been given to indicators of asymptomatic atherosclerosis, such as coronary artery calcium and the ankle brachial index (ABI), which is the ratio of systolic pressure at the ankle to that in the arm. It "is quick and easy to measure and has been used for many years in vascular practice to confirm the diagnosis and assess the severity of peripheral artery disease in the legs," the authors write.
Gerry Fowkes, Ph.D., of the University of Edinburgh, Scotland, and colleagues with the Ankle Brachial Index Collaboration, conducted an analysis of data from 16 studies to determine if the ABI provides information on the risk of cardiovascular events and death independently of the FRS and can improve risk prediction. The studies included a total of 24,955 men and 23,339 women who had ABI measured at baseline and were followed up to detect total and cardiovascular mortality.
The researchers found that the 10-year cardiovascular mortality in men with a low ABI (0.90 or less) was 18.7 percent and with normal ABI (1.11 - 1.40) was 4.4 percent, about a four times higher risk of cardiovascular death for men with low ABI. Corresponding mortalities in women were 12.6 percent and 4.1 percent. The risks remained elevated after adjusting for FRS (2.9 for men vs. 3.0 for women). A low ABI (0.90 or less) was associated with approximately twice the 10-year total mortality, cardiovascular mortality, and major coronary event rate compared with the overall rate in each FRS category. Inclusion of the ABI in cardiovascular risk stratification using the FRS would result in reclassification of the risk category and modification of treatment recommendations in approximately 19 percent of men and 36 percent of women.
"These changes [for men] from higher to lower categories of risk would likely have an effect on decisions to commence preventive treatment, such as lipid-lowering therapy ...," the authors write. "In contrast, the main effect in women of inclusion of the ABI would be that many at low risk with the FRS (less than 10 percent) would change to a higher risk level."
"The ABI is potentially a useful tool for prediction of cardiovascular risk. In contrast to measurement of coronary artery calcium and carotid intima media thickness, it has the advantage of ease of use in the primary care physician's office and in community settings," they write. The researchers add that the equipment is inexpensive, the procedure is simple, and can be performed by a suitably trained nurse or other health care professional.
"The results of our study indicate that, when using the FRS, this [considering ABI for the purposes of cardiovascular risk assessment] may indeed be justified to improve prediction of cardiovascular risk and provision of advice on ways to reduce that risk. A new risk equation incorporating the ABI and relevant Framingham risk variables could more accurately predict risk and our intention is to develop and validate such a model in our combined data set," the authors conclude.
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- Ankle Brachial Index Collaboration. Ankle Brachial Index Combined With Framingham Risk Score to Predict Cardiovascular Events and Mortality: A Meta-analysis. JAMA, 2008;300(2):197-208 [link]
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