A meta-analysis published in the July issue of JAMA finds that a particular ratio of blood pressure measurements, called the ankle brachial index (ABI), has the potential to improve predictions of cardiovascular risk. Currently, the ABI has been used to measure risk of peripheral artery disease and atherosclerosis.

One of the major public health challenges is identifying and preventing heart attack, stroke, and other major cardiovascular and cerebrovascular events in individuals who do not have a known pre-existing cardiovascular disease. It is common for physicians to collect data on cardiovascular risk factors - for example, history of cigarette smoking, blood pressure, total and high-density lipoprotein (LDL) cholesterol levels, and diabetes - that are entered into scoring equations to predict the risk of cardiovascular events. The standard measure, the Framingham risk score (FRS), has limited accuracy because it tends to overestimate risk in low-risk populations and underestimate risk in high-risk populations.

Researchers have been interested in finding other tools that may indicate asymptomatic atherosclerosis, including coronary artery calcium and the ankle brachial index (ABI). Gerry Fowkes, Ph.D. (University of Edinburgh, Scotland) and colleagues with the Ankle Brachial Index Collaboration note that this ratio of systolic pressure at the ankle to that in the arm (the ABI), "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."

To determine if the ABI contains helpful information for determining the risk of cardiovascular events and death (independently of the FRS), Fowkes and colleagues conducted a study of studies, or meta-analysis, using data from 16 published papers. This method provided a sample of 24,955 men and 23,339 women who received baseline ABI measurements and were followed up to measure total and cardiovascular mortality.

The researchers found that men with low ABI (0.90 of less) had four times the risk of cardiovascular death than men with normal ABI (1.11 - 1.40). That is, the 10-year cardiovascular mortality in men with low ABI was 18.7% compared with 4.4% for men with normal ABI. Women had similar results, as those with low ABI had a 12.6% and those with normal ABI had a 4.1% 10-year cardiovascular mortality. After adjusting for FRS, these risk remained large for those with low ABI but were somewhat weakened. People with low ABI are predicted to have about twice the rate of 10-year total mortality, cardiovascular mortality, and major coronary event rates compared with the overall rate in each FRS category. Therefore, if physicians include the ABI when using the FRS to measure cardiovascular risk, according to the authors, there would be a risk category reclassification and a change in treatment recommendations for about 19% of men and 26% of women.

The authors indicate that for men, "These changes from higher to lower categories of risk would likely have an effect on decisions to commence preventive treatment, such as lipid-lowering therapy...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."

Further, "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." Compared to these other measurements, the ABI requires inexpensive equipment, a relatively simple procedure, and a trained nurse or other health care professional could perform the procedure.

Fowkes and colleagues conclude that, "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."

Ankle Brachial Index Combined With Framingham Risk Score to Predict Cardiovascular Events and Mortality: A Meta-analysis
Ankle Brachial Index Collaboration
JAMA (2008). 300[2]: pp. 197 - 208.
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: Peter M Crosta