ABI Can Help With Risk Stratification, Study Shows, While Other Research Questions ABI Cut Points

Shelley Wood

March 17, 2009

From Heartwire — a professional news service of WebMD

March 17, 2009 (San Francisco, California and Chicago, Illinois) — A small but meaningful proportion of people deemed to be at low or intermediate risk of coronary events according to Framingham criteria actually have evidence of peripheral artery disease (PAD) known to correlate with increased risk of CVD events, new research shows [1].

In a second study, researchers propose that the definitions of "normal" and "abnormal" ankle-brachial index (ABI) may need to be reconsidered [2].

In the first study, investigators looked at the incremental value of using of abnormal ABI, elevated CRP, and elevated fibrinogen among participants in the 1999-2004 National Health and Nutrition Examination Survey (NHANES 1999-2004) and found that these more novel risk factors are found in almost 45% of subjects not considered to be high risk by Framingham alone. Dr Timothy P Murphy (Rhode Island Hospital, Providence) presented the results from the NHANES analysis last week at the Society of Interventional Radiology (SIR) 2009 Scientific Meeting.

"These simple tests--such as ABI screening--have the potential to improve the accuracy of CV risk prevention and thereby have significant public-health impact by helping identify people for intensive medical therapy and preventing heart attacks and strokes," Murphy commented in a SIR press statement.

He and his coinvestigators reviewed screening results for 6292 men and women over age 40 and compared it with other risk-factor information used to categorize Framingham risk. Using Framingham criteria, 91% of the NHANES cohort fell into the low- or intermediate-risk groups, they report. But when ABI, fibrinogen, and CRP information was used, a full 44.7% of subjects were found to have abnormal ABI and/or elevated fibrinogen and/or CRP.

Risk Factor Prevalence, by Framingham Risk Group

Additional risk markers Low/intermediate Framingham risk score (%) High Framingham risk score (%)
Normal ABI (≥0.9) 97.0 89.5
Abnormal ABI (<0.9) 3.0 10.5
Normal fibrinogen (<400 mg/dL) 83.1 72.0
Elevated fibrinogen (≥400 mg/dL) 16.9 28.0
Normal CRP (≤3 mg/L) 61.3 53.9
Elevated CRP (>3 mg/L) 38.7 46.1

While the study did not link risk factors to outcome data, Murphy believes the use of these less conventional risk factors may help boost CV prevention efforts.

"Even with abnormal ABI, which was the least prevalent of the three novel risk factors evaluated, that number translated into about 2.1 million Americans age 40 and older who have no known history of heart disease, stroke, diabetes, or atherosclerotic vascular disease," Murphy stated in the press release. "There is also a good chance that that ABI, which actually detects subclinical already-established atherosclerotic disease, may actually perform better in terms of risk prediction than fibrinogen or CRP because it may be more specific."

Reconsidering the ABI Cut Points

In other PAD research this week, researchers for the Walking and Leg Circulation Study (WALCS), writing in the March 24, 2009 issue of the Journal of the American College of Cardiology, say the cutoff used to define "normal" ABI may need to be redefined. Dr Mary McDermott (Northwestern University, Chicago, IL) and colleagues reviewed ABI measures among 666 subjects, of whom 412 had known PAD, then followed them for five years. They report that, as expected, people defined as having a low ABI (<0.90) have significant and progressive declines in function and mobility over the five years of follow-up. But McDermott et al also found that having borderline ABI (0.90-0.99) and low-normal ABI (1.00-1.09) were also associated with developing mobility loss; borderline ABI was also associated with becoming unable to walk for six minutes continuously over five-year follow-up.

The authors point out that people defined as having borderline or low-normal ABI measures are already known to have higher prevalence of subclinical coronary and cerebrovascular atherosclerosis; these new data show that these groups are also at higher risk of suffering functional limitations.

"Even though cardiologists may be specifically focused on the heart, I'm sure they want to have their patients improve their overall quality of life, remain active in their communities, and not require institutionalization," McDermott told heartwire in an interview. "This is important because this is a condition that cardiologists see a lot, and it tells us that even borderline and low-normal ABI values should be attended to because they're associated with this increased risk of functional decline."

Moreover, this is a group that finds it particularly difficult to get the kind of regular physical activity that their cardiologists may be urging them to do. "The physician needs to counsel them about the fact that it's okay if they need to stop in their exercise routine and rest, but that they should start walking again as soon as they are able," McDermott said.

McDermott says that knowledge of the link between PAD and coronary disease, MI, and stroke has been around for 10 to 15 years, but that cardiologists may not always think to screen their patients for PAD. And when they do, McDermott believes a cut point of <1.0, and not <0.9 as current guidelines suggest, is probably a "very reasonable" strategy.

"A truly normal ABI is probably 1.08, so if it's less than 1.00, that's already getting into the mildly abnormal or low range," she told heartwire . As such, physicians need to consider ABI as a continuum of risk, she said.

In an accompanying editorial [3], Dr Heather L Gornik (Cleveland Clinic, OH) calls McDermott et al's study "important new data" for understanding functional impairment and disability in people previously classified as "normal."

"The ABI is a stalwart tool," Gornik writes, "but there is a need for updated consensus guidelines as to its optimal interpretation in vascular practice."

Authors for both studies disclose having no financial conflicts of interest.

  1. Dhangana R, Murphy TP, Ristuccia MB, et al. Prevalence of low ankle brachial index, elevated plasma fibrinogen, and CRP among those otherwise at low-intermediate cardiovascular events risk: Data from the National Health and Nutrition Examination Survey (NHANES) 1999-2004. SIR 2009 Scientific Meeting; March 7-12, 2009; San Diego, CA. Abstract 146. Available at: https://download.journals.elsevierhealth.com/pdfs/journals/1051-0443/PIIS1051044308012372.pdf

  2. McDermott MM, Guralnik JM, Tian L, et al. Associations of borderline and low normal ankle-brachial index values with functional decline at 5-year follow-up: the WALCS (Walking and Leg Circulation Study) J Am Coll Cardiol 2009; 53:1056-1062.

  3. Gornik HL. Rethinking the morbidity of peripheral arterial disease and the "normal" ankle-brachial index. J Am Coll Cardiol 2009; 53:1063-1064.

The complete contents of Heartwire , a professional news service of WebMD, can be found at www.theheart.org, a Web site for cardiovascular healthcare professionals.

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