Impact of Ankle Brachial Index Calculations on Peripheral Arterial Disease Prevalence and as a Predictor of Cardiovascular Risk.

OBJECTIVE

The aim of this study was to estimate the prevalence and predictive accuracy for cardiovascular (CV) morbidity by using different ankle brachial index (ABI) calculation methods in the general population.

METHODS

ABI measurements and questionnaire data were collected from 5 080 randomly selected citizens aged 60 - 90 years. A 10 year follow up with data from Swedish national health registries was carried out. ABI was calculated using as numerator the highest (ABI-HI) or the lowest (ABI-LO) ankle BP obtained in each leg. Subjects were defined as references or having peripheral arterial disease (PAD) based on ABI-LO (Group 1) or ABI-HI (Group 2). Prevalence, mortality, CV events and risk were then analysed for these three groups, and their predictive power by using the area under the curve (AUC).

RESULTS

A total of 4 909 inhabitants were included in the cohort (References: 83.8%, Group 1: 6.7% and Group 2: 9.6%). The prevalence of PAD was 16% using ABI-LO, and 9.6% using ABI-HI. The 10 year all cause mortality for references and Groups 1 and 2 was 27.6%, 48.8%, and 67.2%, respectively. The overall age adjusted hazard ratio (95% confidence interval) for the composite outcome of CV mortality and a non-fatal CV event was 1.25 (1.06 - 1.49) for Group 1 and 2.11 (1.85 - 2.39) for Group 2. The prediction accuracy for ABI < 0.9 in predicting CV event measured with AUC was 0.60 for ABI-HI and 0.62 for ABI-LO.

CONCLUSION

An ABI < 0.9 should be considered a strong risk marker for future CV morbidity. Applying the traditional ABI calculation method of using the highest measured ankle BP, a group of subjects with high CV risk may be overlooked for intervention, and this why the lowest ankle BP should be the preferred for risk stratification. However, as a single predictive tool an ABI < 0.9 cannot adequately discriminate which individual will have a future CV event regardless of calculation method used.