Letter by Jaquinandi et al Regarding Article, “Different Calculations of Ankle-Brachial Index and Their Impact on Cardiovascular Risk Prediction”
To the Editor:
We read with great interest the article of Espinola-Klein et al1 about the methods of calculation of the ankle-brachial index (ABI). The article provides excellent evidence that the lower ankle pressure is a better index than the higher ankle pressure for identifying patients at cardiovascular risk. Among the 831 patients included in this study, the authors reported that the minimal observed ABI was 0.10 in the modified definition. Thus, it would appear that they had no missing values: No patient had infrapopliteal artery occlusion.
This result is somewhat different from the reported 4% prevalence of distal artery occlusions reported in previous works using ABI to estimate cardiovascular disease mortality.2 It is also inconsistent with what is known of the radiological imaging of ankle arteries in peripheral arterial disease patients. Lee et al3 used both magnetic resonance (MR) digital subtraction angiography and 2-dimensional time-of-flight MR angiography in 23 patients aged 26 to 85 years with suspicion for osteomyelitis, nonhealing diabetic foot ulcer, and rest pain. Among the 37 extremities imaged (74 arteries), 18% of values were absent on both MR imaging and angiography (6 anterotibial and 7 posterotibial arteries). In a recent article, Bernstein et al4 showed that the prevalence of distal artery occlusion was 20% in a population of 50 patients aged ≥50 years referred for orthopedic evaluation of leg pain without claudication. In the study by Kreitner et al,5 among the 24 extremities imaged (48 arteries) for grade III chronic limb ischemia, 52% of values were absent on both digital subtraction angiography and 3-dimensional contrast-enhanced MR angiography (9 anterotibial and 16 posterotibial arteries).
On the one hand, routine Doppler recording of heavily calcified arteries, even patent arteries, may be impossible because of the complete reflection of the ultrasound signal by the arterial wall. On the other hand, cutaneous lesions can make the cuff application impossible.
Thus, although the percentage of occluded arteries on radiological imaging varies widely (ranging from 4% to 52%) and that of calcified arteries or cutaneous lesions is certainly low, it is quite unlikely that, when studying large groups (>800 patients among whom at least one quarter had peripheral arterial disease1) no ABI values were missing or unavailable. It is more likely that the lowest reported values were based on recordings not of arterial trunks but of small cutaneous or subcutaneous arterioles. The problem remains of what should be done with the expected missing values that should be observed in such populations. One interest of using the highest pressure to calculate ABI is that one can get rid of these missing values if they are observed in 1 of the 2 trunks. If one aims to use the lower ankle pressure for peripheral arterial disease screening as suggested in the article by Espinola-Klein, in the case of a missing artery, ABI should probably be reported as zero.
Espinola-Klein C, Rupprecht HJ, Bickel C, Lackner K, Savvidis S, Messow CM, Munzel T, Blankenberg S. Different calculations of ankle-brachial index and their impact on cardiovascular risk prediction. Circulation. 2008; 118: 961–967.
Resnick HE, Lindsay RS, McDermott MM, Devereux RB, Jones KL, Fabsitz RR, Howard BV. Relationship of high and low ankle brachial index to all-cause and cardiovascular disease mortality: the Strong Heart Study. Circulation. 2004; 109: 733–739.
Bernstein J, Esterhai JL, Staska M, Reinhardt S, Mitchell ME. The prevalence of occult peripheral arterial disease among patients referred for orthopedic evaluation of leg pain. Vasc Med. 2008; 13: 235–238.
Kreitner KF, Kalden P, Neufang A, Duber C, Krummenauer F, Kustner E, Laub G, Thelen M. Diabetes and peripheral arterial occlusive disease: prospective comparison of contrast-enhanced three-dimensional MR angiography with conventional digital subtraction angiography. AJR Am J Roentgenol. 2000; 174: 171–179.