Ankle-Brachial Index (ABI) Measurement

The ankle-brachial index (ABI) is a noninvasive, simple, reproducible, and cost-effective diagnostic test that compares blood pressures in the upper and lower limbs to determine the presence of resistance to blood flow in the lower extremities, typically caused by narrowing of the arterial lumen resulting from atherosclerosis. ^{[1, 2, 3]} The simplicity and low cost of ABI measurement make it a convenient diagnostic test for early detection of peripheral arterial disease (PAD) or acute arterial injury.

In patients with clinically suspected significant PAD, an ABI lower than 0.90 has been reported to have a sensitivity of 79-95% and a specificity exceeding 95%. ^{[4, 5, 6]} In diabetic patients with neuropathy or ulcers, however, the accuracy of the ABI in early detection of PAD is much more variable, with a sensitivity of 29-100% and a specificity of 58-97% as compared with imaging methods (angiography or color duplex ultrasonography [CDUS]). ^[7] Thus, in diabetics with signs of neuropathy or foot ulcers, a normal ABI result should be interpreted with caution, and further testing may be warranted in order to institute guideline-directed medical therapy (GDMT). ^[8]

GDMT for PAD includes interventions to alleviate symptoms from PAD (claudication), to prevent progression to critical limb ischemia (CLI), and to avert major acute cardiovascular events (MACEs); the presence of PAD is a well-established risk factor for coronary artery disease (CAD).

In the acute trauma setting, the ABI can facilitate identification and risk stratification for those at risk for lower-extremity arterial injury. ^{[9, 10, 11]} In the setting of potential arterial injury after acute blunt or penetrating injury to the lower extremity in stable patients, an ABI lower than 0.90 has been shown to have a sensitivity exceeding 87% and a specificity exceeding 97% for identifying lower-extremity arterial injury. ^[10] Furthermore, the ABI can aid in the assessment of postoperative wound healing and ulcer management. ^[12]

Further information on the use of the ABI is available in the PAD management guidelines developed by the American College of Cardiology (ACC) and the American Heart Association (AHA) and in a prior scientific statement from the same organization. ^{[8, 13]}

Detection of peripheral arterial disease

As a general principle, a test for PAD should only be performed either to confirm its presence or to inform treatment decisions. ABI measurement is primarily useful for the following two purposes:

• Symptomatic patients - Claudication (exertional non-joint-related leg symptoms) or CLI ^[8]
• Asymptomatic patients - Identification of patients with PAD as a measure of overall cardiovascular disease (CVD) risk

The 2016 American College of Cardiology (ACC)/American Heart Association (AHA) guidelines on the management of patients with lower-extremity PAD stated that the resting ABI is the initial diagnostic test for PAD and may be the only test required to establish the diagnosis and institute GDMT. ^[8]  These guidelines also provided definitions of acute limb ischemia (ALI) as well as CLI. ^[8]

ALI is defined as the presence, for less than 2 weeks, of signs and symptoms of acute ischemia of a limb and complications thereof, indicated by the presence of the "six Ps":

• Pulselessness
• Pallor
• Pain
• Paresthesia
• Paralysis
• Poikilothermia

The presence of a pulse should be determined by means of continuous-wave Doppler rather than palpation alone, given the unreliability of pulse palpation in this setting.

An acutely ischemic limb can be subdivided into the following three categories:

• Viable - Pulse still present despite the presence of other symptoms of ischemia; audible arterial and venous Doppler, with no motor or sensory loss
• Threatened - Inaudible arterial but audible venous Doppler, with mild-to-moderate motor and sensory loss; may be further subdivided into marginally threatened and immediately threatened
• Irreversible - Inaudible arterial and venous Doppler, with motor and sensory loss (often rigor); permanent nerve damage and major tissue loss are inevitable

CLI is defined as the presence, for greater than 2 weeks, of the following findings, attributable to objectively proven arterial occlusive disease:

• Rest pain
• Nonhealing wound or ulcer
• Gangrene

Arterial disease can be proved objectively by measuring the ABI, the toe-brachial index (TBI), transcutaneous oxygen pressure, or skin perfusion pressure. A very low ABI or TBI does not necessarily mean that the patient has CLI.

Detection of arterial injury

In the emergency or trauma setting, ABI measurement is useful for the evaluation of a patient who is at increased risk for lower-extremity arterial injury, as follows:

• An ABI lower than 0.90 suggests a need for further vascular imaging: angiography in a stable patient, and operative exploration in an unstable patient
• An ABI higher than 0.90 is associated with a decreased likelihood of an arterial injury; thus, the patient may be observed with serial ABI assessments or may undergo a vascular study on a delayed basis ^[14]

The current utilization of ABI in trauma is limited, as a consequence of the increased use of routine computed tomography (CT) in these settings; it is often deferred in favor of CT angiography (CTA) of the lower limbs, which can be performed while other parts of the body are being imaged.

Contraindications for obtaining the ABI include the following:

• Patients who are unable to remain supine for the duration of the examination
• Patients in whom the use of an occlusive sphygmomanometer cuff may worsen the extremity injury
• Patients with known or suspected deep venous thrombosis (DVT) - Compression of the extremity should be avoided out of concern for breaking and embolizing a thrombus