There are numerous non-invasive tests available to assess lower-extremity peripheral arterial disease (PAD), but there remains some confusion among practitioners as to which non-invasive study should be performed on a given patient. A patient presenting with critical limb ischemia may warrant a different initial examination from a patient presenting with intermittent claudication or an asymptomatic diabetic patient with no palpable pulses. As space does not permit, this article is not a guideline or definitive treatise on the diagnosis of lower-extremity PAD, but a resource on the importance of the ankle–brachial index (ABI) and segmental limb pressures (SLP) in the diagnosis of lower extremity PAD. For a broader and more in-depth view of which test to perform on a given patient, please see the American College of Cardiology (ACC)/American Heart Association (AHA) guidelines (www.americanheart.org/downloadable/heart/1137607531918PAD_ES_MASTER.pdf) or the complementary guidelines from the Intersocietal Commission for the Accreditation of Vascular Laboratories (ICAVL; www.icavl.org/icavl/pdfs/arterial2007.pdf). Please note that many states have restrictions on who can perform non-invasive vascular studies. Please contact your local Medicare provider for more information, or visit the ICAVL website for information on reimbursement requirements by state (www.icavl.org/icavl/reimbursement/terms.htm).
As a final note, ABIs and lower-extremity segmental limb pressure (SLP) measurements alone are not an adequate test to diagnose the presence or absence of lower extremity arterial disease. As such, it is highly recommended that segmental pulse volume recordings (PVRs) or continuous wave (CW) Doppler waveforms accompany these pressure measurements. PVRs or CW Doppler waveforms are also necessary for Medicare reimbursement under CPT codes 92922, 93923, and 93924.
Many practitioners overlook the basic fundamentals when carrying out the ABI and SLP exams. In order to ensure that blood pressure measurements are accurate, patients must lie supine on the examination table with the legs level with or lower than the heart and a pillow elevating the head. Blood pressure cuffs should be selected accordingly such that the width of the cuff bladder exceeds by 20% the diameter of the limb segment.
The current gold standard for obtaining arm and ankle pressures is by a continuous-wave Doppler probe for determining the presence or absence of blood flow. For ankle pressures, the Doppler probe allows the clinician to obtain pressures individually from the dorsalis pedis and posterior tibial arteries, thereby providing an accurate detection of flow-reducing arterial disease in specific vessels, and an estimate of the probability of collateral tibial arterial flow (see Figure 1). Although the use of photoplethysmographic (PPG) sensors in measuring arm and ankle pressures is quick and allows for a shortened examination time, accuracy and specificity are lower.
The Ankle–Brachial Index
After the patient has been resting in the supine position for 10 minutes, appropriately sized cuffs should be applied to both biceps areas. The Doppler probe should be held at 45º to the arm and positioned to obtain the strongest signal at the brachial artery. If using photoplethysmography (PPG), the sensor should sit snugly on the index or middle fingers of the patient, at which point a waveform will be seen pulsing on the instrument screen. Many vascular devices are now designed to automatically inflate and deflate the pressure cuff and record the systolic pressure at which the pulse returns. If this option is not available, inflate the cuff on the right arm until the Doppler sound ceases or the PPG waveform ‘flat-lines’, at which point the pressure should be bled out of the cuff at a slow rate of 2–3mmHg per second until the Doppler sound returns or PPG tracing resumes pulsation. Repeat this procedure for the left arm. The typical patient should have inter-arm pressures within 10–15mmHg of each other. However, it is not uncommon for the left arm pressure to be 15–20mmHg lower than the right arm pressure if Doppler is used—this is often an artifact caused by the patient tensing up upon hearing the Doppler sound. If this difference is seen, a repeat measurement of the right arm pressure will often provide a lower value. Brachial blood pressure differences of ≥20mmHg suggest the presence of stenosis or occlusion of the subclavian, axillary, or brachial artery, which would lead to a lower brachial pressure on that side. The higher of the two arm pressures is the ‘brachial reference’ and is used to calculate ABI.
The appropriately sized cuffs should be applied to both lower calves immediately above the ankles, taking care not to position the cuff too low on the ankle, which could interfere with Doppler probe positioning. A very light touch should be used when applying the Doppler probe to the dorsalis pedis artery to avoid compressing the artery. Complications around the medial malleolus can mean that a Doppler angle of 90º to the skin surface might yield better signals for the posterior tibial artery. The use of PPG in obtaining ankle pressures is similar to that of brachial pressures: the sensors should be affixed securely to both great toes, the ankle cuff inflated until the PPG signals disappear, and the pressure bled off slowly until the pulse returns. The ankle arteries are frequently affected by vessel calcification, particularly in patients with diabetes, and it is not uncommon to inflate the pressure cuff to 250mmHg and still detect arterial pulsation. However, it is generally not necessary to inflate the ankle cuff beyond 50mmHg of the highest brachial pressure.
Interpreting the Ankle–Brachial Index
The ABI can be calculated by dividing the ankle pressures by the higher of the two brachial pressures and recording the value to two decimal places. The general diagnostic values for the ABI are shown in Table 1. The lower the ABI, the more severe the PAD. If ankle vessels are non-compressible or yield an ABI greater than 1.3, the practitioner should consider taking digit pressures at the great toes and calculating a toe–brachial index (TBI) to better assess arterial perfusion to the feet. This is often performed in patients in whom PAD is clinically suspected but results from the ABI are unreliable due to non-compressible vessels, such as in elderly patients or those with long-standing diabetes.
Segmental Limb Pressure
Segmental limb pressure measurements were first described in 1950, where Winsor et al. used a narrow cuff (10x40cm) at the high thigh level with additional cuffs above the knee (10 or 12cm), below the knee (10 or 12cm), and at ankle levels (10cm). In the 1960s, Strandness and Bell found that although the atypically narrow high thigh cuff (generally, the thigh cuff is 12cm wide) resulted in artifactually elevated proximal thigh pressures, accuracy improved for the identification of aortoiliac disease.
Several investigators have recommended using a three-cuff technique incorporating a wide thigh cuff (17–22cm width). Although this approach results in a thigh pressure measurement that accurately approximates the brachial systolic pressure, it prevents the ability to accurately differentiate inflow from outflow disease. These issues have prompted vascular practitioners to employ the four-cuff method in current clinical practice whenever possible (see Figure 2). This method is more specific in identifying disease locations. However, the use of a 12cm-wide cuff at high thigh in the four-cuff method more often than not results in ‘cuff artifact,’ except in very thin patients, because of the discrepancy between the cuff width and the limb diameter. This artifact is considered during SLP interpretation.
The SLP examination is essentially identical to the ABI examination, with additional pressure readings at the calf and, patient height permitting, at sites above the knee and on the high thigh. If the patient’s legs are not long enough to place two pressure cuffs above the knee without overlap or coverage of the patella, a single 12cm cuff should be placed at mid-thigh. If the Doppler is used to obtain SLP, the probe should be positioned on the artery with the higher ankle pressure, either dorsalis pedis or posterior tibial, and kept in place while the pressure cuffs at the calf, above the knee and thigh are inflated and deflated. Pressure measurements should always be performed while moving sequentially from the lower limb segment toward the thigh. This is due to reactive hyperemia that occurs distal to the site of arterial occlusion, which can result in inaccurate blood pressures. If PPG sensors are used to obtain SLP, the sensors should be placed on the great or second toe while the pressure cuffs at the calf and above the knee and thigh are inflated and deflated.
The PPG gain should be appropriately adjusted so that the opening arterial pulse pressure wave can be differentiated from artifact waves. A gain setting that is too high can cause artifacts that may mask the return of arterial pulsation during pressure-cuff deflation. Similar to ABI measurements, the leg pressure cuffs should be inflated to 10–15mmHg above the point where the Doppler or PPG signal is no longer discernable and the analog waveform tracing becomes flat-lined before initiating cuff deflation. This ensures that the artery is completely occluded—an important step since a significant difference can occur between the closing and opening pressures, or the points at which the arterial pulse ceases and reappears. The opening pressure obtained is used as the pressure value for each limb segment.
Interpreting Segmental Limb Pressure Values
SLPs should be compared vertically and side to side. This is because a pressure–flow gradient develops and a stenosis becomes hemodynamically significant when the diameter of a peripheral artery is reduced by >50–60%. The high thigh pressure should exceed the brachial pressure by approximately 20–30mmHg when using the standard 12cm-wide high thigh cuff. Side-to-side pressure at the same levels should be compared; a gradient exceeding 20–30mmHg between the high thigh pressures is suggestive of flow-reducing disease on the side with the lower pressure. A high thigh to brachial artery index (HTI) is calculated by dividing the high thigh pressures by the higher of the two brachial pressures. An HTI value >1.2 indicates an absence of flow-reducing iliac artery disease, while an HTI value <0.8 suggests iliac stenosis exceeding a 50–60% diameter reduction. Other comparisons of pressures between adjacent cuffs on a limb or side to side should yield no more than a 20–30mmHg pressure difference. Such a difference between sequential or opposing cuff sites generally indicates clinically significant arterial obstruction between or underneath the cuffs. In cases of multilevel PAD, more than a single significant pressure gradient may present in the same limb. This finding should correlate with an ABI <0.5, indicative of severe/critical disease.
Points to Consider
High thigh pressures that are 50–60mmHg above the brachial reference pressure are not uncommon in obese patients. This cuff artifact can mask inflow disease if not taken into consideration. As vessel calcification can also create abnormal segmental pressure, it is helpful to correlate SLP values with waveforms taken at multiple levels using either Doppler or pulse volume recording. There are certain medical conditions that can preclude the performance of pressure measurements, such in cases where patients have deep vein thrombosis, a history of lower limb stent replacement, or recent saphenous vein arterial bypass grafting or are mastectomy patients.
Symptoms of deep vein thrombosis include pain, tenderness, and possible swelling in one or both legs. This may also present with increased warmth in the skin of the affected leg, erythema, and a palpable cord or red streak along the course of a vein.
Endovascular stents are commonly used in the peripheral arteries to treat occlusive disease, and pressure measurements should not be taken over a stent due to risk of stent fractures. Furthermore, in situ arterial vein bypass grafts should not be compressed in the early post-operative period. It may not be possible to take both brachial pressures in mastectomy patients if axillary node dissection was performed; one arm pressure will suffice in such a situation. For patients who have undergone bilateral axillary node dissections, pressure measurements at the level of the wrists are recommended. As in any medical examination, the length of the examination and the accuracy of the test results will depend on the knowledge and experience of the practitioner carrying out the examination. Adherence to the nationally recognized guidelines will help ensure optimum results.