Utility of the Ankle-brachial Index in Clinical Practice: A Q&A with Harry Agis

Harry Agis, MD, is a vascular surgeon and a partner in the New Jersey Vein Institute, Morristown, NJ He speaks with Internal Medicine World Report Editor-in-Chief, Simon Douglas Murray, MD about the utility of the ankle-brachial index (ABI) in screening for peripheral artery disease and atherosclerosis and then will touch briefly on a few other subjects that might be relevant to internists with regards to vascular surgery.

How is ABI used in clinical practice and how do you do it?

The ABI is really a good screening test that is underutilized by the primary care practitioner. It is fairly simple to perform and widely available, and certainly should be part of the armamentarium of any clinician. It is a better and more objective way to assess whether there is peripheral vascular disease. When we look at patients with suspected peripheral artery disease of course we do a thorough clinical history, taking into account risk factors for peripheral arterial disease especially smoking, and diabetes, Subsequently we perform a focused physical exam of the vascular system, feeling for peripheral pulses in the dorsalis pedis and posterior tibial arteries is often the best way to quickly assess circulation in the legs. If you feel the pulses in both, then most likely that person does not have flow limiting peripheral vascular disease. On the other hand, as you know there are people in whom you can’t feel either or both pulses, whether it is due to edema in the legs, or it’s just difficult to palpate the pedal pulses or perhaps they are weak, in this group the ABI gives us an objective method of assessing circulation to the lower extremities.

Does the patient have to have both dorsalis pedal and posterior tibial pulses present for the test to be valid?

You can do the ABI in one of the arteries that you can insonate with the continuous wave Doppler probe and calculate the ABI on the basis of one measurement, but you should note that you could not insonate the other artery. Sometimes one of the arteries is occluded.

What is the technique of actually doing the ankle-brachial index?

In the supine position the blood pressure in both arms are is taken and the systolic brachial pressure is noted. The BP cuff is then placed around the ankle of both legs and the blood pressure is taken again noting for the systolic pressure. In order to do it correctly, you need to use a continuous wave Doppler which is much more sensitive and is the preferred method. We will generally take the higher of the 2 readings to calculate the ABI. We do this in both legs. In case we only are able to identify one of the two pulses then we use that one. The ratio of the highest systolic pressure at the dorsalis pedis or posterior tibial level to the higher of the two brachial pressures is termed ABI. A normal ABI is 1.0 anything below one is abnormal, so an ABI of 0.5 to 0.9 is consistent with moderate to mild arterial disease. An ABI below 0.4 is consistent with severe arterial disease.

An ABI of 1.0 -1.3 is considered normal and no further testing is indicated unless the patient has symptoms consistent with arterial disease like claudication. An ABI above 1.3 usually implies arterial calcification and blood vessels that cannot be compressed, so it is not reliable. This can occur in diabetic patients and patients with end stage renal disease among others. It has been shown that a high ABI is also predictable of coronary events.

You said earlier that the ankle to arm ratio should be about one. Doesn’t gravity play some role in making the leg blood pressures higher than the arm? Secondly I would think that the larger vessels in the legs compared to the arms would make the pressure higher.

Gravity plays no role in this since the patient is lying supine for testing. Secondly many of the vessels in the leg below tibial peroneal branches are much smaller than you think. The common femoral artery, the superficial femoral and popliteal arteries are rather large, and the tibial vessels are small in general, that is why any revascularization of the tibial arteries is a challenging task for the vascular surgeon, and perhaps one of the least favorite operations of vascular surgeons.

As an internist I was thinking that the ABI might be a good clinical screening tool for atherosclerosis in general not just in peripheral artery disease. Atherosclerosis is a disease that affects the coronary arteries, the carotid arteries and the distal extremities and it often progresses step wise from cardiac to carotid to distally. If there is disease peripherally then there is a high likelihood of finding in the carotid s or coronary arteries. ABI is a noninvasive way to gather one more piece of evidence for the puzzle. The only require equipment is a blood pressure cuff and an inexpensive Doppler. It is even reimbursable under Medicare.

There have been several studies suggesting that a low ankle-brachial index correlates with coronary disease with a reasonable degree of certainty, and that low ABI can be a predictor of cardiovascular morbidity and mortality. The same way as I mentioned earlier that a high ABI of 1.4 and above can predict the development of coronary events. The difficulty in interpreting these studies lies in the methodology along with co morbidities and how conclusions are made.

Yes, but if the ankle-brachial index were low it might prompt the clinician to look further at the carotids and the coronary arteries for disease there. We can't very well be doing ultrasounds and stress test on everyone.

The ABI is a reliable way of screening for peripheral arterial disease of the legs and perhaps even as a predictor of atherosclerosis elsewhere, because clinically we know that people with peripheral arterial disease most likely have disease elsewhere. This is quite helpful actually. Now if the ankle-brachial index is abnormal and the patient is symptomatic the next step is to do pulse volume recordings (PVR) and segmental pressures. This is done with a special machine. This machine sequentially takes blood pressures at different places in the leg and records a waveform of the artery. A normal arterial waveform is triphasic, meaning that it has a sharp upstroke, a dicrotic notch and a gradual downward slope diastolic component. The biphasic waveform loses the dicrotic notch and might be indicative of mild arterial disease, on the other hand a monophasic waveform is indicative of severe arterial disease.

By measuring segmental pressures we can better pinpoint where the blockage may be. If we need to get a better idea of the anatomy of the vessels we do a duplex scan, or MRA or CAT. Both these tests are good and whether to use one or the other may depend on what your radiologist is good at reading. These scans can really give multidimensional imaging of the blood vessels in question and pinpoint the lesion. Angiography today is reserved for cases where further definition is needed and to use as a road map for revascularization procedure.

I want to switch gears here and ask you about diagnosing carotid atherosclerosis vis-à-vis ultrasound or MRA.

The initial study to identify carotid artery disease should be a duplex scan of the carotid arteries. This modality allows you to identify each of the extracranial arteries and see in real time what the vessel looks like, we identify areas of stenosis, calcification of the vessels and areas of ulcerated plaques and assess the velocities in a given artery. The interpretation of the study relies on pre-determined velocity criteria. A good vascular lab with state of the art equipment and Registered Vascular Technologists can identify with much accuracy the areas of stenosis. We often send patients to surgery on the basis of the duplex scan alone.

Now that said, if you're thinking of some other problem other than atherosclerosis of the carotid such as tumor on the carotid body or a problem other than atherosclerosis causing obstruction, or if we want to rule out complete occlusion of the vessel we may want to get better imaging with MRA or CTA. If we suspect intracranial disease with siphon lesion, MRA is indicated.

The duplex scan can diagnose a hemodynamically significant stenosis of (of more than 50%) of the internal carotid artery accurately.

A more detailed analysis of the study is beyond the scope of this discussion.

I am confused with what the difference is among the venous Doppler, arterial ultrasound, duplex scan, and Doppler.

You are right sometimes the use of all these terms can be confusing.

Duplex scanners are based on real time B- mode imaging with a built-in Doppler, so you can localize the arterial segment in question and look at the anatomical abnormality, you can then examine the flow characteristics with the Doppler.

A venous duplex scan uses the same technology, and is basically used to image the venous system directly for evidence of venous thrombosis or occlusion. It is also regularly used to determine venous insufficiency in either superficial or deep system. It is very useful to guide some of the modern procedures that we perform for varicose veins, like laser or radiofrequency ablation and to follow these patients post operatively.

Of course the duplex scan has its limitations, for example imaging deep in the abdomen on an obese patients can be challenging, and also it loses sensitivity behind the clavicle to evaluate the subclavian artery or vein.

Arterial Doppler is a commonly used generic term to request an analysis of the segmental pressures of an extremity and to analyze the PVR. It is a very good tool to follow an abnormal ABI for example, or to follow patients after revascularization, the limitation of this technique is that it doesn’t precisely localize the diseased vessel, for further definition you need a duplex scan.

One final question. How you evaluate the patient with swollen legs. This is a common complaint in internal medicine practices.

This is a common reason for consultations. As you know there are many causes of swollen legs ranging from Congestive Heart Failure to parasitic infection due to filiarisis. Other important causes are DVT, lymphedema either congenital or acquired, obstruction of the vein at the level of the pelvis, lymphatic damage due to surgery or radiation therapy, drugs, and even lipedema in obese patients. When I am evaluating the patient my overriding principal is that I want to be sure that they do not have a life threatening problem. A careful history and physical exam may lead to a specific diagnosis but the duplex scan if negative rules out DVT, insufficiency and PAD. If I am unable to define what the problem is, based on duplex scan and physical exam, I usually recommend a trial of compression stockings and re- evaluate in 6 weeks. If the swelling has not gone down I do a CT of the pelvis and abdomen looking for tumors which may compress the vessels. 9 out of 10 times this is negative but we are obliged to look. Missing cancer in the pelvis would be a big problem.

By doing the investigation this way we at least limit the number of CTs we do. If the scan is negative then other cause for the swollen limb need to be pursued.

Agreed. I want to thank you for all the time you spent with us. It’s been very informative.