Arterial disease is a common problem in the US, and affects the entire body, not just the heart. Peripheral arterial disease, (PAD) is arterial disease found outside the heart, mostly in the arms and legs. A good method to test for PAD is the ankle-brachial index (ABI). This standard exam compares blood pressure in both the ankle and arm, which gives the clinician a valuable amount of information about the blood flow to the lower extremity.
Why would a physician perform or order an ABI?
Typically, a physician will screen for peripheral arterial disease using the ABI if there is a potential to intervene to prevent the progression of PAD and decrease the risk for other cardiovascular disease. Image 1 above represents what the inside of a diseased artery may look like.
What does the ABI tell the Doctor?
The ABI will tell the doctor about both the risk for symptoms (such as poor circulation leading to pain, ulceration, cold feet, and potentially loss of limb) and about the progression of disease. A normal value for the ABI is a ratio of arm to ankle blood pressure of 1.0. In other words, the blood pressure is the same in both the ankle and the arm. Higher blood pressure in the leg than in the arm indicates possible disease. Higher blood pressure means more resistance in the arteries, and the higher the difference between the two, the more severe the disease. The further from 1.0 the blood pressure ratio is, the more serious the disease. Certain symptoms correspond to different values and there is a progression of disease as the ABI value decreases. Intermittent claudication corresponds to values between 0.6-0.8. Intermittent claudication is defined as pain or cramping in the legs after walking a certain distance. Rest pain occurs when the ABI value is between 0.4-0.6. Pain or cramping in the legs when at rest is a sign of significant lower extremity arterial disease. Ischemic ulcerations generally occur when the ABI is less than 0.4. Ischemic or ischemia means a “restriction in blood supply to tissues, causing a shortage of oxygen and glucose needed to keep tissue alive.” Without blood supply, the tissue dies. Dead tissues will eventually turn into a wound.
If the ABI is greater than 1, then it may signify calcification of the arteries of the lower extremity. If the vessel wall is calcified or hardened then it won’t be compressible as a normal artery would be, giving an elevated or false reading.
What other tests are typically performed with an ABI?
Often, the ABI is ordered with “segmental pressures.” The technician puts blood pressure cuffs at different levels of the thigh and leg as depicted in image 2. Segmental pressures are measured at high thigh, above the knee, below the knee, ankle, midfoot, and toe. By reading the blood pressure at different “segments” of the leg, it will tell the doctor the level where the artery is blocked or occluded. The normal pressure is 70-120 mm Hg and any drop in in pressure greater than 30 mm Hg indicates disease in the blood vessel above.
As the blood flows through the arteries, it creates waves that can be recorded. Normal waveforms are triphasic, meaning it has 3 peaks. The waveforms become widened and blunted with severe disease.
Transcutaneous oximetry (TcPO¬2)
A noninvasive procedure called transcutaneous oximetry measures the oxygen level of the tissue beneath the skin, and helps to assess wound healing potential. Research has shown that for a non-diabetic patient, a wound can be healed with 30 mm Hg, but diabetic patients need 40 mm Hg of oxygenation.
1. Sieggreen, M, and Kline RA. Arterial insufficiency and Ulceration: Diagnosis and Treatment Options. Clinical Management Extra, June 2004.
2. Image courtesy of BioMedix Vascular solutions, Inc. 2010
3. Image courtesy of BioMedix Vascular solutions, Inc. 2010