Sonography - Lower Extremity

Lower Extremity Arterial Evaluation
Ankle – Brachial Index (ABI)

Mira L. Katz, Ph.D., R.V.T.
University of North Carolina at Chapel Hill
Chapel Hill, North Carolina

Sonography - Lower Extremity

Type of Examination

Ankle-Brachial Index: Lower Extremity


Objective

To identify and document the presence of lower extremity arterial disease


Indication(s)

  • Claudication
  • Rest pain
  • Ischemic ulcers, gangrene
  • Baseline study prior to surgical/radiological procedures


Limitations/Contraindications

Casts, etc.


Equipment & Supplies

  • Blood pressure cuffs (cuff width: 10-12 cm)
  • Duplex imaging system: Logiq 700
  • Transducers
  • Ultrasound gel
  • Towels


Patient Preparation & Positioning

  • The ankle-brachial pressure examination is explained to the patient and any questions are answered.
  • A history is obtained from the patient focusing on risk factors, signs, and symptoms of lower extremity arterial disease.
  • The risk factors associated with lower extremity arterial disease are: age, hypertension, diabetes mellitus, elevated cholesterol, tobacco smoking, documented atheroslcerosis in the coronary or carotid system, previous lower extremity arterial intervention, and a family history of atherosclerosis.
  • If the patient is symptomatic, unilateral or bilateral lower extremity involvement should be documented. A history of claudication should include the leg involved, the area of pain (calf, thigh, buttock), the distance walked prior to onset of pain, the total walking distance, and the duration of the symptoms. A history of rest pain should document the foot involved and the duration of the symptoms. The history form should also document any evidence of lower extremity arterial disease (lack of pulses, ischemic ulcers, gangrene).
  • The patient is in the supine position with their legs straight. The patient’s head may rest on a pillow, but the patient’s legs should be at the level of the patient’s heart.


Technique

General Technical Points

  • Before beginning the examination, there should be a 15-minute rest period to allow the patient’s blood pressure to stabilize and the legs to recover from walking to the examination room.
  • The measurement of the ankle pressure will be affected by the size of the blood pressure cuff. A blood pressure cuff with a bladder width of 10-12 cm is usually used to measure ankle pressures. When the width of the cuff is small compared to the girth of the limb, the pressure in the cuff may not be completely transmitted to the arteries, and measured pressures are often falsely elevated. Pressures may be falsely elevated in obese patients, and a pressure may be falsely lower in extremely thin patients. The cuff/limb ratio should be kept in mind when the patient’s legs are either abnormally large or abnormally small.
  • A longitudinal imaging plane should be used to obtain the audible arterial Doppler signals. In addition, a small sample volume size should be used and a 60 degree angle.
  • The pressure applied to the skin by the ultrasound transducer must maintain good contact but cannot be excessive or it may obliterate the arterial Doppler signal.
  • At the end of the arterial examination, the blood pressure cuffs and the ultrasound gel should be removed from the patient. Any excess gel should be removed from the transducer, and it should be cleaned using a disinfectant.


Pressure measurement

  • Blood pressure cuffs are placed bilaterally on the upper arm (brachial pressure), and at the ankle just above the medial malleoli.
  • Using the ultrasound transducer locate the arterial Doppler signal distal to the blood pressure cuff. The Doppler signal from the brachial artery is used to obtain the arm pressure, and the Doppler signal from the Dorsalis pedis and Posterior tibial ateries are used to obtain the ankle pressure.
  • At each location, the blood pressure cuff is inflated about 20 to 30 mmHg above systolic pressure (the audible arterial Doppler signal will disappear).
  • The blood pressure cuff should be slowly deflated, noting the pressure at which the audible arterial Doppler signal returns.
  • The ankle pressures are recorded using the Doppler signals from both the Dorsalis pedis and Posterior tibial arteries.


Interpretaion of ABI

  • See accompanying Table.
  • The brachial pressures should be obtained in both arms. If a difference of ≥ 20 mm Hg occurs between arms, an arterial obstruction (usually the sublcavian artery) is suspected on the side with the lower systolic pressure.
  • Because systemic pressures vary from person to person, and in the same patient from time to time, absolute pressures are not used to categorize or follow patients.
  • The ankle-brachial index (ABI) is expressed as a ratio and is calculated by dividing the ankle pressure by the highest brachial pressure. This pressure index is used to follow patients.
  • ABIs are usually divided into four main categories: normal, claudication, rest pain, and tissue loss. Most patients’ clinical symptoms and their ABIs fit into these four categories, but there tends to be some overlap between groups.
  • Under constant conditions and with careful measurements, an ankle-brachial index (ABI) change of more than 0.l0 or 0.15 should be considered significant.
  • Modest variability in measuring ankle pressures is likely to be associated with normal patient or observer variability.
  • Lower extremity pressures may be artifactually elevated because of medial calcinosis, medial sclerosis, or both, with pressure indices exceeding 1.25. When this occurs, ABIs are not reliable.
  • If arterial disease is identified by the ankle-brachial pressure index, the following information should be reported: the lower extremity involved (right, left, bilateral), and the ABI (including the range).
  • The report should specify which arteries were used to calculate the ABI and if an artery (Dorsalis pedis or Posterior tibial) was not located either due to technical difficulties or possible occlusion.


Table 1.
Interpretation Of Ankle-Brachial Pressure Index (ABI) Worksheet
Clinical Presentation Ankle-Brachial Index
Normal > 0.95
Claudication 0.50-0.95
Rest pain 0.21-0.49
Tissue loss < 0.21