Comparing the ankle (pedal arteries) systolic blood pressure to the brachial systolic pressure is a simple screening test designed to detected peripheral vascular disease and to try and quantify how much blood is getting to the feet. Measurement of this ankle/brachial index can indicate the presence of stenosis or occlusion in the aorto-iliac and leg arteries and the severity of the disease. A continuous wave hand held Doppler unit is used to detect the brachial and distal posterior tibial and dorsalis pedis pulses and the blood pressure is measured using blood pressure cuffs and a conventional sphygmomanometer. The brachial blood pressure is divided into the highest of the PTA and DPA pressures. The brachial artery pressure is usually equal to or slightly less than the pedal artery pressure so a normal resting ABI is equal to 1 or slightly greater than 1. Significant arterial occlusion or multi-segmental disease will result in an ABI of less than 1.
At rest a patient who suffers from claudication may have a normal resting ABI of 1 or greater. After measurement of the resting ABI these patients should be exercised on a treadmill with retesting of the ABI performed after exercise. A marked drop in ABI is often seen post exercise. Other medical conditions such as back related lumbar or sciatic pain and joint and nerve pain can often mimic claudication. The exercise ABI test can confirm or exclude the diagnosis of intermittent claudication and peripheral arterial disease.
The exercise ABI test involves walking on a treadmill at a speed of 3.0km/hr at an incline of 5 degrees for 5 minutes (250 meters) or until the patients is forced to stop because of claudication, breathing difficulties or fatigue. The patient referred for exercise testing should be of adequate coronary and respiratory health to undertake the test.
Stress exercise ABI can be required for younger patients or for patients requiring diagnosis of sports related conditions such as popliteal entrapment, external iliac artery endofibrosis and compartment syndrome. This must be indicated on the referral so that more time is allocated to the test.
PATIENT PREPARATION AND EXAMNATION TECHNIQUE:
The patient should wear appropriate clothing to allow for easy exposure of the arms and lower legs for the application of blood pressure cuffs. The patient should ensure that blood pressure and all other medication has been taken and that they are well hydrated. Any asthmatic and/or angina medication should also accompany the patient.
A baseline measurement of your arm and ankle blood pressures at rest is taken. You will then be asked to walk on the treadmill for 250m or as long as you can comfortably walk until your symptoms become too uncomfortable to continue. After exercise your arm and ankle pressured will be immediately assessed again. If you have any difficulty walking, please inform the sonographer prior to the treadmill test. The study is performed at a speed of 3.0km/hr at in incline of 5 degrees for a period of 5 minutes. This can be varied depending on the patient’s level of fitness. Symptoms are recorded during the study.
Your examination will take approximately 15-20 minutes. Examinations requiring stress exercise (for younger patients or sports related conditions) 30-45 minutes may be required.
Often resting/exercise ABI is combined with duplex ultrasound of the lower limb arteries which requires a longer appointment.
Resting ABI criteria:
- Normal = 1.00 or greater
- Mild claudication = 0.6 – 0.98
- Severe claudication = 0.30 – 0.60
- Rest pain/ischemia = <0.30
The ABI can also be guide to whether healing of distal leg tissue loss is likely.
- Pedal pressure of > or = to 100mm Hg suggests probable healing in the diabetic patient.
- Pedal pressure of > or = to 60mm Hg suggests probable healing in the non-diabetic patient.
Diabetic and renal patients will sometimes have heavily calcified semi-incompressible and incompressible (>300mmm Hg) tibial arteries which means the ABI is not a useful diagnostic test in this instance. A resting ABI greater than 1.35 should be discounted.
A 20% drop in ABI from the baseline value immediately upon cessation of exercise indicates a positive test. The lower the post exercise ABI the greater the significance of the peripheral arterial disease. The interpretation of the test requires the correlation of symptoms experienced during the study with pressure readings.