When to order additional testing beyond an ankle-brachial index (ABI)
Three scenarios require further evaluation beyond obtaining an ankle-brachial index (ABI):
- When a pedal vessel is not compressible above 220 mmHg.
- When a patient with normal or mildly reduced ABI ratios presents with claudication.
- When an ABI ratio falls outside of the normal range (e.g., outside a 0.9–1.4 range).
We’ll get into the recommended testing for when an ABI falls outside the normal range, but first, let’s dive into the first and second scenarios in a little more detail.
Scenario 1: A pedal vessel is not compressible above 220 mmHg
When you have at least one pedal vessel that is not compressible at a pressure above 220 mmHg, this indicates medial calcinosis. Medial calcinosis is a hardening of the arterial walls rather than a collection of plaque within the lumen, and it is often seen in patients with diabetes.
If a pedal vessel is not compressible above 220 mmHg, the next step is to obtain a toe-brachial index (TBI). This requires a machine that can produce an analog waveform.
Obtain a TBI
A TBI can determine the presence or absence of blood flow in the toes and gives a ratio similar to an ABI. We use TBIs when the dorsalis pedis artery (DPA) or the posterior tibial artery (PTA) are not compressible, as often seen in patients with diabetes. If there is good blood flow to the toes, then the leg arteries are patent.
A TBI does not directly reflect the degree of peripheral arterial disease (PAD) in the legs; doctors use it as a baseline for serial ultrasound follow-up exams if the ankle vessels are not compressible. Toe-brachial indices focus on feet perfusion by taking pulse recordings of the toes with photoplethysmography (PPG) and taking systolic pressure readings of the big toes.
Foot perfusion can be easily influenced by factors such as room temperature or microvessel disease. For this reason, TBIs are not always as useful as ABIs for determining general PAD.
But, a TBI is better than not having any quantitative information, especially if the concern is a foot ulcer. As well, a PPG waveform can be used to determine the presence or absence of blood flow in the toe digit without strictly depending on the pressure.
A TBI is performed with small PPG light sensors and small toe-sized blood pressure cuffs. The cuff is wrapped around the base of the big toe and the sensor is applied to the toe pad by either a clip, tape, or Velcro strap. From the sensor on each big toe, you receive an analog waveform on the screen. The waveform is just a light sensor, so there is no sound for this reading.
Figure 1. When obtaining a toe-brachial index, a small photoplethysmography (PPG) light sensor takes pulse recordings, and a small blood pressure cuff measures the blood pressure of the big toe.
Photoplethysmography waveforms do not follow the peaks and pits spectrum, and are instead judged mostly by their amplitude. The higher the amplitude, the better the patient’s circulation.
Figure 2. Photoplethysmography (PPG) waveforms are judged mostly by their amplitude. The higher the amplitude, the better the patient’s circulation.
A TBI is calculated in the same way as an ABI, with toe pressure over the highest brachial systolic pressure. A normal TBI often settles around 0.7 but can vary between laboratories.
Figure 3. A toe-brachial index (TBI) is calculated by dividing the patient’s toe pressure by their highest brachial systolic pressure (left or right arm).
Patient case where a TBI was indicated
On a patient’s ABI report, the PTA and DPA pressures were labeled as noncompressible (NC). Since the pedal vessels were noncompressible, a TBI was performed and the indices were normal.
When looking at the report, you can see that the ABI Doppler waveforms are multiphasic. This indicates normal resting arterial perfusion at the ankles. With at least one vessel that is not compressible above 220 mmHg, we can predict some medial calcinosis, but we know from the TBI that the feet are well perfused.
Figure 4. On the ankle-brachial index (ABI) and toe-brachial index reports from a patient with noncompressible pedal pulses, the ABI Doppler waveforms are multiphasic and the photoplethysmography waveforms have a high amplitude, which indicates that the feet are well perfused. The patient likely has medial calcinosis.
Scenario 2: A patient with normal or mildly reduced ABI ratios presents with claudication
The second scenario that requires further evaluation beyond ABIs is when a healthy patient with normal or mildly reduced ABIs (such as 0.8) presents with claudication symptoms. In this scenario, pre- and post-exercise ABIs are indicated.
Perform exercise ABIs
Ankle-brachial index testing is normally performed when the patient is at rest. But, testing of a patient with normal ABIs with claudication symptoms should include blood pressures taken before and after exercise. Exercise such as a treadmill walk or repetitive heel raises is sufficient.
A drop in ABI greater than 0.15 between pre- and immediate post-exercise pressure readings indicates PAD. Remember, depending on where the disease is located, this can be a unilateral or bilateral drop.
Figure 5. When an exercise ankle-brachial index (ABI) is performed, ankle and brachial pressures are taken before and immediately after exercise. A drop in ABI that is greater than 0.15 after exercise indicates the presence of peripheral arterial disease (PAD).
Both waveform and pressure readings are obtained before exercise, but only pressure readings are recorded after exercise. Waveforms are not recorded after exercise because they naturally become more monophasic as the limb vessels expand for more blood flow. So, they do not reflect the true extent of disease.
Another reason waveforms aren’t recorded after exercise is that there isn’t sufficient time. The stress test requires you to record the highest ankle vessel pressure on each side (PTA or DPA) plus the higher brachial pressure (left or right) all within one minute since the values are most accurate immediately after exercise.
Patient cases where exercise ABIs were indicated
Bilateral iliac disease
A patient had normal ABIs and multiphasic waveforms, which indicates normal resting arterial perfusion at the ankle for this patient. However, after exercising, this patient’s bilateral ABI ratios dropped to a level indicating critical arterial insufficiency. The patient was found to have bilateral iliac disease.
Figure 6. A patient who had normal ankle-brachial index (ABI) ratios before exercise experienced a significant drop in both ratios after exercise and was found to have bilateral iliac disease.
Popliteal artery entrapment
Exercise ABI testing is also indicated for more rare forms of PAD, such as popliteal artery entrapment. This testing is especially useful in younger, athletic patients where this condition may occur.
Popliteal artery entrapment syndrome involves the repetitive compression of the popliteal artery by hypertrophic calf muscles which can cause claudication in younger, athletic, and otherwise healthy patients. These patients usually have normal resting ABI ratios that will drop with exercise.
When popliteal artery entrapment is suspected, PPG waveforms should also be obtained. They are recorded during maneuvers of dorsiflexion and plantar flexion with knee hyperextension.
In this case, the patient’s PPG waveforms were normal at rest. However, with maneuvers they became flat, which is indicative of popliteal artery entrapment.
Figure 7. A toe-brachial index (TBI) report from a patient with popliteal artery entrapment syndrome. The resting photoplethysmography (PPG) waveforms are normal, but the waveforms become flat with dorsiflexion and plantar flexion.
Scenario 3: An ABI ratio falls outside of the normal range
The flow of testing to perform when an ABI falls outside of the normal range follows this simple algorithm:
- If an ABI is greater than 1.4, obtain a TBI.
- If an ABI is between 0.9–1.4, you’re either done or perform exercise ABIs.
- If an ABI is less than 0.9, perform duplex and / or exercise ABIs.
- If an ABI is 0.6 or less, perform duplex, but not exercise ABIs.
Table 1. Summary of additional testing to perform after an ankle-brachial index (ABI) ratio falls outside of the normal range.
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