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Thursday, September 22, 2016

From the SIR Residents and Fellows Section (RFS)

Teaching Topic: Position Statement on Noninvasive Imaging of Peripheral Arterial Disease by the Society of Interventional Radiology and the Canadian Interventional Radiology Association

Dhanoa D, Baerlocher MO, Benko AJ, Benenati JF, Kuo MD, Dariushnia SR, Faintuch S, Midia M, Nikolic B. Position statement on noninvasive imaging of peripheral arterial disease by the Society of Interventional Radiology and Canadian Interventional Radiology Association. J Vasc Interv Radiol. 27: 947-51.

This manuscript reviews and provides recommendations for noninvasive lower-extremity imaging of PAD. This includes both functional and anatomic tests. The functional or physiologic tests include the ankle-brachial index (ABI), segmental limb pressures, pulse volume recordings (PVRs), segmental Doppler waveforms, and oxygen testing. The anatomic tests include duplex ultrasound (US), computed tomography (CT), and magnetic resonance (MR) imaging. Because of the complexities and degree of discussion needed for each study, CT and MR imaging will be discussed in a future manuscript. Given changes to our delivery of healthcare in the United States, the manuscript serves an important role in defining the appropriate use of noninvasive imaging to improve patient selection and documenting post-procedure outcomes.

Clinical Pearls

What imaging modalities constitute a complete noninvasive examination of peripheral arterial disease?

A typical noninvasive examination should always include an ABI with concomitant pulse volume recordings (PVRs), continuous-wave Doppler analysis, segmental pressures, and exercise testing.

Why is a position statement on noninvasive imaging of PAD needed?

The implementation of the Affordable Care Act has brought about drastic changes to the reimbursement models for medicine. Alternative methods have emerged to enact the intended paradigm shift towards value-based and outcome-oriented delivery of healthcare instead of the standard, traditional merit-based fee-for-service model. Thus, the appropriate use of noninvasive imaging to improve pre-procedural patient selection, as well as to objectively document post-procedure outcomes, is of critical significance.

Questions to Consider

How are ABIs calculated and how are results utilized to grade PAD?

ABIs are calculated by dividing the ankle systolic blood pressure by the brachial artery systolic blood pressure. Both upper extremity BP are obtained and the higher of the two are utilized. For the ankle systolic BP, the greater of the dorsalis pedis or posterior tibial artery should be used.

If there is a >15mmHg discrepancy between the upper extremity systolic blood pressures, hemodynamically significant disease should be considered to be present proximal to the brachial artery with the lower systolic blood pressure.

Stielger et al. proposed an ABI-Based Grading Scale as follows:
  • >1.3 : Falsely high value (suspicious for medial sclerosis) 
  • 0.9-1.3 : Normal 
  • 0.75-0.9 : Mild PAD 
  • 0.4-0.75 : Moderate PAD 
  • <0.4 : Severe PAD 
What are the important limitations to ABIs?

ABIs can be falsely elevated in patients with heavily calcified arteries. In these circumstances, it is recommended that toe brachial index be utilized as calcifications are rarely found at the great toe. A TBI of >0.65 is considered normal and TBI < 0.4 is considered severe PAD.

What are the four phases of a normal typical waveform in segmental PVR?

The four phases are:

1. A rapid systolic upstroke
2. Rapid diastolic downstroke
3. Prominent dicrotic notch (*The dicrotic notch denotes the closing of the Aortic Valve)
4. Normalization to baseline before the next cycle

How do you interpret arterial Doppler waveforms?

Normal arterial Doppler waveforms are triphasic consisting of a sharp systolic upstroke, reversal of flow below baseline and then a short forward component in late diastole. Mild PAD leads to a bisaphic waveform in which the short forward component in late diastole is lost. As PAD progresses to severe disease, the waveform continues to flatten and becomes monophasic with loss of the flow reversal. The waveform becomes rounded with a slow upstroke and slow downstroke creating the classic postobstructive tardus parvus signal.

Post Author:
Andrew Niekamp, MD
Diagnostic Radiology Resident, PGY-3
UT Houston

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