What You Should Know About Using NIV Studies
When it comes to peripheral vascular disease, you can use many modalities to detect and evaluate this disease. Arteriography is the gold standard and provides excellent anatomic detail, but it is invasive and requires ionizing radiation and administration of contrast. It also provides very limited physiologic or functional information.
Indeed, it’s important to be aware of the role of noninvasive testing measures (see “When NIV Studies Are Warranted”). After all, Pellerito, et. al., pointed out it’s advantageous to use noninvasive vascular (NIV) evaluation prior to angiography in order to screen out those individuals who have adequate flow, thus decreasing the morbidity of angiography by restricting it only to those who require it.2 With this in mind, let’s take a look at testing measures such as the ankle/brachial index (ABI), toe/brachial index (TBI), segmental pressures, pulse volume recording (PVR), Doppler waveforms and transcutaneous oxygen measurements (TCOM)/ transcutaneous partial pressure of oxygen (Tcpo2).
Taking A Closer Look At The ABI And TBI
First up is the ABI (also known as the ischemic index, ankle/arm index, or ankle/wrist ratio), which you can obtain by dividing the systolic pressure of one of the pedal arteries by the brachial artery systolic pressure. You can easily get the ABI when you are performing segmental pressures. Normally, the ankle systolic BP should be less than or equal to the brachial systolic BP. A normal ABI value is 0.9 -1.0. Values of 0.5 to 0.8 indicate one primary arterial occlusion with mild to moderate disease. When you see values that are less than 0.5, your patient most likely has severe, multilevel occlusive disease.3
However, keep in mind these values do not take into account calcification of vessels, which prevents complete compression when testing and can give falsely elevated values. So you should remember that a low ABI is almost certainly pathologic while a normal ABI could be a falsely elevated value in the face of non-compressible vessels. Also be aware an ABI of 1 with good velocity flow in the dorsalis pedis and posterior tibial arteries documents only the adequacy of flow at the level of the ankle.4 It is very possible there are distal occlusions in the foot.
Therefore, you should compare ABI results to the TBI, segmental pressures and PVR for accurate interpretation. ABIs are indicated for any patient with diabetes who has newly detected diminished pulses or wounds that are not progressing as expected. They are also indicated for any patient who has leg pain of unknown etiology. You should perform baseline exams for type 1 diabetes patients over age 35, type 2 diabetes patients over 30 and in any patient who has had diabetes for more than 20 years.
Toe pressures are important predictors in the clinical course of rest pain, skin ulcerations and gangrene in diabetic patients. Several studies have found that a toe systolic pressure of 40mmHg or less is significant. You can use the TBI like the ABI when evaluating flow to the forefoot. When you combine it with toe pulse waveforms, you can identify those patients who have a good prognosis for spontaneous healing. According to Bowker, these patients will have toe systolic pressure greater than 30 mmHg.5
What About Segmental Pressures?
You can obtain segmental pressures by placing pressure cuffs at the high thigh, above the knee, below the knee and at the ankle and toe levels. The recordings reflect the volume of the arterial pulse as it passes under the cuff. (You wouldn’t normally take midfoot pressures because osseous structures surround the arteries and falsely elevate pressures.)