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.) The cuffs should be the same width or 25 percent larger than the width of the part they encircle. If the cuff is too narrow, the pressure will be falsely elevated. Using a Doppler probe, you would inflate the cuff so pedal sounds are no longer audible. Then slowly deflate the cuff and record the pressure in the cuff when you first hear the sound. This represents the pressure of the artery where the cuff is located.5 According to Bowker and Pfeifer, the pressure changes correlate directly with flow.5 So an index of 0.5 represents only 50 percent of the expected blood flow. A gradient of 40mmHg or greater between the ipsilateral or contralateral levels suggests an occlusion or highly stenotic segment. There is discrepancy with this value as Hoffman suggests 30mmHg and Pellerito suggests 20mmHg.2,4 These values can be misleading if collateral flow is so extensive that a 20-30 mmHg drop does not occur or in a situation in which there are non-compressible vessels due to calcification.1,4,5 Getting The Lowdown On Pulse Volume Recordings And Doppler Waveforms You can obtain the PVR by placing cuffs around the toe, midfoot, ankle, BK, AK and upper thigh. The cuffs filled to 60-65mmHg and the resulting recording reflects blood that pulses beneath a sensor cuff and produces a waveform reflective of arterial pulse. This closely corresponds to direct intra-arterial recordings at that level. The flatter the curve, the slower the flow, the less the volume. Analyze the results for changes between segments and for differences between the contralateral limb. A normal PVR will show fast flow, dicrotic notching and a “teepee” appearance of the waveform. The absence of dicrotic notching occurs with an increase in proximal resistance that you will see when there is less than 50 percent stenosis or good collateral flow. Greater than 70 percent stenosis or low collateral flow gives an “igloo” appearance of the waveform or an oscillatory or flat line in the case of complete occlusion. In this instance, tissue necrosis is likely. When evaluating waveforms in a limb without pathology, you will see a rapid systolic upstroke and usually a peaked appearance. The magnitude of the waveform can be affected by several factors and one should not attempt to make comparisons in the peak height. The reverse segment represents distal resistance, followed by elastic recoil of the vessel wall, which you may not see in a normal patient at the ankle level. You will see deterioration of the waveform just proximal to a lesion and it is always abnormal distal to the lesion. As flow deteriorates, waveforms become flattened and then undulating before they totally disappear. Be aware flow may exist despite absent waveforms because Doppler probes rarely detect flow at less than 6 mL/min. It is for this reason that comparing pressures with waveforms helps to avoid errors in interpretation.5 Physicians will often perform a Doppler evaluation or get an ultrasound of the lower extremity following an abnormal evaluation in cases of known arterial occlusive disease or arterial injury. The goal of the exam is to provide the location, number and severity of arterial lesions. The frequency of the emitted beam is altered by any object moving faster than 6cm/second, such as blood flow in a vessel. The sound beam is reflected from red blood cells and the signal is used to measure segmental systolic pressure and produce flow velocity waveform patterns for analysis. The faster the blood flow, the steeper the recorded waveform and the louder the pitch.2,4,5 The results are read as triphasic, biphasic or monophasic. You can use peak systolic velocity and waveform analysis to quantify and localize the disease. You may hear two distinct sounds with the first being forward flow and the second being backward flow. Normal arteries may produce three sounds. In organic occlusive disease, the intima is disrupted, the lumen is narrowed and the vessel is calcified. This results in blood flowing slower and backward flow being damped out. The pitch will be lower and you’ll only hear one sound (i.e. monophasic). Hoffman stated when performing elective cases like bunion surgery, it is important to do a Doppler evaluation of the deep plantar and digital arteries of the first and second toes in patients with suspected arterial insufficiency.4 This is especially important when you are dealing with diabetes patients who might have occlusive disease distal to the ankle. When Should You Emphasize TCOM? TCOM is indicated in wound evaluation, hyperbaric oxygen therapy, plastic surgery and vascular surgery. You can also use TCOM to determine the level of amputation and adjunctively to help diagnose PVD.6 Accuracy in testing depends on the local factors (skin thickness, capillary formation and density, presence of inflammation or edema) and systemic factors (lung function, FI02, blood Hg level, cardiac output).4,6 Keep in mind you can’t use TCOMs on the plantar aspect of the foot, on curved surfaces like toes or malleoli or when the patient has cellulitis.4 You would obtain readings by placing electrodes in designated areas. The central reference position is 5 cm below the middle left clavicle. The peripheral positions are 10cm AK, 5 cm BK, medial foot and the lateral foot. You would apply a buffer solution to electrodes and the attached probes begin heating underlying skin. After equilibration, you should obtain readings at three- to five-minute intervals and you can add to your baseline testing by including an oxygen challenge with inhalation of 100 percent oxygen. In order to detect positive increases in value, you would need to compare baseline readings to oxygen challenge. A positive response is an increase of at least twice the baseline reading. Successful wound healing has been shown to occur with >40mmHg. Failure to heal is indicated by <20mmHg with necrosis at 10mmH or less.6,7,8 Grolman, et. al., found that patients with a TCOM increase of 10 mmHg or greater had a 70 percent chance to heal ischemic wounds with adjunct therapy.9 They also report that TCOM was an excellent predictor of the severity of tissue hypoxia associated with peripheral arterial occlusive disease. Final Notes NIV is a valuable tool in screening for peripheral arterial disease. The cost of NIV studies is substantially less in comparison to arteriography. Sykes states that overall NIV testing is less than $500 versus angiograms which cost over $2,000.1 It is important to emphasize the role of recognizing peripheral arterial disease and instituting appropriate treatment regimens. A recent study, the Peripheral Arterial Disease (PAD) Detection, Awareness and Treatment in Primary Care, confirmed the relative lack of awareness of PAD by patients and their physicians. Eighty-three percent of patients with prior PAD were aware of their condition, although only 49 percent of physicians were aware of their patients’ diagnosis. Classic claudication was uncommon in 5 to 15 percent of patients.8 The study also confirmed the lack of treatment for patients with PAD in comparison with those having other cardiovascular diseases.8 Dr. Giacalone is a first-year resident and Dr. Khan is a second-year resident at the University of Texas Health Science Center. Dr. Steinberg is an Assistant Professor in the Department of Orthopaedics/Podiatry Service at the University of Texas Health Science Center.
 

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References 1. Sykes M and Godsey J. “Vascular Evaluation of the Problem Diabetic Foot.” Clinics in Podiatric Medicine and Surgery. 15(1): 49-74, January 1998. 2. Pellerito J. “Current Approach to Peripheral Arterial Sonography.” Radiologic Clinics of North America. 39(3): 553-67, May 2001. 3. Dawson, et. al. “Treating Intermittent Claudication Secondary to Peripheral Arterial Disease.” Pharmacy and Therapeutics. 24(12): 616-623, December 1999. 4. Hoffman AF. “Evaluation of Arterial Blood Flow in the Lower Extremity.” Clinics in Podiatric Medicine and Surgery. 9(1): 19-56, January 1992. 5. Bowker J and Pfeifer M. Levin and O’Neal’s The Diabetic Foot. Edition 6, Mosby, St. Louis, Missouri, 2001 6. Rich K. “Transcutaneous Oxygen Measurements: Implications for nursing.” Journal of Vascular Nursing. 19(2): 55-59, June 2001. 7. Axelrod, et. al. “Cost of Routine Screening for Carotid and Lower Extremity Occlusive Disease in Patients with Abdominal Aortic Aneurysms.” Journal of Vascular Surgery. 35(4): 754-758, April 2002. 8. Dieter, et. al. “The Significance of Lower Extremity Peripheral Arterial Disease.” Clinical Cardiology. 25: 3-10, January 2002. 9. Grolman, et. al. “Transcutaneous Oxygen Measurements Predict a Beneficial Response to Hyperbaric Oxygen Therapy in Patients with Nonhealing Wounds and Critical Limb Ischemia.” American Surgeon. 67(11): 1072-1080, November 2001. 10. De Graaff, et. al. “The Usefulness of a Laser Doppler in the Measurement of Toe Blood Pressures.” Journal of Vascular Surgery. 32(6): 1172-1179, December 2000.