To ensure full understanding, the SLP exam by itself is not a reimbursable procedure; but combined with either segmental Doppler or PVR waveforms (covered in the next installment), is reimbursable under CPT code 93923.
As in performing the ankle brachial index (ABI) exam, it is important that the patient be supine WITH THE LEGS LEVEL WITH OR LOWER THAN THE HEART and that appropriately-sized blood pressure cuffs are selected for each limb segment (width of the cuff bladder must exceed 20% of the diameter of the limb segment)
SLP Methodology
Segmental limb pressure measurements were first described by Winsor et al in 1950 using a narrow cuff (10 x 40 cm) at the high thigh level with additional cuffs at the above knee (10 or 12 cm), below knee (10 or 12 cm), and ankle levels (10 cm). The procedure was popularized by Strandness and Bell in the mid 1960s. These investigators noted that while use of the narrow high thigh cuff (typically the thigh cuff is 12 cm wide) resulted in artifactually elevated proximal thigh pressures, accuracy for identification of aortoiliac disease improved. Several investigators have recommended using a 3-cuff technique which incorporates a single, wide thigh cuff (17 to 22 cm width). The wider cuff results in a thigh pressure measurement that approximates the brachial systolic pressure. This technique yields accurate pressures in the remaining segments of the limb but obviates the ability to accurately differentiate inflow from outflow disease. Because of these issues, vascular practitioners now employ the 4-cuff method whenever possible.
Although more specific in identifying disease locations, the 4-cuff method, using a 12 cm wide cuff at high thigh, almost always (except for very thin patients) results in “cuff artifact” because of the discrepancy between the width of the cuff and the diameter of the limb. This artifact is taken into account during SLP interpretation.
SLP Examination Technique
The SLP exam is simply an ABI exam with additional pressure readings obtained at the calf, and (patient height permitting), above knee, and high thigh sites. NOTE: If the patient’s legs are not long enough to place two pressure cuffs above the knee without the cuffs overlapping, or covering the patella, place a single 12 cm cuff at mid-thigh.
As with the ABI examination, segmental limb pressures can be obtained by using a continuous wave (CW) Doppler probe at the posterior tibial (PT), or the dorsalis pedis (DP) artery, or by using photoplethysmographic (PPG) sensors on the great or second toe, to monitor presence or absence of arterial flow.
If using CW Doppler to obtain pressures, place the Doppler probe on the artery (PT or DP) that had the higher ankle pressure, and keep the Doppler probe on that artery while you inflate and deflate the calf, above knee, and thigh pressure cuffs.
Because inaccurate blood pressures can result from the reactive hyperemia that occurs distal to the site of arterial occlusion, care should be taken to always perform pressure measurements moving sequentially from the lower limb segment to the thigh.
If using PPG sensors to obtain pressures, place the PPG sensors on the great or second toe, while you inflate and deflate the calf, above knee, and thigh pressure cuffs. Take care to adjust the PPG gain appropriately so that the opening arterial pulse pressure wave can be differentiated from artifact. (a gain setting that is too high can result in artifact which may mask the return of arterial pulsation during deflation of the pressure cuff).
As in obtaining brachial and ankle pressures, inflate the pressure cuffs to a value 10 to 15 mmHg pressure ABOVE the value where the Doppler or PPG signal is no longer audible and the analog tracing becomes flat lined before beginning cuff deflation. This technique ensures that the artery has been completely occluded. This is important, as there can sometimes be a significant difference between the “closing” (the point at which the arterial pulse ceases) and “opening” (the point at which the arterial pulse reappears) pressures. The opening pressure is used as the pressure value for each limb segment
Contraindications to SLP examinations
There are two notable contraindications to performance of SLP’s:
Clinical signs or symptoms suggesting deep vein thrombosis (DVT), such as pain, tenderness, and/or swelling in one or both leg. Pain may occur only while standing or walking. Additionally check for increased warmth in the skin of the affected leg , erythema, and/or a palpable cord or red streak along the course of a vein.
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Known or suspect history of lower limb stent placement or recent saphenous vein arterial bypass grafting. Endovascular stents are commonly deployed in the peripheral arteries for treatment of occlusive disease. As stent fractures are now garnering significant attention, do NOT attempt to obtain a pressure measurement over a stent. In-situ arterial vein bypass grafts should not be compressed during the early postoperative period.
In Mastectomy patients you may not be able to take both brachial pressures; in this event one arm pressure will suffice. In cases of bilateral mastectomies, consider obtaining pressure measurements at the level of the wrists.
Interpretation of SLP values
A stenosis becomes hemodynamically significant (a pressure-flow gradient develops) when the diameter of a peripheral artery is reduced by more than 50-60%. Given this, limb pressures should be compared vertically and side-to-side. As noted in the first installment, the ankle pressures normally exceed the higher of the two brachial pressures. Therefore, the high thigh pressure should exceed the brachial pressure. Using the standard 12 cm wide high thigh cuff, the pressure at this level should be 20-30 mmHg higher than the highest brachial pressure. Remember to compare side-to-side at the same level. A gradient exceeding 20-30 mmHg between the high thigh pressures suggests flow-reducing disease on the side with the lower pressure. A high thigh to brachial artery index (HTI) can be calculated in a manner similar to the calculation of the ABI. A high thigh index > 1.2 indicates the absence of flow-reducing iliac artery disease while an index < 0.8 suggests iliac stenosis exceeding 50-60% diameter reduction.
Similarly, there should be no more than 20-30 mmHg pressure decrease between adjacent cuffs on a limb or when comparing pressures side to side. A pressure difference of 30 mmHg between sequential or opposing cuff sites is usually indicative of clinically significant arterial obstruction between or underneath the cuffs. In cases of multi-level PAD you may observe more than one significant pressure gradient in the same limb. This finding should correlate with an ABI <0.5.
Important considerations
In obese patients, it is not unusual to have high- thigh pressures 50 to 60 mmHg above brachial pressure due to cuff artifact. This artifact can mask inflow disease if not taken into consideration. Vessel calcification can also create abnormal segmental pressures, which is why it may be helpful to correlate the SLP values with waveforms taken at multiple levels using either CW Doppler or pulse volume recording (PVR).
As with any medical examination, the time involved and the accuracy of the test results will be dependent on the knowledge and experience of the person(s) performing and interpreting the examination. Adherence to the above nationally recognized guidelines will help ensure optimum results. A more complete document with illustrations can be accessed at the Unetixs website.
Courtesy of Unetixs Vascular, Inc. (800) 486-3849 • www.unetixs.com
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