A Closer Look At Perfusion Assessment Techniques

In recent years, the concept of multidisciplinary wound care has gained increasing momentum. Wound care journals cite example after example of podiatrists teaming with various disciplines including vascular specialists, nutritionists, endocrinologists, infectious diseases specialists and nurses from different care settings.¹ This type of collaborative effort has certainly benefited patients. In addition, practitioners have gained access to a wealth of information outside of their traditional practice parameters.    Perfusion assessment is one such area. Beyond the simple pulse examination, podiatrists have access to a number of noninvasive vascular laboratory studies. By learning how to use and interpret these studies, podiatrists are better equipped to treat patients and communicate with vascular specialists.    One of the simplest ways to assess blood flow in the office is with a continuous wave Doppler. A Doppler uses individual crystals to send and receive ultrasound signals of a single frequency. This particular assessment of blood flow lacks depth precision and the ability to pinpoint location that is associated with pulsed wave Doppler ultrasound. However, continuous wave Doppler is useful in measuring blood flow velocity. The practitioner is also able to distinguish triphasic, biphasic and monophasic signals. A triphasic waveform demonstrates a rapid systolic upstroke with a reverse flow in early diastole, followed by forward flow. In a diseased vessel, the dicrotic notch indicating reverse flow is lost and a biphasic signal is audible. Finally, a monophasic signal indicates more severe disease and further perfusion assessment is likely needed.

Current Insights On The Use Of ABI And TBI

Another simple perfusion assessment tool is the ankle brachial index (ABI). Using a handheld Doppler and blood pressure cuff, one can obtain the ABI, the ratio of the systolic ankle pressure divided by the highest systolic brachial pressure. Physicians should obtain the ankle pressures by evaluating both the dorsalis pedis and posterior tibial arteries. Often, one only reports the higher ankle pressure, which may not be reflective of significant occlusive disease in the other vessel.    Although there is slight variability from center to center, ABI interpretation is largely standardized. An ABI > 1.3 is abnormal and consistent with calcified vessels, but is unreliable as a single measure of perfusion. An ABI from 0.91 to 1.30 is normal while 0.70 to 0.90 indicates mild arterial disease and 0.40 to 0.69 indicates moderate arterial disease. An ABI < 0.40 (or absolute ankle pressure < 50 to 70 mmHg) represents severe arterial disease and is one of the hemodynamic definitions of critical limb ischemia.²    Although ABIs are reportedly 90 percent sensitive and 95 percent specific for angiographically defined peripheral arterial disease (PAD), there are limitations to the test’s use.³ Ankle brachial indices may be abnormally elevated in patients with diabetes and end-stage renal disease due to non-compressible vessels. Normal resting values may become abnormal after exercise in symptomatic patients and are often insensitive to mild aortoiliac inflow disease. Finally and perhaps most importantly, ABIs do not provide information about perfusion in the foot. Patients may have profound small vessel arterial disease despite normal resting values.    Alternatively, the toe brachial index (TBI) provides useful information about small vessel perfusion and wound healing potential in the distal periphery. A TBI ≥ 0.7 is normal while an absolute pressure of 50 to 70 mmHg indicates moderate arterial disease and < 30 to 50 mmHg indicates critical limb ischemia.² Measurements < 30 mmHg indicate very poor healing potential and are predictive for amputation with 75 percent sensitivity and 86 percent specificity.⁴    The segmental blood pressure examination is simple, noninvasive and provides information to localize arterial disease and determine severity. In conjunction with Doppler ultrasound instrumentation, pneumatic cuffs measure blood pressure at the arm and at single or multiple sites on the thigh, calf, ankle, foot and toe. Typically, the test analyzes five vascular anatomic levels: • aortoiliac • common femoral/profunda artery • superficial femoral/popliteal artery • tibial vessel (infrapopliteal) • small vessel (foot)    A segmental pressure drop > 20 mmHg between cuffs or in comparison to the contralateral limb is consistent with obstruction at that level.

What Pulse Volume Recordings And Arterial Duplex Ultrasound Can Tell You

Alternatively, waveforms can interpret disease in the setting of calcified vessels. For example, in the graphic above, waveform analysis demonstrates a significant decreased amplitude at the right calf in comparison to the right low thigh (and the left calf) consistent with severe superficial femoral/popliteal disease on the right. Additionally, metatarsal and toe tracings can help evaluate small vessel perfusion.    Pulse volume recordings (PVR) often occur in conjunction with segmental pressures. Pulse volume recording is a form of plethysmography that measures the changes in blood volume as the arteries expand and contract. A blood pressure cuff inflates slightly so flow is not occluded. As blood pulses, the arteries expand and contract, causing an increase or decrease in air volume within the cuff. This is reflected in the waveform and is particularly useful when evaluating patients with diabetes or calcified vessels, in whom absolute pressure measurements tend to be inaccurate.    Unlike segmental pressures and PVR, arterial duplex ultrasound provides more anatomic information regarding perfusion. Flow characteristics such as peak velocities and velocity shifts help to localize stenotic or obstructive lesions. Additionally, one can visualize blood vessel structure to help forecast interventional strategies. It is an accurate and well-accepted method for assessing the peripheral arterial system both preoperatively and postoperatively. Relative to contrast angiography, duplex ultrasonography has 97 percent sensitivity, 96 percent specificity, 95 percent positive predictive value, 98 percent negative predictive value and an overall 96 percent accuracy.⁵

Assessing The Value Of Transcutaneous Oximetry And Skin Perfusion Pressure

Historically, assessment of distal perfusion was limited to waveform interpretation and segmental pressures. Practitioners attempted to predict wound healing based largely on waveform pulsatility, which allowed a great amount of subjectivity. However, contemporary wound care dictates more sophisticated, localized perfusion assessment. The angiosome concept refers to six geographic areas in the foot fed by the three infrapopliteal vessels: the anterior tibial, posterior tibial and peroneal arteries. One can now perform targeted interventions to restore direct in-line blood flow to areas of tissue loss.⁶ Different assessment tools have developed to guide these revascularization interventions and measure responsiveness to therapy.    Transcutaneous oximetry (TcPO₂) is one such tool to assess angiosome specific perfusion. The TcPO₂ measures the oxygen tension 1 to 2 mm deep in the skin from the local capillary perfusion. A value > 70 mmHg is normal while < 40 mmHg predicts impaired wound healing and < 30 mmHg is critical limb ischemia.⁷ In addition to predicting wound healing, TcPO₂ also assesses candidacy for and responsiveness to hyperbaric oxygen therapy. If TcPO₂ levels do not improve with oxygen challenge, there is likely little benefit to hyperbaric treatment.    Clinicians can also use TcPO₂ values to predict amputation healing. A value of > 40 mmHg suggests spontaneous healing is likely. When readings are < 40 mmHg, one may perform an oxygen challenge with 100% normobaric oxygen. If there is > 10 mmHg increase, amputation healing is likely and < 10 mmHg improvement suggests amputation failure.⁷ As with any other means of perfusion assessment, there are limitations to the test’s use. Low TcPO₂ levels (< 40 mmHg) occur with high altitude, pulmonary disease, heart failure, significant edema, local inflammation and callused skin. Practitioners must account for factors such as these when interpreting study results.    Skin perfusion pressure (SPP) is another useful measure of distal perfusion. One would apply a laser Doppler probe to the skin under a blood pressure cuff. The cuff inflates to the point of occluding flow and then gradually deflates. As the cuff deflates, the laser Doppler emits light and detects scattered light within the tissue. When the Doppler detects moving blood cells, a change in frequency occurs. This is called a Doppler shift. An algorithm converts the optical information into the skin perfusion pressure by capturing the onset of capillary flow return and determining the pressure (in mmHg) at which flow onset occurs.    Although it is not as useful for predicting responsiveness to hyperbaric therapy, skin perfusion pressure is generally more sensitive in its ability to predict wound healing than TcPO₂.⁸ A value of > 50 mmHg indicates normal skin perfusion while values of 40 to 50 mmHg indicate probable wound healing. Thirty to 40 mmHg is a gray zone for healing and < 30 mmHg is critical limb ischemia. Unlike TcPO₂, skin perfusion pressure measures are not affected by factors such as thickened skin, edema or calcified vessels. However, the test can be painful for patients with occlusive disease as it requires occlusion of flow with a blood pressure cuff.

Final Thoughts

The non-invasive vascular laboratory provides a number of perfusion assessment tools. Greater understanding of these tools enhances the collaborative nature of wound care with our vascular specialists. Ultimately, clinicians are thereby better equipped to guide therapy and improve outcomes with individualized care plans.    Dr. Oltmann is a resident in the Orthopedic Surgery Department at the Cleveland Clinic in Cleveland.    Dr. Maier is the Director of the Lower Extremity Wound Clinic at the Cleveland Clinic in Cleveland. He is a Fellow of the American College of Certified Wound Specialists. References 1. Kirksey L. Heath care disparity in the care of the vascular patient. Vasc Endovascular Surg. 2011; 45(5):418-2. 2. Dormandy JA, Rutherford RB. Management of peripheral arterial disease (PAD). TASC Working Group. TransAtlantic Inter-Society Consensus (TASC). J Vasc Surg. 2000; 31(1 Pt 2):S1-S296. 3. Khan T, Farooqui F, Niazi K. Critical review of the ankle brachial index. Current Cardiology Reviews. 2008; 4(2):101-104. 4. Bonham P. Get the LEAD out: non-invasive assessment for lower extremity arterial disease using ankle brachial index and toe brachial index measurements. J Wound Ostomy Continence Nurs. 2006; 33(1):30-41. 5. Alexander JQ, Leos SM, Katz SG. Is duplex ultrasonography an effective single modality for the preoperative evaluation of peripheral vascular disease? Am Surg. 2002; 68(12):1107–1110. 6. Soderstrom M, Alback A, Biancari F, Lappalainen K, Lepantalo M, Venermo M. Angiosome-targeted infrapopliteal endovascular revascularization for treatment of diabetic foot ulcers. J Vasc Surgery. 2013; 57(2):427-35. 7. Fife CE, Smart DR, Sheffield PJ, et al. Transcutenous oximetry in clinical practice: consensus statements from an expert panel based on evidence. Undersea Hyperb Med. 2009; 36(1):43-53. 8. Lo T, Sample R, Moore P, Gold P. Prediction of wound healing outcome using skin perfusion pressure and transcutaneous oximetry. Wounds. 2009; 21(11):310-316.