A Closer Look At Perfusion Assessment Techniques
- Volume 27 - Issue 5 - May 2014
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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