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A Closer Look At Non-Invasive Testing Methods For PAD

Whenever one suspects Peripheral Arterial Disease (PAD), the clinician must perform the five non-invasive vascular testing methods that are commercially available and widely implemented. They include the ankle brachial index ABI), the toe-brachial index(TBI), pulse volume recording(PVR), transcutaneous oxygen monitoring(TCPO2) and skin perfusion pressure(SPP).

Ankle Brachial Index:

The ankle brachial index is the most well-known, non-invasive vascular testing tool. ABI test is performed with a Doppler and a blood pressure cuff. One calculates the ABI by dividing the ankle pressure by the brachial systolic pressure. An ABI of < 0.9 is abnormal and ABI values have a linear correlation with wound healing potential in lower extremity wounds. Patients with DM may have calcified and hardened lower extremity arterial walls that cannot be readily compressed and occluded with blood pressure cuffs. This produces falsely elevated ankle pressure readings that are often in the “normal ABI range” (0.9 to 1.2) or sometimes in the non-physiological range of above 1.3.

Pros.: The ABI is a relatively quick and cost-effective test for screening of PAD with known sensitivity and specificity of 90 percent or higher.

Cons.: Calcified leg arteries in DM or dialysis patients may yield falsely elevated ABI results.

Toe-Brachial Index:

One would calculate TBI by dividing the blood pressure of the great toe by the systolic brachial blood pressure. Toe pressure of > 50mmHg is considered normal. Toe pressure < 30 mmHg is considered severely ischemic.

Pros.: The digital arteries in great toes are considered to be less affected by medial arterial calcification.

Cons.: There are site limitations such as the inability to measure toe pressure due to toe wounds or toe amputation

TCPO2 and SPP:

Transcutaneous oxygen monitoring (TCOM) and skin perfusion pressure (SPP) are valuable tools in the wound care setting. As these methods are unaffected by calcified arteries and a higher pressure reading clinically correlates with increased wound healing potential. In addition, both devices allow strategic sensor placement in various locations around foot and ankle wounds. When combined these tools provide specific information relative to leg ischemia, wound healing potential, optimal amputation level and incision site determination for the lower extremities. Normal values are > 50 mmHg. Wound healing potential drops as TcPO2 values decline. Traditionally, 30 mmHg is correlated with a diagnosis of severe PAD or critical limb ischemia (CLI).

Pros.: Transcutaneous oxygen monitoring is a clinically validated tool that reveals a linear correlation between higher partial pressure oxygen reading and wound healing potential. The test is not affected by calcified leg arteries.

Cons.: There are physical limitations. One can not place the probe over the plantar foot as the skin is too thick for oxygen permeation

SPP: Skin perfusion pressure is an alternative technology to TCOM for assessing the perfusion status of skin or “skin capillary blood pressure.” Using the laser Doppler and pressure cuff in combination, provides the SPP measurement in mmHg. Normal perfusion for lower extremity SPP values is > 50 mmHg. A SPP measurement between 30 and 50 mmHg is diagnostic of PAD while an SPP measurement of < 30 mm Hg is diagnostic of severe PAD or CLI.

Pros.: The SPP is a clinically validated tool with a strong correlation to wound healing potential.The test is not affected by calcified leg arteries. Clinicians can measure SPP in plantar skin and in edematous limbs.

Cons.: The patient movement can create noise in measurement and it may be difficult to measure patients with involuntary shaking/spasm.

The following are the various products used to measure ABI, TBI and TCPO2.

Test L150R Versalab LE Versalab Dx Versalab Auto Periflux PF5000  
Ankle pressure ABI X X X X X ABI values below 0.9 are indicative of PAD. Values above 1.40 should always be confirmed with toe pressure or other techniques. (TASC II)
Toe/ankle pressure, TBI/ABI   X X X X ABI values below 0.9 are indicative of PAD. Values above 1.40 should always be confirmed with toe pressure or other techniques. Diabetics and other patients with suspected calcified vessels should always be tested using toe pressures. Toe Pressure Index under 0.7 are abnormal. (TASC II)
PVR   X X X X Qualitative assessment relying on the interpretation of the graphical output and the location of the PVR measurement. (TASC II)
tcpO2         X Values below 30 mmHg are considered critical. (TASC II)
Heat controlled laser Doppler         X Values above 20 perfusion units during heating and an increase greater than 150% compared to the baseline value predict wound healing. (Wound care practice, edited by Paul Sheffield et al. Best Publishing Company 2004, p 117-156)
SPP         X Skin Perfusion Pressure (SPP) reflects the local pressure in the microcirculation. It has been successfully employed for amputation level determination, in particular major amputations
PC controlled   X X X X Operated from PC, data storage and report generation

ABI= ankle-brachial index, TBI=toe-brachial index, PVR = pulse volume recording,
tcpO2=transcutaneous oxygen, SPP=skin perfusion pressure