How To Diagnose Peripheral Arterial Disease
- Volume 20 - Issue 4 - April 2007
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A Guide To The Toe-Brachial Index
The toe-brachial index is analogous to ABI in that one would calculate TBI by dividing the blood pressure of the great toe by the systolic brachial blood pressure. Clinicians can measure toe pressure by placing a small toe cuff around the great toe and attaching a plethysmography probe at the pulp of great toe tip.
Pros. The digital arteries in great toes are smaller in diameter compared to the large leg arteries and 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 great toe wounds or toe amputation. The test has a lack of specificity and a low TBI vaguely indicates that the limb is ischemic at some level “upstream” in the leg arteries.
Interpretation of TBI. The cut-off values of toe pressure and TBI are arbitrary and vary in the literature. In general, a toe pressure of 70 to 110 mmHg or TBI > 0.5 to 0.75 is considered normal and anything below is diagnostic of PAD. A toe pressure lower than 30 mmHg or TBI < 0.2 is considered severely ischemic and diagnostic of critical limb ischemia (CLI). Wound healing potential drops as TBI decreases from the normal values.
Can Pulse Volume Recording Be Beneficial From A Diagnostic Standpoint?
Pulse volume recording uses blood pressure cuffs (inflated to 65 mmHg) around the lower limbs. This effectively compresses limb veins while the transducer connected to the cuff detects the pressure difference in pulsatile arterial inflow during systole. This facilitates documentation of the waveform.
Pros. The PVR is not affected by calcified arteries. It is not operator dependent.
Cons. The PVR is a morphologic test without numerical values that makes interpretation and communication among clinicians difficult. One should always pair PVR with other non-invasive vascular tests.
Interpretation of PVR. Depending on the waveform shape, PVR is categorized as triphasic, biphasic or monophasic (stenotic). A normal PVR waveform shows a rapid rise and fall with sharp peaks — similar to what one might see with an EKG — while flatter, nonpulsatile flow may represent ischemia and a diagnosis of PAD.
How To Assess Microcirculation With TCOM and SPP
Transcutaneous oxygen monitoring (TCOM) and testing for skin perfusion pressure (SPP) are valuable tools in the wound care setting as they enable one to assess microcirculation of skin. 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 the clinician combines these tools with the knowledge of the angiosome concept, he or she may obtain specific information relative to leg ischemia, wound healing potential, optimal amputation level and incision site determination for the lower extremities.9,10
Identifying The Pros And Cons Of TCOM
Transcutaneous oxygen monitoring, which was developed in the neonatal intensive care unit for the monitoring of neonates, measures tissue oxygenation level or transcutaneous partial oxygen pressure (TcPO2) in mmHg. Transcutaneous oxygen monitoring uses Clarke electrode sensors to measure oxygen molecule permeation through the skin as heating elements warm the epidermis. One would place the sensors on the patient’s skin surface with disposable plastic fixation rings and subsequently apply a few drops of saline within a reservoir of these rings.
Pros. Transcutaneous oxygen monitoring is a clinically validated tool that reveals a linear correlation between higher partial pressure oxygen reading and wound healing potential.8 The test is not affected by calcified leg arteries.