How To Diagnose Peripheral Arterial Disease

Peripheral arterial disease can result in a range of serious complications and possible death. Accordingly, this author offers a closer look at non-invasive testing and assesses the pros and cons of these tests at the microcirculations and macrocirculation levels

Peripheral arterial disease (PAD) affects 10 to 20 percent of the United States senior citizen population, approximately 8 million people. These PAD patients experience significant and costly morbidity such as leg amputation and death.1 There is a 20 percent incidence rate of PAD in patients 75 years of age and older. Despite the prevalence of the disease and associated cardiovascular risk, only 25 percent of these patients with PAD undergo treatment.2

   Early diagnosis and treatment of PAD not only improves one’s quality of life, it can save lives. According to the REACH Registry, approximately 16 percent of PAD patients have other incidental atherosclerotic changes in the brain, such as cerebrovascular disease (CVD), and/or in the heart with coronary artery disease (CAD). These deadly “clogged artery” problems are collectively called “polyvascular diseases.”2 Consequently, PAD patients have a mortality risk that is two to three times greater than patients who do not have PAD.3 It should be emphasized here that PAD is not just a “leg cramping problem” but an ominous sign of death that requires our close attention.

   Diabetes also presents as a separate and significant risk factor of PAD. As obesity becomes the norm in America, diabetes will similarly become more commonplace. Currently, the diabetic population represents 20.8 million patients, about 7 percent of the U.S. population.4 The recently released 2006 American Diabetes Association consensus statement presented clinical data that one in three patients with diabetes mellitus (DM) are known to have PAD. The statement urged clinicians to participate actively in the diagnosis and treatment of this disease.

Recognizing The Challenges Of Diagnosing PAD

However, diagnosing PAD can be confusing for many clinicians. Although we often associate PAD with intermittent claudication (cramping calf pain caused by walking), this classic symptom accounts for only 10 percent of the entire PAD patient population over the age of 50. Surprisingly, up to 50 percent of the PAD patients are known to be asymptomatic while 40 to 50 percent of PAD patients may present with atypical, non-specific complaints in their legs.5

   To complicate the issue, diagnosing PAD can be challenging from both a visual and clinical standpoint. A recent systematic review of PAD clinical studies found that a physical examination of the lower extremities (e.g., palpation of pulses and “classic findings” such as atrophic skin change or lack of hair growth) alone is unreliable and “not independently sufficient to include or exclude the diagnosis of PAD.”6 The study concluded with a strong recommendation for the use of non-invasive vascular tests to screen patients for PAD upon each exam.6
   In consideration of the current statistics, do not ignore the importance of non-invasive vascular tests of the lower extremities. The Podiatry Insurance Company of America (PICA) group recently reported two legal cases in which podiatric physicians “failed to make a correct diagnosis” of severe PAD and revealed a “lack of documentation” of the pre-op vascular workup.7 Both cases resulted in lawsuits initiated by patients who underwent below-knee amputations. One can easily avoid these unfortunate events by using the right tools to assess our patients effectively and non-invasively.

What You Should Know About Non-Invasive Vascular Testing

Whenever one suspects PAD, the clinician must perform non-invasive vascular testing. Given the current medical evidence, one may expect to encounter PAD in patients with any of the following histories:

   • polyvascular diseases (e.g., CAD and CVD) such as myocardial infarction (MI) or stroke;
   • diabetes, especially if it is of a longstanding and poorly-controlled nature, and with other DM complications (e.g., prior amputations, nephropathy, neuropathy or retinopathy);
   • chronic, non-healing wounds longer than four weeks; and/or
   • non-specific leg complaints such as cramping, achiness or generalized pain.
   CPT codes 93922 (single-level, bilateral extremities) and 93923 (multiple levels, bilateral extremities) are the appropriate CPT codes for the reimbursement of non-invasive vascular testing. One should pair these CPT codes with the appropriate ICD-9 codes that indicate a diagnosis representing PAD or ischemic conditions. For example, one may use the ICD-9 code 440.23 for atherosclerosis of the extremities with ulcer. It is also important to document the medical necessity for these CPT codes, test results and interpretations, and have a hard copy of the diagnostic studies you perform.

   Currently, Medicare does not reimburse for these studies when one uses them as screening tools. Accordingly, in order to receive Medicare reimbursement for these studies, a patient must present with ischemic symptoms. Unfortunately, up to 50 percent of patients with PAD are asymptomatic so these patients will be overlooked. Clinicians need to determine whether to provide non-reimbursable screening tests, knowing that PAD is grossly under-diagnosed and under-treated. Future studies should address this topic as a source of possible cost savings to the Centers for Medicare and Medicaid Services (CMS) in the form of limb salvage.

A Closer Look At Non-Invasive Testing Methods For PAD

There are 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 (TCOM) and skin perfusion pressure (SPP).

   The ankle brachial index is the most well-known, non-invasive vascular testing tool. Clinicians often perform an ABI test with a handheld Doppler probe and a blood pressure cuff. One calculates the ABI by dividing the ankle pressure by the brachial systolic pressure.

   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.5 One can perform this test with inexpensive equipment.

   Cons. Calcified leg arteries in DM or dialysis patients may yield falsely elevated ABI results. The test is operator dependent (i.e., pulses in ischemic or edematous feet and ankles are difficult to locate).

   Interpretation of ABI. An ABI of <0.9 is abnormal and is diagnostic of PAD. In these cases, one should refer the patient to an internist for intensive medical management. When the lower extremity symptoms are severe or if a chronic wound is present, consultation by vascular specialists (i.e., vascular surgeons, interventional cardiologists or interventional radiologists) becomes necessary.

   However, there are pitfalls of “normal” ABIs with calcified arteries. ABI values have a linear correlation with wound healing potential in lower extremity wounds. As Padberg illustrates, ABI is highly unreliable in patients with diabetes and/or those undergoing dialysis due to chronic renal failure (CRF).8 These patients 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.

   Simply put, a low ABI (<0.9) indicates PAD while “normal” ABI values (>0.9) may be misleading and unreliable in the diagnosis or ruling out of PAD, especially in DM and CRF patients.

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.

   Cons. There are physical limitations. One cannot place the probe over the plantar foot as the skin is too thick for oxygen permeation and clinicians cannot use TCOM to measure an edematous foot/ankle.

   In regard to technology limitations, there is a significant margin of error for the reliable prediction of wound healing potential when assessing tissue oxygenation level in room air and outside of the hyperbaric oxygen chambers. The test is also cumbersome. Testing site preparations, calibration of the machine and the actual testing procedure have a cumulative duration of 30 to 45 minutes per patient.

   Interpretation of TCOM. 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), and necessitates immediate referral to vascular specialists.

What You Should Know About Skin Perfusion Pressure

Skin perfusion pressure is an alternative technology to TCOM for assessing the perfusion status of skin or “skin capillary blood pressure.” To measure SPP, one would ensure supine positioning of the patient on the exam table and place a laser Doppler optical sensor over the specific skin site. Proceed to wrap a computer-automated pressure cuff around the limb/sensor. The computer operates the laser Doppler and pressure cuff in combination, guiding it through a gentle inflation/deflation process that detects sufficient arterial compression and identifies the point at which blood flow resumes. This provides the clinician with the SPP measurement in mmHg.

   Pros. The SPP is a clinically validated tool with a strong correlation to wound healing potential.11,12 The test is not affected by calcified leg arteries. It has fewer physical limitations in comparison to the TCOM. 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.

   Interpretation of SPP. 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.13 Wound and amputation site healing potential are correlated in a sigmoid curve (see “Assessing The Probability Of Healing With SPP” below). Wound healing potential drops dramatically when the SPP is below 40 mmHg.

What The Author Recommends For The Diagnostic Workup

In our wound care center, when we assess new patients with lower extremity wounds, we start with the patient history and physical examination, and subsequently perform both SPP and PVR tests bilaterally on the feet and ankles.

   In our center, we use the SensiLase® System (Väsamed) for comprehensive non-invasive vascular testing of the lower extremities. The SensiLase measures SPP in mmHg (microcirculation test) and records PVR waveforms (macrocirculation test) with its laser Doppler sensor and the computer-automated cuff.

   We established that the combined use of SPP and PVR tests allows us to reserve ABI tests in our practice. The latest clinical study comparing various non-invasive vascular tools to the multi-slice magnetic resonance angiography (MRA) found SPP testing to be most sensitive in the diagnosis of PAD with a sensitivity of 85 percent while the ABI had a sensitivity of 30 percent.14 In other words, the ABI failed to diagnose 70 percent of the PAD patients tested in this investigation. This study illustrates the need to incorporate a microcirculation testing method, either TCOM or SPP, as a part of the clinical workup for those who frequently see DM and dialysis patients in the clinic.

   Given that SPP testing has distinct advantages over TCOM such as less physical limitations and a higher rate of accuracy and reproducibility, we have abandoned room air TCOM tests. Further, coupling SPP testing with PVR testing provides greater clinical information. In addition, we are able to efficiently determine which of our patients will benefit from hyperbaric oxygen treatment, thereby allowing us to reserve TCOM for in-chamber oxygen challenge as needed.

Understanding The Angiosome Concept In Regard To SPP Testing

The novel “angiosome” concept, introduced by Taylor in 1982, was further developed by Attinger.9,10 Angiosomes, three-dimensional blocks of tissue fed by source arteries, are somewhat analogous to the relationships between peripheral nerves and dermatomes. The human body is composed of at least 40 angiosomes with the foot and ankle being fed by three source arteries and six angiosomes.10

   The angiosomes of the foot and ankle can be summarized as follows:

   • The anterior tibial artery (and dorsalis pedis) angiosome supplies the anterior leg and the dorsum foot.

   • Posterior tibial artery angiosomes supply the medial ankle and the plantar foot, including the heel.

   • Peroneal artery angiosomes supply the lateral ankle and the lateral heel.

   The heel is unique in that it is redundantly supplied by two source arteries, the calcaneal branches of the posterior tibial artery and the peroneal artery. As Attinger described, the knowledge of the angiosome concept can be enormously helpful in choosing the best incisions for debridement in treatment of foot and ankle wounds.

   Skin perfusion pressure can be a powerful diagnostic tool and wound healing predictor when one appropriately places the sensors around the foot and ankle, effectively incorporating the angiosome concept. Testing for SPP has another advantage over TCOM with the ability to measure plantar foot microcirculation.

How To Incorporate Non-Invasive Testing Into Wound Assessment

Consider the following case studies from our wound care center.
In the first case (see photo on the left), a patient with diabetes presented with no wounds but did request a matrixectomy for a painful ingrown toenail. In regard to vascular testing of the right foot, there was a monophasic PVR and a SPP of 25 mmHg (plantar). This indicated severe PAD (CLI).

   We subsequently referred the patient to a vascular surgeon. The patient avoided a matrixectomy. If the phenol matrixectomy had been performed in this ischemic foot, it may have lead to a gangrenous great toe and a partial foot amputation.

   In the second case (see right photo), a diabetic patient on dialysis presented with a small chronic wound on the left plantar foot. The physical exam findings were identical for both feet with non-palpable pulses and atrophic skin change. The PVR was weakly biphasic for both feet. The SPP was 63 mmHg (normal) in the left foot and 34 mmHg (ischemic) in the right plantar foot.

   We referred the patient to a vascular surgeon. The surgeon determined the limb was non-reconstructable. The left foot wound eventually healed with conservative wound care and offloading but it progressed very slowly over a three-month period as expected.

   In the third case (see left photo), a diabetic patient on dialysis presented with a non-healing great toe wound with exposed bone. There was a monophasic PVR, the SPP was 41 mmHg on the plantar foot (slightly ischemic) and 21 mmHg on the dorsal foot (severely ischemic). This indicated a compromised anterior tibial artery/dorsalis pedis angiosome and a diagnosis of critical limb ischemia.

   We immediately referred the patient to a vascular specialist. Following a successful angioplasty, the wound eventually closed with minor toe amputation and antibiotics.

Insights On Common Clinical Scenarios
The following are typical clinical scenarios in the realm of wound care. In these cases, one can see how the use of non-invasive testing can help facilitate appropriate treatment and/or referral.

   • “Normal, non-ischemic.” The periwound SPP is > 50 mm Hg with a biphasic/triphasic PVR waveform. The wound is well perfused. One may confidently perform aggressive and thorough debridement procedures to remove non-viable tissues. One may also apply aggressive compression therapy if edema is present.

   • “Ischemic.” The periwound SPP is < 30 mmHg and/or PVR waveforms are monophasic/stenotic. In addition to the wound being ischemic, the aforementioned measurements are diagnostic of severe PAD or CLI. I recommend refraining from sharp debridement and compression altogether to minimize the potential of gangrenous change and wound enlargement. I would inform the patient of the test result and make an immediate referral to a vascular specialist for vascular intervention.

   • “Marginal ischemia.” The SPP is between 30 and 50 mmHg with a PVR showing a mono to weak biphasic waveform. These results indicate marginal vascular compromise and are diagnostic of PAD. I may perform sharp debridement and compression therapy conservatively. It is still important to make the diagnosis of PAD so one can facilitate an appropriate referral to the patient’s PCP for intensive medical management, such as smoking cessation, anti-hypertensive therapy, anti-platelet therapy, anti-cholesterol therapy and lowering of A1c for patients with diabetes.

   As Sheehan, et. al., indicate, if the wound does not improve in the first few weeks of treatment, it may not improve for the next 12 weeks.15 Therefore, if there is no improvement on the follow-up visits, I would assume the wound is relatively ischemic and that the patient may benefit from consultation with a vascular specialist.

Understanding The Dynamics Of Macrocirculation And Microcirculation

Peripheral arterial flow can be divided into two categories: macro- and microcirculation. Macrocirculation involves three major arteries (anterior tibial, posterior tibial and peroneal arteries) deep within the ankle. The arteries are pulsatile with large diameters up to 3 mm. Vascular surgeons often manipulate and bypass these arteries, which are visible to the naked eye.

   Conversely, microcirculation occurs in the arterioles within the skin capillary bed. Arterioles are non-pulsatile, located superficially within the epidermis and are only visible with a microscope (as arterioles have a diameter of approximately 0.012 mm).

In Summary

It is in our patients’ best interest to ensure that we as clinicians have a heightened awareness of PAD/CLI and the ability to diagnose these conditions with non-invasive tools. For primary care physicians, ABI is an inexpensive and effective screening tool of PAD for non-diabetic patients. As a wound care physician, I have found that combining SPP with PVR is one of the most effective diagnostic tests for my practice. Patients tolerate these tests well and the tests are valuable in the assessment and documentation of lower extremity PAD.

   As an adjunct to the physical exam, these non-invasive vascular tests provide us with greater clinical information than the physical exam alone, thereby enhancing our ability to render better clinical judgments for our patients, improve documentation and protect our practices from a medicolegal standpoint.

   In some cases, we will not be reimbursed for conducting such tests until CMS acknowledges the prevalence of PAD. However, providing a means for early PAD detection and initiating medical and/or surgical management and facilitating appropriate referrals for the condition before it becomes problematic or life-threatening is our duty as physicians.

Dr. Suzuki is the Medical Director of the Tower Wound Care Center at Cedars-Sinai Medical Towers in Los Angeles. He is a consultant, researcher and lecturer on wound care and limb salvage in the U.S. and in Japan. The author can be contacted via e-mail at kazu88@gmail.com.

For related articles, see “Can Endovascular Atherectomy Be Beneficial In Diabetic Limb Salvage?” in the November 2006 issue of Podiatry Today and “Understanding The Dangers Of PVD And PAD” in the February 2002 issue.

Also be sure to visit the archives at www.podiatrytoday.com.

References:

References
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11.    Adera H et al. Prediction of amputation wound healing with skin perfusion pressure. J Vasc Surg 1995; 21(5):823-828.
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13.    Castronuovo J et al. Skin perfusion pressure measurement is valuable in the diagnosis of critical limb ischemia. J Vasc Surg. 1997; 26(4):629-637.
14.    Okamoto et al. Peripheral arterial occlusive disease is more prevalent in patients with hemodialysis. Am J Kidney Disease 2006; 48:269-276.
15.    Sheehan et al. Percent change in wound area of diabetic foot ulcers over a 4-week period is a robust predictor of complete healing in a 12-week prospective trial. Diabetes Care 2003; 266:1879-1882.