Addressing the Need for Noninvasive Vascular Testing in the Outpatient Clinic: A Case Study

Editor’s Note: The basis for this article was an actual diagnosis of a patient admitted to the author’s outpatient wound clinic.

The Case: A 30-year-old male working as a machinist suffered an injury to the left great toe 12 months prior to presenting at the wound clinic (Figure 1). Previously treated with a variety of topical products at another wound center without improvement, the patient admittedly “smoked a pack a day since the age of 17.” His father died at age 42 of an acute myocardial infarction. The patient had no known health problems and took no medications. Physical exam revealed a normal pattern of hair growth on the leg with a shallow ulcer on the dorsal great toe at the nail bed and mild erythema of the toe. No dorsalis pedis or posterior tibial pulses were palpable.

What is it that would cause a traumatic wound of the toe to fail to heal for 12 months? The goal of any patient evaluation is to identify the answer(s) to that question.

From a statistical standpoint, the most likely answer for most of wound care patients is tissue hypoxia. The most common reason for tissue hypoxia is arterial disease. So, the protocol in the wound clinic should be that all patients living with nonhealing, chronic wounds undergo vascular screening in order to determine whether tissue hypoxia is present. In this case, transcutaneous oximetry (TcPO₂) was initiated first. Surprisingly, the TcPO₂ values on the dorsum of his foot were only 8 and 10 mmHg with no response to sea level oxygen. These readings were so bad for someone of that age that the clinical staff decided to confirm them with a skin perfusion pressure (SPP) evaluation, which was recorded at 35 mmHg — borderline for wound healing and suggestive of arterial disease (Figure 2). Worse, his pulse volume recording (PVR) was almost a flat line (Figure 3), indicating he had almost no pulsatile flow through the large vessels in his calf. Considering his father’s circumstances, we wanted to assess his ability to ambulate. On careful questioning, he admitted to  “cramping in his calves when he walked across the machine shop.” Sure enough, a magnetic resonance angiogram revealed that only one of the three vessels below the knee was open. He underwent peripheral bypass surgery a few days later, his vascular obstruction not being amenable to angioplasty.

Long story short: This 30-year-old had peripheral arterial vascular disease severe enough to prevent him from healing a toe injury without the recognition of any previous clinician. The toe ulcer, which began with minor trauma, had been open so long that by the time the underlying problem was identified, he had osteomyelitis of the great toe.

This case is an example of why vascular screening is advisable for every patient living with a chronic, nonhealing, lower extremity ulcer. It also provides an opportunity to discuss the differences between TcPO₂ and SPP. The reason the SPP appeared better than the TcPO₂ is probably because the patient had created collateral vessels to perfuse the area. The collateral vessels that supplied his foot may have kept his perfusion up somewhat, but were not sufficient to keep the tissue oxygen levels within a range for healing. 

This article will discuss methods for establishing vascular screening protocol in the wound clinic.

Why Screen for Vascular Disease?
The clinicians at the previous clinic who failed to diagnose the vascular disease committed what may be the cardinal sin in wound care: the failure to recognize arterial disease. If anyone were to ask where to find those patients living with undiagnosed vascular disease, the answer would be: “in the wound center.” Why? Because patients who are most likely to experience arterial insufficiency are those living with diabetes, those who smoke, and those who develop nonhealing wounds. In other words, we see them each day in the wound clinic.

The five-year survival rate of patients after a major amputation is worse than that of most cancers. After a major amputation (below the knee or higher), the five-year survival rate for nondiabetic patients is 50%. For a diabetic patient, the five-year survival rate is only 30%. For diabetic patients living with renal failure, the chances of being alive five years after a major amputation are only 14%. Major amputation can be considered a “fatal disease” from this perspective. That said, it’s incumbent among wound clinic clinicians to diagnose peripheral arterial disease and get patients revascularized if they are candidates.

To that end, guidelines published by the Wound Healing Society state that all patients living with lower extremity ulcers should be assessed for arterial disease.¹ However, the ideal way to perform noninvasive vascular screening has not been clearly defined by clinical research. A variety of clinical options exist. One can look at the “macro” circulation (large blood vessels) or the “micro” circulation (small blood vessels or tissue perfusion/oxygenation). For many years, the most common method of screening was to assess the macro circulation by assessing the ankle-brachial index (ABI). ABI values of 0.7 or less are considered abnormally low, with values of 0.3 or less representing severe obstruction. (Reimbursement note: ABI is not separately reimbursed; results must be accompanied by printed waveform analysis in order to meet Medicare reimbursement requirements.) The problem is that diabetes can cause one’s ABI to be falsely elevated due to incompressibility of calcified blood vessels, so ABI values correlate poorly with healing prediction in the diabetic population. A better way to assess vascular supply or healing prediction is needed.

Measuring TcPO₂
TcPO₂ measures the oxygen partial pressure in the skin. The technology dates back to the 1960s and the device consists of a heating element and a sensor attached to the skin via a gas-permeable membrane in contact with an electrolyte solution. The electrode is heated, usually to 44° C or 45° C, to dilate blood vessels and allow oxygen to diffuse to the electrode. When a constant polarizing voltage is applied to the gold cathode, oxygen molecules at the cathode are reduced, the silver in the anode becomes oxidized, and a current is generated in proportion to the number of oxygen molecules in the solution. This means the machine is measuring the actual number of oxygen molecules (PO₂) in the periwound area, not oxygen saturation. In fact, before pulse oximetry became available, transcutaneous oxygen measurements (TCOMs) were used as a way to noninvasively monitor arterial PO₂ in neonates. TcPO₂ is not a way to determine arterial PO₂ in adults because of skin thickness, among other reasons. However, it can provide an estimate of arterial perfusion to the area. The results are reported in mmHg, just as an arterial blood gas. The machine has to be calibrated, the seal has to be airtight, and there is a steep learning curve for technologists. It is an “artsy” test. Examples of devices are those manufactured by Radiometer (Denmark) and Perimed (Stockholm, Sweden).

Fifteen studies (1,137 patients) have demonstrated TcPO₂ provides better overall predictive capability than Doppler studies, ABI, segmental pressures, or laser fluximetry. However, many factors can cause TcPO₂ to be low besides arterial disease. Anything that creates a barrier to diffusion of oxygen will lower TcPO₂. Callus, edema, skin diseases such as scleroderma, scar tissue, or placing the electrode over a bony prominence may decrease the readings. Factors that decrease cutaneous blood flow, such as hypotension, dehydration, or vasoconstriction due to cold, may also decrease TcPO₂. Arterial hypoxia will decrease the reliability of the findings and infection may cause them to be increased or decreased as a result of local oxygen consumption or vasodilatation.

TcPO₂ has some serious limitations. It cannot be used on most digits because the electrodes are 9 mm wide and thus will not “seal” around most toes. Reliability is affected by callus, so it may not be useful on the plantar foot. It cannot be used in the wound bed itself since the seal around the electrode must be airtight (otherwise the electrode will read the oxygen molecules in the air). It is an estimation of the oxygen supply to the periwound. Patients who are on supplemental oxygen probably cannot have meaningful TcPO₂ studies. TcPO₂ values are best thought of as “mapping” the extremity (the more values the better). It is important to have more than two sites to use for decision-making, and the more electrodes the better. This can get expensive, however, because the equipment is not cheap.

The Oxygen Challenge
To determine whether a low TcPO₂ value is due to a diffusion barrier, the patient is allowed to breathe 100% oxygen via a tight-fitting facemask. Some sort of “challenge” is absolutely necessary as a method of determining whether a low value is due to arterial disease or a diffusion barrier. Without some sort of challenge, a complete TcPO₂ test has not been conducted. The “leg drop” test can be used as a test for arterial disease. If TcPO₂ values increase with the leg hanging down, or decrease with leg elevation, then arterial disease is likely. Figure 4 depicts a “positive leg drop test” in a 58-year-old male with rest pain who sleeps in a chair. Oxygen challenge is also a commonly used challenge. If the TcPO₂ values increase to >100 mmHg with sea level oxygen breathing, the patient is not likely to have arterial disease. If the TcPO₂ values fail to increase significantly with oxygen breathing, the patient likely has large vessel disease.

Baseline air TcPO₂ values can be diminished for reasons other than arterial disease, but this does not mean the TcPO₂ values are “false.” A patient with a venous ulcer who has a value of 10 mmHg is not likely to heal spontaneously, but if values increase to 200 mmHg with sea level oxygen breathing, the treatment is edema reduction with compression, not arterial revascularization.

Selecting Patients for HBOT
Transcutaneous oximetry has hundreds of references to support its use, including randomized controlled trials. It is often found in facilities that offer hyperbaric oxygen therapy (HBOT) because it is often used as a way of predicting which patients will benefit from HBOT. An evidence-based guide to TcPO₂ interpretation has been published,² but there is still much misunderstanding about how to use TcPO₂ for HBOT patient selection. Following are some related myths:

Myth No. 1: If the TcPO₂ value doubles, the patient will respond to HBOT.

The Truth: Doubling, tripling, and quadrupling of TcPO₂ values were each evaluated in a large retrospective series of diabetic foot ulcers with none of the responses were shown to have value in predicting benefit from HBOT.³ This is because even some patients with very low baseline air values heal with HBOT. For example, a patient with a baseline value of 5 mmHg could have that value “double” with oxygen breathing and still have a value of only 10 mmHg. This is still a terrible “oxygen response” indicative of severe arterial disease. For this reason, there was no predictive value in “doubling,” or other similar rules.

Myth No. 2: An increase in the TcPO₂ value with sea level air is the best way to predict benefit from HBOT.

The Truth: Some studies show patients with a very good response to sea level oxygen were more likely to heal after HBOT. Why would that be? Patients who achieve high sea level oxygen values are less likely to have significant arterial disease. One could also predict that students with the highest SAT scores will do better in college than students with the lowest scores. However, if the goal is “limb preservation,” a clear focus needs to be on the patients with a poor sea level oxygen response. These patients are likely to have arterial disease and, if possible, will need angiography and revascularization (as in the case discussed in this article). If the focus of the “hyperbaric program” is to find patients with the best “sea level oxygen response” and get them in the chamber, providers are likely doing two things: 1) Selecting the patients who least need HBOT (by selecting only the patients without arterial disease), and 2) ignoring the patients who most need an aggressive vascular evaluation. The “poor sea level oxygen responders” need further arterial workup. Besides that, it is the “in-chamber” TcPO₂ value that will really determine whether or not HBOT will work. The most reliable way to predict benefit from HBOT is the in-chamber TcPO₂. Diabetic patients whose values during HBOT are >200 mmHg have a significant likelihood of benefitting from HBOT. Even patients with poor sea level oxygen response may have excellent in-chamber TcPO₂ values. Although patients whose “in-chamber” values are <50 mmHg are not likely to benefit from HBOT, this does not mean they should not be given HBOT, particularly since they have few other options.³ If TcPO₂ values increase at least 30 mmHg after revascularization (endovascular or surgical), this suggests the revascularization procedure was successful. These patients have a high likelihood of subsequently healing an open wound.

Skin Perfusion Pressure
Another noninvasive vascular screening option is the measurement of the capillary opening pressure after occlusion. This involves using a cuff to occlude blood flow before slowly deflating the cuff to allow the gradual return of blood flow. The maximal velocity of the returning blood flow is the skin perfusion pressure (SPP) as measured by a laser device that senses the first blood cells moving through the capillaries. This measurement is also in mmHg, but in this case it is a unit of pressure similar to blood pressure. An example of this device is the SensiLase System (Vasamed, Eden Prairie, MN). The Perimed (Ardmore, PA) TcPO₂ device has an SPP unit as an option as well. SPP technology has a number of advantages over TcPO₂ in that it is not affected by vessel calcification, it is not (much) affected by edema, it can be used on the plantar foot and the digits, it is not affected by callus or thickened skin, and no calibration of the machine is required.

Lo and colleagues compared SPP and TcPO₂ in terms of ability to predict healing outcome in 100 patients living with lower extremity wounds.⁴ A threshold of <30 mmHg was selected as the cutoff below which the test was considered significantly abnormal and indicative of a wound unlikely to heal. SPP alone successfully predicted outcome in 87% of the patients compared to TcPO₂ at a rate of 64% (P< 0.0002). SPP was more sensitive in its ability to predict wound healing relative to TcPO₂ (99% vs. 66%).⁴ SPP is also highly reliable in the prediction of healing after amputation and can be used to diagnose limb ischemia⁵ (and thus select patients who need further vascular testing). SPP testing may also be useful for planning foot and ankle surgery and ensuring the patient can tolerate compression therapy for venous ulcers.

SPP Interpretation Guideline (mmHg)
30 or less = Chronic limb ischemia; wound healing unlikely
30-40 = GRAY ZONE in wound healing; likely to moderate ischemia
40+ = Wound healing probable; mild to moderate ischemia
40-50 = Mild ischemia                     
50+ = Normal skin perfusion

SPP has some minor disadvantages. Blood flow occlusion by the cuff may be painful and the area measured must be an area over which a cuff can be placed, so the face or the trunk cannot be assessed. Such areas are more amenable to TcPO₂ assessment. For SPP assessment, patients must be placed supine and must be able to extend their legs, so patients with severe contractures or patients who are not able to lie supine may not be able to be tested with SPP. It is often reported that SPP can be performed more quickly than transcutaneous oximetry. However, if multiple sites are evaluated up and down the leg, the time to perform SPP and TCOM can be comparable.

Many people find it confusing that both TcPO₂ and SPP report their results in the same units of measure. However, transcutaneous oximetry measures oxygen molecules (rather like a blood gas) and SPP measures pressure (rather like blood pressure). Both are reported in mmHg, but they do not measure the same thing. This is why the man in the case presented could have different values for his TcPO₂ and his SPP.

The patient’s oxygen partial pressure to the skin was only 8-10 mmHg. However, he had probably developed small collateral vessels that kept his SPP at 35 mmHg. There was one more piece of information that the SPP device was able to provide that helped diagnose what was really happening to the large vessels, and that was the pulse volume recording (PVR) of the waveforms.

A plethysmograph is an instrument for measuring changes in volume within an organ (usually resulting from fluctuations in the amount of blood it contains). Each time the blood pulses through the leg (or the toe), the volume of the leg changes slightly. The cuff can detect these tiny volume changes and provide information on the pulsatility of this waveform. Wound care providers are familiar with the “triphasic” or three “hills” seen in a healthy pulse waveform on an arterial catheter. A less healthy waveform has two phases, and a pulse with only one phase suggests even worse flow. There are also observable changes within the waveform itself, such as steepness of upstroke, shape of the peak, steepness of downstroke, and presence/absence of the dicrotic notch. If no waveform can be detected, it is because so little flow enters the organ that the volume inside the cuff has not changed significantly. That was the case with the patient whose flow to the foot was so poor there was no pulsatile waveform at all (flat line).

Coverage Policy: Which Patients, How Often, By Whom?
Although they measure different things, both SPP and TcPO₂ are billed using CPT codes 93922 and 93923. Many Medicare Administrative Contractors (MACs) will not cover vascular screening for patients living with chronic venous stasis ulcers, even though a high percentage of these patients also have arterial disease and arterial screening is required before venous compression can be initiated. Preoperative screening prior to elective surgery may also be a medically justifiable reason to perform noninvasive arterial studies, but is often not covered by third-party payers or Medicare. There are also Medicare limits on the frequency of noninvasive screening. So, follow-up testing to assess the success of vascular interventions or to predict the success of HBOT may fall outside the payment guidelines depending on how soon it is performed after the initial study. Conduct a review of MAC policies and remember that private payers may follow different rules, even when they are providing Medicare HMO plans.

There are compelling reasons to screen wound care patients for arterial disease regardless of whether it can bill for the screening. These reasons include:

1) Local coverage determinations for advanced therapeutics may require testing to prove arterial disease is not present. If testing is not performed, reimbursement for treatments such as HBOT and cellular products can be put at risk.

2) Healthcare payment reform is shifting from a volume-based to a value-based system. Arterial screening is a quality measure available through the US Wound Registry (USWR) to satisfy the requirements for participation in the Physician Quality Reporting System (PQRS). So, while you may or may not be able to charge for a specific screening test, you can report your screening activities as part of PQRS and thus be “compensated” in a different way.

3) Wound care clinicians who are participating in Medicare or private payer risk-sharing programs may find arterial screening saves money in the long run because patients have better outcomes. “Doing the right thing” with regard to vascular screening aligns with risk-sharing programs.

Clinicians who wish to include vascular screening among the quality measures they report for PQRS participation can download the electronic clinical quality measure (eCQM) from the USWR at www.uswoundregistry.com/specifications.aspx. The programming for the vascular screening eCQM can be installed in any electronic health record certified for stage II Meaningful Use. Reporting the vascular screening quality measure can not only enhance patient care (by encouraging screening), but can ensure practitioners are poised to ensure payment in a quality-based system.

Medicare generally does not allow billing of vascular screening unless the study is performed by a certified vascular technician. In the past, some regions have allowed certified hyperbaric technicians (CHTs) to perform transcutaneous oximetry testing since this is part of their training, but a facility could not bill for an SPP performed by a CHT even though the same code is used for both because CHTs are not trained to use this method and SPP is not used to predict hyperbaric treatment success.

The American Medical Association has edited the Healthcare Common Procedure Coding System code 93923 defined as “complete bilateral noninvasive physiologic studies of upper or lower extremity arteries, three or more levels” (eg, for lower extremity). It lists:

•ABI at distal posterior tibial and anterior tibial/dorsalis pedis arteries, plus segmental blood pressure measurements with bidirectional Doppler waveform recording and analysis, at three or more levels;

•or ABI at distal posterior tibial and anterior tibial/dorsalis pedis arteries, plus segmental volume plethysmography, at three or more levels;

•or ABI at distal posterior tibial and anterior tibial/dorsalis pedis arteries, plus segmental transcutaneous oxygen tension measurements, at three or more levels:

•or single-level study with provocative functional maneuvers (eg, measurements with postural provocative tests or measurements with reactive hyperemia).

The Bottom Line
Appropriate noninvasive vascular screening can reduce unnecessary amputations and help clinicians make better use of advanced technology, thus saving money and lives. The problem is that even if clinicians are committed to screening, payers may not cover it for those who most need it, and the technical component may not be billable if performed by the staff members who are trained to do it if their credentials do not meet payer requirements. The equipment is relatively expensive and purchase may only be justified if the test is reimbursable. Clinicians may be left with choosing between what is right and what is covered. Clinicians should work with regional carriers for better coverage policies regarding noninvasive testing. If not used properly, TcPO₂ (or even SPP) can be unintentionally misused as a way to select patients with the best vascular status for HBOT, rather than as a way to reduce amputation rates. Following the stepwise decision tree at right will ensure no patients with significant peripheral arterial disease are overlooked. This is what happened to the patient in the case presented here, who was followed for a year in a wound clinic without benefitting from therapy because he had undiagnosed ischemia. Patients who are not candidates for revascularization or who remain ischemic after revascularization (a common occurrence) can undergo HBOT. In this way, HBOT becomes part of a limb-preservation program, reducing the rate of amputation among highest-risk patients. 

Rational Approach to Noninvasive Vascular Screening in the Wound Clinic
1. Vascular screening is clinically indicated for all patients living with nonhealing lower extremity wounds. (Whether screening is billable is a separate issue.)
2. Patients with low baseline TcPO₂ values breathing air should be challenged with 100% oxygen. Patients whose values increase dramatically upon respiring sea level oxygen are unlikely to have large vessel disease. Patients with a low SPP but a normal PVR likely have microvascular disease.
3. Patients whose TcPO₂ values fail to increase with sea level oxygen or who have a low SPP in the presence of a dampened PVR can be referred for anatomical studies to determine whether correctable disease is present. 
4. After vascular status is maximized, TcPO₂ or SPP should be performed again. Patients who still have low baseline values should undergo in-chamber TcPO₂ testing to determine whether HBOT is likely to be of benefit.

Caroline E. Fife, MD, FAAFP, CWS, FUHM, is chief medical officer at Intellicure Inc.; executive director of US Wound Registry; medical director of St. Luke’s Wound Clinic, The Woodlands, TX; and co-chair of the Alliance of Wound Care Stakeholders. 

References

1. Hopf HW, et al. Guidelines for the treatment of arterial insufficiency ulcers. Wound Repair Regen. 2006;14(6):693-710.

2. Fife CE, Smart DR, Sheffield PJ, Hopf HW, Hawkins G, Clarke D. Transcutaneous oximetry in clinical practice: consensus statements from an expert panel based on evidence. Undersea Hyperb Med. 2009;36(1):43-53.

3. Fife CE, Buyukcakir C, Otto GH, Sheffield PJ, Warriner RA, Love TL, Mader J. The predictive value of transcutaneous oxygen tension measurement in diabetic lower extremity ulcers treated with hyperbaric oxygen therapy: a retrospective analysis of 1,144 patients. Wound Rep Regen. 2002;10:198-207.

4. Lo T, Sample R, Moore P, Gold P. Prediction of wound healing outcome using skin perfusion pressure and transcutaneous oximetry: a single-center experience in 100 patients. Wounds. 2009; 21(11);310-316.

5. Adera HM, et al. Prediction of amputation wound healing with skin perfusion pressure. J Vasc Surg. 1995; 21:823-829.