Non-invasive Vascular Testing and the Wound Care Clinic

  Some months ago a 30-year-old white male came to see me with a non-healing injury to the great toe. He had been followed for a year at another wound center in my city, using a variety of topical products. He had a 20-year history of smoking and was employed full time as a machinist, had a new baby and otherwise appeared well. We have a policy in our wound center that all patients with chronic non-healing lower extremity wounds are screened for vascular disease. However, the staff wondered if this healthy-appearing young man really needed to undergo vascular testing. It was an unusually busy day, and testing takes time.

  Fortunately, we followed our protocol that day. The transcutaneous oximetry (TCOM) values on the dorsum of his foot were only 8 and 10 mmHg (50-70mmHg would have been normal) with a poor response to sea level oxygen. I thought to myself, “Surely these results are in error.” I went back into the room to get a little more history. I already knew that his father had died of an MI at the age of 42, but this time I asked more pointed questions about how far he could walk. On careful questioning, he admitted to cramping in his calves when he walked across the machine shop. I decided to get a skin perfusion pressure test, which also allowed me to evaluate his lower extremity waveforms (more on that below). His skin perfusion pressure was 35 mmHg, borderline for wound healing prediction and an indication of arterial disease. However, worse than that, his pulse volume recording was a flat line. Sure enough, an angiogram revealed that only one of the three vessels below the knee was open. A few days later he underwent peripheral by-pass surgery. This 30-year-old man had arterial vascular disease severe enough to prevent him from healing a toe injury. I will use this case to illustrate the pros and cons of two methods of non-invasive vascular testing, but also WHY non-invasive vascular testing is so important.

Why Screen?

  There are 5 reasons to perform non-invasive vascular testing, although the reasons overlap. They include:
  1. Prediction of healing.
  2. Screening for arterial disease.
  3. Amputation level prediction.
  4. Determining response to revascularization.
  5. Predicting benefit of Hyperbaric oxygen therapy (HBOT).

Amputation: A Fatal Disease

  The wound center, which saw this patient for a year and failed to diagnose his vascular disease committed what may be the cardinal sin in the wound care world, namely, failure to recognize arterial disease. Why is this a big mistake?

  After a major amputation (below the knee or higher), the 5-year survival rate for non-diabetics is 50%. For a diabetic, the 5-year survival rate is only 30%, and only 14% of diabetics with renal failure are alive 5 years after a major amputation. Thus, the 5-year survival of patients after a major amputation is worse than that of most cancers. Major amputation can be considered a “fatal disease.” The patients who are likely to require an amputation are those with arterial insufficiency, and who are they? They are patients with diabetes, who smoke, and who develop non-healing wounds. In other words, they are the patients we see every day.

  Guidelines recently published by the Wound Healing Society (WHS) state that all patients with lower extremity ulcers should be assessed for arterial disease. (www3.interscience.wiley.com/cgi-bin/fulltext/118605281/PDFSTART). However, the ideal way to perform non-invasive vascular screening has not been defined by clinical research. A variety of clinical options exist. One can look at the “macro” circulation (the large blood vessels) or the “micro” circulation (the 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). The ABI is the ratio of the ankle systolic blood pressure to the brachial systolic blood pressure. It is technically easy to perform, takes only about 15 minutes and requires only a blood pressure cuff and a hand held Doppler. ABI values of 0.7 or less are considered abnormally low, with values of 0.3 or less representing severe obstruction. (And by the way, ABI is not separately reimbursed; results must be accompanied by printed waveform analysis in order to meet Medicare reimbursement requirements.) The problem is that the ABI can be falsely elevated in diabetics due to the incompressibility of their calcified blood vessels, so ABI values correlate poorly with healing prediction in diabetics. We need some better way to assess the vascular supply or healing prediction.

Transcutaneous Oximetry (TCOM)

  Transcutaneous oximetry measures the oxygen partial pressure in the skin (TcPO2). The technology is old (1960’s). 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 or 45 Centigrade, dilating the blood vessels and allowing 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 solution. This means that the machine is measuring the actual number of oxygen molecules (PO2) in the periwound area, NOT OXYGEN SATURATION! In fact, before pulse oximetry was available, TCOMs were used as a way to non-invasively monitor arterial PO2 in babies. TcPO2 is not a way to determine arterial PO2 in adults because of the thickness of adult skin, among other reasons. However, it can provide an estimate of arterial perfusion to the area. The results are reported in “mmHg” just like an arterial blood gas. It is important to remember that the machine has to be calibrated, the seal has to be air tight, and there is a steep “learning curve” for the technologists. It is an “artsy” test. Examples of devices are those manufactured by Radiometer (Denmark) and Perimed (Stockholm, Sweden).

  Fifteen studies (1137 patients) have demonstrated that 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 which creates a barrier to diffusion of oxygen will lower TcPO2, so callous, edema, skin diseases such as scleroderma, scar tissue, or placing the electrode over a bony prominence may decrease the readings. Factors, which decrease blood flow, may also decrease TcPO2 such as hypotension, dehydration or vasoconstriction due to cold. 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. 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. If the TcPO2 values increase to >100mmHg with sea level oxygen breathing, then the patient is not likely to have arterial disease.

  Thus, it is important to remember the following things:
  1) 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 because the equipment is not cheap.

  2) Some sort of “challenge” is important as a method of determining whether a low value is due to arterial disease or a diffusion barrier. If you are not using a challenge, you have not done a complete test. Oxygen is the most commonly used challenge. Leg elevation is also used as a test for arterial disease (if values decrease with elevation, then arterial disease is likely).

  3) 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 oxygen, the treatment is edema reduction with compression, not arterial revascularlization.

  An evidence-based guide to TcPO2 interpretation has been recently published.¹ Transcutaneous oximetry has many hundreds of references to support its use, including randomized controlled trials. Interestingly, it is used most often in facilities, which offer hyperbaric oxygen therapy because it is often used as a way of predicting which patients will benefit from HBOT. Diabetic patients whose values during HBOT are >200 mmHg have a significant likelihood of benefitting from hyperbaric oxygen therapy. Patients whose “in-chamber” values are <50mmHg are not likely to benefit.² “Non-hyperbaric” wound centers have been slower to adopt TcPO2. However, it has long been used for prediction of healing, including prediction of amputation level. It can also predict response to revascularization. TCOM values that increase at least 30mmHg after either endovascular or surgical revascularization are associated with a successful revascularization procedure and with a high likelihood of healing an open wound.

  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 callous, 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 air tight (otherwise the electrode will read the oxygen molecules in the air). It is an estimation of the OXYGYEN SUPPLY to the periwound. Patients who are on supplemental oxygen probably cannot have meaningful TcPO2 studies.

Skin Perfusion Pressure (SPP)

  Another non-invasive vascular screening option is the measurement of the capillary opening pressure after occlusion. This involves using a cuff to occlude blood flow, and then slowly deflating the cuff allowing the gradual return of blood flow. The maximal velocity of the returning blood flow is the skin perfusion pressure, as measured by a laser device, which 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).

  SPP technology has a number of advantages in that it is:
  • Not affected by vessel calcification.
  • Not (much) affected by edema.
  • Not affected by location (can use this on the plantar foot and the digits).
  • Not affected by callus or thickened skin.
  • No calibration is required.

  Lo and colleagues compared SPP and TCOM in terms of ability to predict healing outcome in 100 patients with lower extremity wounds. 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). Skin perfusion pressure was more sensitive in its ability to predict wound healing relative to TcPO2 (99% vs. 66%).³ SPP is also highly reliable in the prediction of healing after amputation, and it can be used to diagnose limb ischemia4 and thus select patients who need further vascular testing. In addition to the uses above, SPP testing may be useful for planning foot and ankle surgery and ensuring that the patient can tolerate compression therapy for venous ulcers.

  SPP Interpretation Guideline (in mmHg)
  • 30 or less = Chronic limb ischemia (CLI), Wound healing unlikely.
  • 30 to 40 = GRAY ZONE in Wound healing /Likely to Moderate Ischemia.
  • 40+ = Wound healing probable / Mild to Moderate Ischemia.
  • 40-50 = Mild Ischemia.
  • 50 or more = Normal Skin Perfusion.

Do TcPO2 and SPP Measure the Same Thing?

  Many people find it confusing that both of these technologies 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 patient I used in my introduction could have different values for his TCOM and his SPP. Different things were being measured. My patient’s oxygen partial pressure to the skin was only 8-10 mmHg. However, he had probably developed small collateral vessels, which had kept his perfusion pressure at 35 mmHg. There was one more piece of information, which the SPP device was able to provide which helped me understand 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. We are all 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 poorer flow. There are also observable changes within the waveform itself such as steepness of upstroke, shape of the peak, steepness of down stroke and presence/absence of the dichrotic 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 my patient whose flow to the foot was so poor there was no pulsatile waveform at all.

   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, I have found that the time to perform SPP and TCOM can be comparable.

Complimentary Technologies

  I have more than 20 years experience with TCPO2 and more than 10 years of experience with SPP. I find the two technologies highly complementary. I use both devices every day, often selecting one over the other for a given patient, and occasionally using one to check the results of the other as I did in the example cited above. Frankly, SPP is the easier technology to learn and implement, and likely less costly to introduce to the clinic. It will not be useful in determining which patients will benefit from HBOT. For a clinic just beginning to perform non-invasive testing, the most logical approach may be to initially screen with SPP, reserving TcPO2 for in-chamber testing. However, I find I need both technologies.

Coverage Policy: Which Patients, How Often and By Whom?

  Although they measure different things, both SPP and TCOM are billed using CPT codes 92922 and 93923. I am not a reimbursement expert and am not qualified to provide advice regarding billing and coding. Therefore, I can only point out some potential problems, which the reader needs to research further with a reimbursement expert.

  For example, there are regional variations regarding which Medicare beneficiaries can undergo vascular screening and unfortunately they do not necessarily correlate with evidence-based guidelines for screening. You need to review your local carrier coverage policy. Most evidence based guidelines agree that all patients with non-healing leg ulcers should be screened for arterial disease. However, many carriers will not cover vascular screening for patients 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. Pre-operative screening prior to elective surgery may also be a medically justifiable reason to perform non-invasive arterial studies, but is often not covered by third-party payers and Medicare.

  There are also Medicare limits on the frequency of non-invasive 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. Medicare reimbursement for non-invasive vascular testing is usually restricted to physicians or to credentialed vascular technicians. However, some regions may allow certified hyperbaric technicians (CHTs) to perform transcutaneous oximetry testing since this is part of their CHT training. At this time, CHT certification does not include SPP training. Thus, it could be argued that CHTs may not be able to perform SPP evaluations even though the same procedure code is used to bill both tests.

The Bottom Line

  Appropriate non-invasive 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, payors may not cover it for the patients who most need it, and the technical component may not be billable if performed by the staff who are trained to do it if their credentials do not meet payor requirements. The equipment is relatively expensive and purchase may only be justified if clinicians can bill for performing the test. Clinicians may be left with choosing between what is right and what is covered. Clinicians should work with their regional carriers for better coverage policies regarding non-invasive testing.

A Rational Approach to Screening

  1. Vascular screening is clinically indicated for all patients with non-healing lower extremity wounds. (Whether screening is BILLABLE is a separate issue).

  2. Patients with low 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.

  Dr. Caroline Fife, is currently co-editor of Today’s Wound Clinic, is the director of Clinical Research at the Memorial Hermann Center for Wound Healing, Houston, Tex., and Chief Medical Officer of Intellicure, Inc. She can be reached at cfife@intellicure.com.

References

1. 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. 36(1):43-53, 2009.

2. 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 1144 Patients. Wound Rep Regen. 10:198-207, 2002.

3. 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.

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