New, non-invasive technologies are the key to better outcomes and efficient management of wound care patients
Worldwide, diabetes and peripheral arterial disease continue to rise, with more than 30 million Americans now carrying the diagnosis of Type 2 diabetes mellitus1, and more than 8 million Americans diagnosed with PAD.
I recently had the opportunity to lecture on the topic of “New Technologies in Non-Invasive Vascular Evaluation” at the Undersea and Hyperbaric Medical Society conference in Dallas, Texas on September 7, 2019, and it was evident from the audience response that there is keen interest in the subject.
It’s easy to understand why, because never has there been a more pressing need to quickly and reliably assess the vascular status of our patients. Seasoned wound clinicians know that without a clear and early assessment of vascular status, it is impossible to make appropriate and timely wound care decisions and assure optimal outcomes.
However, as Caroline Fife, M.D. and other thought leaders have pointed out, too often, clinicians evaluate vascular status in a hit or miss fashion, to the detriment of patients. While there are wound care centers with pro-active vascular evaluation policies, where at minimum, ABI testing is performed on new patients presenting with a lower extremity wound and any PAD risk factors, just as many wound care programs do not have consistent policies for such testing.
One of the reasons for this is that commonly available vascular screening methodologies, such as Ankle Brachial Index, and the related Toe Brachial Index suffer from significant limitations in the diabetic populations common in our wound centers. Factors such as vascular calcification which are common in these patients render ABI data difficult to interpret. TBI addresses some of these limitations, but it can be very difficult to reliably administer the test in a significant percentage of patients.
More importantly, ABI and TBI do not tell us anything about microvascular flow in the wound bed, and certainly nothing about tissue oxygenation – an essential factor for wound healing.
Transcutaneous Oxygen Mapping, or TCOM, long accepted as the “Gold Standard” for assessing tissue oxygenation (and collaterally, microvascular blood flow) and thereby assessing potential for wound healing, also suffers from very significant limitations in clinical practice.
One key limitation of TCOM is that it cannot be used to directly assess perfusion of plantar skin; the site of some of our most challenging wounds. In addition, the TCOM electrodes in common use are not suitable for assessing perfusion in the toe, due to their size.
In addition to these technical limitations, TCOM is an expensive technology to purchase and maintain, with typical devices costing more than $50,000 to purchase. My group recently received a $76,000 quote for maintenance costs for six devices. They also require a significant training investment to use effectively, and testing can be time consuming, consuming valuable technician time and bed minutes in the clinic.
In an era of decreasing margins, and increasingly stringent demands for efficient, optimal utilization, the limitations and expenses associated with ABI and TCOM are forcing clinical leaders, and clinic owners to look to new technologies offering a better cost-benefit ratio.
As I pointed out in my recent lecture, two technologies are generating enthusiasm for new paradigms in non-invasive vascular evaluation in the wound clinic and bringing hope that wound care programs can grow beyond the current technical limitations and cost burdens imposed by existing technologies. The two technologies I focused on are Combined Skin Perfusion Pressure and ABI with PVR (Vasomed PAD-IQ), and Hyperspectral Imaging (Kent Imaging “Snapsho2t,” and Hypermed “Hyperview”).
While Hyperspectral Imaging and Skin Perfusion Pressure are very different from a technological perspective, each of these new devices address the imperative to quickly, repeatedly, and cost-effectively assess the vascular status of our patients, so that treatment decisions can be optimized from the point of care, on the first visit.
Hyperspectral imaging technology is not fundamentally new, but the use of hyperspectral imaging to assess skin blood flow is a recent development. Many wound care clinicians have seen one or more of these devices demonstrated at industry trade shows, such as SAWC, and those who have seen them are typically intrigued by what they see.
These units are just a bit bigger than an iPAD, and are capable of capturing an image that provides direct, quantitative visualization of oxygenated hemoglobin, deoxygenated hemoglobin, and oxygen saturation, superimposed on a visual image of the wound and surrounding skin.
Hyperspectral images can be obtained in seconds, without expendable costs, by a technician or provider with as little as five minutes of training, and they are compelling to look at, but are they useful clinically? The literature is meager right now, but several groups are publishing clinically oriented papers assessing the utility of the technology for wound care and vascular medicine.
These studies have looked at the predictive value of hyperspectral imaging in patients with vascular ulcers and diabetic foot ulcers, and what has been consistently evident is that deoxy-hemoglobin imaging data correlates reasonably well with TcPO2:
TcPO2 and DeOxyHgb (r2 = 0.63, P < 0.0001)2
Hyperspectral imaging also correlates with angiosomal anatomy:
“Deoxyhemoglobin values for the plantar metatarsal, arch, and heel angiosomes were significantly different between patients with and without PAD (P <.005)”3
Finally, and perhaps most meaningfully, in a study of 73 diabetic foot ulcers in 66 patients, over a 24 week period, hyperspectral imaging correctly predicted healing with 80% sensitivity, and non-healing with 74% Specificity.4 Thus as a predictor of healing, hyperspectral imaging compares quite favorably with TCOM, whose generally accepted sensitivity for prediction of healing is 72%.
Having evaluated one of these units in a wound clinic setting, I find the information helpful, but difficult to integrate into the practice patterns necessitated by LCD strictures, and commonly accepted treatment pathways. I intend to watch these devices evolve, and champion their promise. They likely represent a future evolutionary step in wound care and vascular medicine, but I don’t think they are a compelling replacement for TCOM at this time.
If Hyperspectral Imaging is not ready for prime time, Skin Perfusion Pressure may be ready to step into the breach and meet the need for timely and cost-effective evaluation of tissue perfusion in the critical DFU and vascular ulcer population. While not a new technology, SPP is now available in the form of an integrated, elegant device called PAD-IQ, capable of providing rapid, repeatable, predictive test results in the challenging lower extremity wound population.
By combining ABI testing with Pulse Volume Recording (PVR), PAD-IQ can deliver 100 sensitivity for identification of PAD, and 100% negative predictive value for absence of the disease. Integrative SPP allows clinicians to perform accurate and repeatable “perfusion maps,” much like those obtained using multi-channel TCOM testing, but unlike TCOM, users can assess perfusion at the toe, as well as plantar skin.
Having used one of these devices in the wound clinic for 6 weeks, I can say unequivocally that I am ready to give up my TCOM device and embrace a new way of assessing skin perfusion. The PAD-IQ is a well-designed device that meets multiple needs in one unit. Nurses and technicians were quickly trained and performing studies. The perfusion sensor is easy to apply, and the interface is about as elegant as any I have seen on a medical device. Printouts are easy to read, and information rich; the kind of reports one is proud to send to a referral source, or consultant. Results are unambiguous, and graphically clean.
Compared to TCOM, a PAD-IQ test can be completed in less than half the time. Like TCOM, interpretation of SPP is straightforward. Using a cutoff of 30mmHg (capillary opening pressure, not TcPO2), one can readily predict non-healing status, and determine the need for intervention.
How does SPP compare to TCOM in clinical trials? Lo, et. al. found that SPP alone successfully predicted wound outcome in 87% of the cohort compared to TcPO2 at a rate of 64% (P < 0.0002). In addition, SPP was more sensitive in predicting wound healing than TcPO2 (90% versus 66%; P <0.0001).5
These are impressive results, and I feel just what we need to move the management of lower extremity ulcers, and particularly diabetic foot ulcers forward. Too many programs are confounded by unsystematic, subjective vascular evaluation heuristics.
It is as though we have been evaluating vascular status in these technically complex, challenging cases using a “magic 8 ball.” I believe that, until we are assessing every lower extremity wound patient with PAD risk factors early, and definitively, we will be “running in place” as a specialty, generating inconsistent, suboptimal outcomes.
It is time, I feel, to make vascular testing simple, reliable, and something that happens at the bedside at the point of consultation in the wound clinic. I for one am ready to embrace change. Please let me know your thoughts and insights. I’ll be presenting on this topic next year at ACHM, where I look forward to sharing much more data.
Marcus Gitterle, M.D., FACCWS
Chief Medical Officer
WoundCentrics, LLC
__________________________________________
1Centers for Disease Control, 2019
2Jafari-Saraf L, Wilson SE, Gordon IL. Hyperspectral image measurements of skin hemoglobin compared with transcutaneous PO2 measurements. Annals of Vascular Surgery. 2012;26(4):537–548
3Chin JA, Wang EC, Kibbe MR. Evaluation of hyperspectral technology for assessing the presence and severity of peripheral artery disease. Journal of Vascular Surgery. 2011;54(6):1679–1688
4Bolton, L., Hyperspectral Imaging: Early Warning of Low Tissue Perfusion Wounds, 2012;24(10):A8-A105 Lo, T, et al, Prediction of wound healing outcome using skin perfusion pressure and transcutaneous oximetry: a single center experience in 100 patients, Wounds, 2009 Nov: 21(11) 310-6
5Lo, T, et al, Prediction of wound healing outcome using skin perfusion pressure and transcutaneous oximetry: a single center experience in 100 patients, Wounds, 2009 Nov: 21(11) 310-6