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Hello ladies and gentlemen. It is a pleasure and honor to be here today and talk about Non Invasive Modalities for Monitoring of Vascular Function in the Diabetic Foot. My name is Manish Bharara and I am a post doctor research fellow with Dr. David Armstrong at The Center for Lower Extremity Ambulatory Research.
Production of this present lecture was made possible by a generous grant from HyperMed, the leader in hyperspectral medical imaging.
We all know diabetes is a global problem. It is a disease with multi-system complications. But today we will be focusing on lower extremity complications of diabetes. Diabetic foot is the most commonly occurring complication. In reference to complications of the foot, distinct underlying factors of it include peripheral vascular disease and peripheral neuropathy. If you look at the epidemiology, there are 21 million people in the United States affected by diabetes and just under 200 million people worldwide are effected by diabetes. 25% of these diabetics will develop a wound or ulcer during their life time, and more than 50% of these wounds will get infected.
Approximately 20% of these ulcers will lead to an amputation.
Instances of diabetic foot disease is growing worldwide, contributing to a huge socio-economic burden. Every 30 seconds a limp is lost somewhere in the world as a consequence of diabetes as reported on the cover page of Lancet.
If you look at the economic burden in the US alone, more than $16 million dollars are spent on chronic wound care.
Now, this brings us to an important question. Why is the diabetic foot important?
It is important because foot ulcers and its associated complications are an important cause of morbidity and mortality.
Five year mortality rate for a patient having peripheral vascular disease and amputation is second only to major forms of cancer such as lung cancer, pancreatic cancer, and colon cancer.
It is now important to consider factors which are significantly associated with the presence of foot ulceration. These include duration of diabetes, male gender, poor glycemic control, neuropathy, biomechanical abnormalities which include foot deformity, limited joint mobility, and high plantar pressures and history of previous ulceration or amputation.
Letâs discuss how does diabetes lead to amputation? It is not a one step process. There are multiple steps involved. All of which are targets for intervention. Letâs consider these steps.
Neuropathy is an important cause of foot ulcers and traumatic amputations due to loss of protective sensation patients often do not recognize injury until the full thickness that ulcers occur.
Currently there is no FDA approved treatment to reverse neuropathy. However, we can delay its onset and prevent complications. Recent work by doctors Lavery and Armstrong has suggested that home monitor of foot temperatures using handheld thermometers can reduce the rates of recurrence of ulcers by 4-10 times.
Peripheral vascular disease is an important factor; however, alone itâs not a risk factor for foot ulcers. It helps decide the level of amputation.
Infection is the most emergent of all these factors. It develops when a skin breaks down. However, with good wound healing principles and proactive approach this emergent situation can be prevented.
What does a typical lab and foot examination include?
It includes patient history, visual inspection, dermatological inspection, screening for neuropathy, vascular examination, and biomechanical assessment.
Today we will be focusing on vascular examination and different modalities that can help us fulfill the requirements for a good vascular examination.
We have just considered the stairway amputation concept and individual roles of peripheral neuropathy and peripheral vascular disease leading to a foot ulcer. It is important to emphasize that there is often an attributing factor which could be reparative stress or foot deformity, which leads to foot ulcer and subsequent amputation.
Neuropathy is typically classified into three categories which is motor, sensory, and autonomic. There is a battery of tests used to evaluate these conditions. But today we will not be focusing on this. Instead we will be talking about different modalities for testing peripheral artery disease.
There are two components for testing peripheral vascular disease, which include microvascular and macrovascular. Today we will be discussing in detail ankle brachial index, duplex ultrasonography, segmental pressures, skin profusion pressures, pulse volume recordings MRI, angiography, laser Doppler, TcPO2, and hyperspectral imaging.
We have just looked at the two components for vascular disease. The macrovascular component is also referred to as disease of large vessels or peripheral arterial disease. Whereas, microvascular component involves capillary loss and endothelial dysfunction. For a patient with foot ulcers paravascular disease leads to wound hypoxia.
Peripheral arterial disease is typically characterized by atherosclerosis for patients with long standing diabetes.
About 12 million people in the United States are affected by PAD and most of them are asymptomatic.
Typical symptoms include claudication and critical limb ischemia.
There is a risk for PAD even in pre-diabetic state.
Strongly associated with neuropathy and it worsens over time; typically detected at later stage with non healing skin ulceration.
PAD is 4-5 times more prevalent in diabetic than in non diabetic population. It leads to rheological changes in blood and can impair the nutritional supply to lower extremities, making the foot more susceptible to ulceration in the presence of a triggering factor such as minor trauma or reparative stress.
PAD is an important risk factor for post operative complications in patients with ulceration, indicative of poor healing and infection proliferation.
Microcirculatory complications are also, sometimes referred to as microcirculatory disease or microangiopathy. This is better discussed with neuropathy.
There is a high variation in the anatomy of microcirculatory components at different body sites and between individuals. Clinically microcirculatory complications are most significant in relation to retina and kidney. That is when addressing retinopathy and nephropathy; the systemic process progressively involving the majority of capillary beds.
There is a relatively lower incidence at upper extremities. This is due to high blood flow rates and absence of pressure gradients unlike at lower extremities.
It is often difficult to separate the response of nutritional and thermoregulatory vessels. Therefore, the majority of the modality that we discuss today, assess a combine response of nutritional and thermoregulatory vessels.
Letâs now focus our attention on the important assessment parameters for microcirculatory and macrocirculatory complications.
The macrovascular component typically involves an assessment of arterial supply to the lower extremities. Important parameters include foot pulses, ankle pressure, toe pressure, and pulse volume.
The microvascular component involves assessment of perfusion status in smaller vessels and capillaries. Important parameters include blood flow, blood volume, surface oxygen, intracellular oxygenation, and cellular respiration.
Until recently peripheral vascular disease was not considered as an independent risk factor for development of foot ulcer. Therefore, this classification system which includes neuropathy, deformity and history of ulceration or amputation was considered a paradigm for risk stratification.
However, recent data from Dr. Lavery published in Diabetes Care suggests that vascular disease may be an important independent risk factor. Vascular disease plays a significant role in evaluation of risk for ulcer occurrence, amputation, and hospitalization. This has led to a new classification to access the evolution of risk which now includes peripheral vascular disease.
This is an important step forward to further our understanding of pediophiological causes leading to diabetic foot ulcers and designing clinical management strategies focusing on care for high risk patients.
Before we discuss each technique individually, letâs focus on important characteristics of an ideal measurement technique. An ideal measurement technique should be non invasive, non ionizing, simple, objective, should have high response times, have high reproducibility, and must have scientific evidence behind it. Additionally, there is some recent evidence from some thermometry studies suggesting that weight bearing measurements may be important for diabetic foot evaluation because weight bearing evaluation reflects normal hemodynamic situation for ulcer development.
Especially for medical imaging modalities for whole field capability is very important because it facilitates measurements relative to contralateral limb or adjacent unaffected areas.
Letâs now focus our attention to the wide variety of testing techniques that are out there for evaluating the diabetic foot. Weâll begin by looking at the typical vascular evaluation which involves palpation of foot pulses, visual inspection of skin, and interdigital spaces.
Presence of pulses indicates adequate circulation which is important for sufficient debridement, control of infections, and good healing rates. This is accomplished either manually or with handheld Dopplers to look at the monophasic, biphasic, and triphasic weight forms.
It must however, be emphasized that this is a subjective procedure and varies with clinicians experience. It has high false positives and false negative rates; typically followed by Ankle Brachial Pressure Index.
Ankle Brachial Pressure Index is the ratio of systolic blood pressure at the ankle and brachial artery measured in supine position.
It is a recommended procedure for screening patients with diabetes, symptoms of PAD, and secondary risk factors for lower extremity complications.
Normal range for ABPI are from 0.91-1.30 and ABPI less than 0.9 is diagnostic of PAD.
It is a useful parameter to access degree of ischemia assessed by comparison of contralateral limb.
Typically, ABPI is 95% sensitive and 100% specific relative to angiography. However, there are high false positives due to arterial calcifications, especially in diabetics.
This procedure is typically supported by Duplex ultrasonography and arteriographic assessment.
Letâs now consider some of the tools when we have false elevated ABPI for diabetics. Letâs first consider TBPI, toe brachial pressure index.
Why can we use TBPI? Because digital arteries are less calcified. Similar to ABPI, it is calculated as ratio of toe pressure to brachial pressure. As a reference, toe pressures should be 60% of brachial pressure.
TBPI greater that 0.6 is normal and TBPI less than 0.6 is diagnostic of PAD. Toe pressures less than 40 mmHG is indicative of impaired wound healing.
However, the technique does have limitations. Itâs impossible to measure if digits are affected, just thick callused, or if toe is absent due to amputation.
Letâs now talk about segmental pressures. Segmental pressures are the extension of the same principle. Blood pressure cuff placed on lower extremities at different levels.
Typically blood pressure increases distally due to aorta. Blood pressure at thigh region is greater than brachial pressure by approximately 20-40 mmHG. It is very useful for identification and localization of pathology.
Again, it is impossible to measure with patients with amputation.
We just talked about toe pressures and segmental pressures. This slide illustrates a typical clinical measurement of TBPI and segmental pressures.
This slide illustrates typical segmental pressures measurement in a diabetic patient with peripheral vascular disease. Unlike a healthy subject, blood pressure decreases distally due to aorta, leading to ABPI of 0.54 in the left and 0.44 in the right foot which is indicative of moderate PAD.
Just at the beginning of this presentation when we talked about macrocirculation assessment, we talked about blood volume assessment. Pulse volume recording is a useful technique to measure changes in lower extremity blood volume at each cardiac cycle.
Itâs a qualitative tool that is not affected by calcified arteries. Itâs very useful for measuring normal pulsatile arterial flow in case of obstruction, which could be mild, moderate, or severe. We can identify any stenosis in the limbs.
Normal waveform is triphasic with a characteristic dichrotic notch indicating the elastic recoil of the artery. It becomes biphasic with stenosis and monophasic with severe disease.
Doppler waveform with PVR distinguishes multi-level disease from isolated tibial disease.
Pulse volume recordings capability is typically integrated in to the segmental pressures measurement system. The figure in the slide indicates a normal triphasic form in the left and abnormal wave form in the right indicative of stenosis and leading to obstruction in normal lower extremity flow.
Moving forward onto the next modality, letâs consider skin perfusion pressure. Skin perfusion pressure indicates pressure at which blood flow resumes in capillaries forming a hypolemic even. Hypolemia is a normal physiological response of a microcirculation followed by an event of exclusion or ischemia or trauma stimulus.
This is a non invasive and real time vascular laboratory procedure. Typically used for assessment of critical limb ischemia and predicting wound healing. It is unaffected by falsely elevated systolic pressures. Other applications include wound care, surgical planning, and pre/post operative assessments; typically facilitated by three different techniques. First, radioisotope clearance; this involves histamine induced hyperemia and measurement of isotope clearance.
Second, photoplethysmography involves cuff occlusion and measurement of reappearance of pulsatile blood flux.
And last is laser Doppler involves cuff occlusion or thermal hyperemia and measurement of movement of red blood cells.
Is SPP clinically useful?
Yes. There is compelling evidence supporting its use for diagnosis of critical limb ischemia, severity of PAD and predicting wound healing.
In a study of 53 patients, accuracy of SPP less than 30 mmHG is a diagnostic test for CLI is reported at 79.3%.
In a study of 211 patients, there is good correlation between SPP and ABPI, SPP and TBP, and SPP and TcPO2 have been reported. Similar studies report SPP greater than 40 mmHG as a diagnostic test but sensitivity of 72% and specificity of 88%. Furthermore, SPP, TBP and healing rate show good correlation for wound healing prediction.
Similar studies, ideally well designed randomized controlled trials are needed to establish its role along with other techniques in a complimentary sense.
This technique is non invasive, itâs not site restrictive, it involves limited operator variability, its objective, sensitive, and can be used in suboptimal tissue conditions and are not affected by calcified arteries.
However, the technique does have a limitation, motion artifact- especially in patients with tremors.
We briefly touched upon laser Doppler when we were discussing SPP. What is laser Doppler?
It basically facilities measurement of blood flux, which is defined as the product of red blood cell velocity and red blood cell concentration. The technique is capable of providing evidence of microangiopathy and autonotomic dysfunction during reactive hyperemia that may be induced thermally or physically.
The figure on your right shows a typically laser Doppler system. In a weight bearing assessment of microvascular function in diabetics with clinical detectable PVD and or neuropathy, Cobb et al. demonstrated evidence of reduction in reperfusion rates of dynamically loaded plantar tissue.
It is important to emphasize that this was a first weight bearing study of its kind.
We also briefly touched upon correlation between SPP and TcPO2. So what is TcPO2?
Itâs sensitive, non-invasive, and functional measurement of dermal perfusion and tissue nutritional status. Typically, a Clark type platinum electrode provides current proportional to PO2 which is oxygen pressure mmHG, following thermally stimulated hyperemia.
This is measured in capillary pressure, capillary exchange rate, cellular respiration and skin resistance to oxygen diffusion stabilize.
TcPO2 is used to deign revascularization surgery, determine amputation level, and determine therapy.
TcPO2 greater than 40 mmHG, is indicative of adequate circulation and TcPO2 of less than 30 mmHG is indicative of ischemia.
PAD results in reduced PO2. Why? Because in diabetics with microangiopathy, there is decreased nutritive blood flow to AV shunting with results in this reduction.
Is this indicative of subclinical autonomic neuropathy?
There is little evidence currently to support the use of TcPO2 to supplement diagnosis of subclinical autonomic neuropathy. However, this may be accomplished by combining TcPO2 and thermometry for this purpose.
Itâs important to emphasize that there is considerable biological variability in TcPO2 assessment. This may be due to variation in thickness of epidermis in diabetic neuropathy. TcPo2 generally correlates well with wound healing potential. Interestingly, TcPO2 could also be used as a ratio at extremity to TcPO2 at chest, which is also defined as the Regional Perfusion Index.
TcPO2 is generally used in dynamic studies to evaluate hyperemic response, which measures Oxygen Reappearance Time and Oxygen Recovery Time.
TcPO2 is best used in a complimentary sense for clinical decision making.
Until now we have discussed techniques of applying these measures for assessing the vascular function of the diabetic foot. However, recent advances have facilitated employing medical imaging modalities to supplement the clinicianâs visual inspection of the feet. Use of these techniques can advance our current understanding and result in better clinical management of these complications.
We will now go through duplex color Doppler, magnetic resonance arteriography, and geography and hypospectral techniques in the next few slides.
Duplex Color Doppler is a non invasive vascular imaging tool. It helps in identification of atherosclerotic lesions in lower extremities. It is highly sensitive to silent lesions; however, it is not sensitive to morphological changes in blood vessels and it is problematic with obese individuals.
MRA, which is Magnetic Resonance Arteriography, facilitates imaging of vasculature for anatomical and physiological changes. Typical applications include: surgical planning, especially in revascularization and lesion localization with high visual detail. Contrast enhanced MRA reduces additional vascular imaging procedures by 42%. However, high cost limit routine clinical use.
Moving to angiography. Although this is an invasive procedure but it is discussed in this presentation because it is a current gold standard for evaluation of intraluminal obstruction and diagnosing occlusive arterial disease. It provides morphological data and visualization of vascular tree.
Limitations include: it is invasive, involves radiation exposure, its toxic, and expensive. Other limitations include: it requires comprehensive evaluation of diabetics with other co-morbid disease and current drugs. Typically used for surgical planning.
Digital subtraction angiography can provide image enhancement and degree/length of stenosis.
Letâs now talk about Hyperspectral imaging?
What is hyperspectral technology?
Spectrology in a nut shell is a technique of measuring any physical quantity in terms of frequency or wavelength. Hyperspectral technology is the extension of spectroscopy, where multiple images of the same thing are taken to identify signature spectral using multi-dimension data set.
It is typically used for assessment of physiological changes in terms of acquired wavelength. Itâs a rapid camera based system which is specifically tuned to image oxyhemoglobin and de- oxyhemoglobin that is oxygen delivery and oxygen extraction. Itâs a non contact, non invasive, whole field, portable, safe and objective modality. It may provide subclinical indicators of vascular status. Typically data acquisition time is 15 seconds. It provides a visual map isolating areas of active ulceration or at risk of ulceration.
From a clinical stand point the choice of oxy and de-oxy hemoglobin is justified as underlying microvascular disease predisposes skin tissue to ulceration. The hypospectral system captures images at each wave length. All the 15 wave lengths used are biological specific and it provides a multi-dimension data set, which is used to extract information and provides measurements of oxy and deoxy hemoglobin at each pixel in the image.
Considering a typical hypospectral image, spacial distribution of deoxy-hemoglobin provides a visual map isolating areas of active ulceration and by drawing a region of interest around the ulcer its healing potential can be determined.
Some potential uses of this technology include prediction of ulcer healing. This further includes assessment of perfusion, tissue viability, the data assists in surgical planning especially level of amputation, flap rotation, and debridement and timely intervention may provide an early referral to the patient for vascular surgery.
It would stand to reason that this data may provide postoperative physiological changes evaluation and some scientific evidence needs to be collected to validate this hypothesis.
Secondly, the data from hypospectral imaging may be used to predict development of an ulcer. This provides pre-emptive care for our diabetic patients, provides pre-op evaluation for preventive surgery, device can be used as a screening tool especially when considering identification of patients with high risk. Hypospectral measures metabolic status so; therefore, it may be capable of measuring pre-ulcerous inflammation and there is some ongoing interesting work looking at a combination of hyperspectral and thermometry datasets.
Letâs consider the clinical evidence supporting this technology. In a six-month follow up study of type 1 diabetic patients, sensitivity of 93% and specificity of 86% has been reported; the positive and negative predictive values being reported at 93% and 86% respectively.
There are numerous other clinical studies in progress using specific assessment protocols.
The most importantly, being a federally funded prospective, double blind multi-center clinical study to predict ulcer healing potential and to predict development of ulcer.
Letâs consider results from the ulcer healing potential in the next slide.
This slide shows an arbitrarily chosen value of oxyhemoglobin at 45 in relative units with 100% sensitivity and 100% specificity provides a modest mold for wound healing, which means that all patientsâ with values of oxyhemoglobin at greater than 45 healed within 6 months.
Weâve just looked at some compelling evidence from the healing prediction data. Now letâs consider some images. On the right we have a visual image from a patient with an active ulcer. On the left we have a hypospectral image from the first visit of the patient.
If we consider data at six months, we have a healed ulcer and the oxyhemoglobin value at the base line of 67.
Letâs consider another case from a patient with a non healing ulcer.
First visit we have an active ulcer with an oxyhemoglobin value at 40. At six months, we still have an unhealed ulcer.
As we discussed, hyperspectral imaging data provides evidence of function changes in nutritive blood flow which is useful in predicting ulcer healing, surgical planning and identifying high-risk patients.
Letâs now consider a case study from a patient identified with superficial inflammation under the 2nd and 3rd metatarsal head region. This area appears as a hard spot and has an abnormally high oxyhemoglobin value. When compared with the contralateral foot there is a high absence percentage difference between the two sides. This patient did develop an ulcer of the same spot.
Here is the preliminary data reporting from subjects of the absolute prediction study. High absolute percentage value of the two bilateral sites is a strong predictor for ulcer development.
In this slide we consider the final case study where hyperspectral imaging is used to decide level of amputation, especially with the intent of limb salvage. Sufficient in the profusion in the distal region of the toe results in appropriate surgical intervention.
Today, we have considered multiple modalities to assess vascular pathologies in diabetic foot disease.
These are all targets for intervention. We already have a well-defined problem at hand. The important question is how can we diagnose it and how can we prevent its traumatic consequences?
In a nutshell, all the techniques we discussed provide assessment of perfusion and metabolism.
Key is comprehensive patient evaluation, patient history, visual inspection and interpretation of test results.
It would stand to reason to use these modalities in a complementary sense based on scientific evidence and medical intervention in context.
To summarize the take home points from this presentation:
It is very important for the clinician to identify ischemic component of a non healing ulcer.
PAD is a marker of systemic vascular disease which may involve coronary, cerebral, and renal components.
By identifying the subclinical disease, it may be possible to use preventative measures and avoid limb loss.
Use of one or more vascular assessment procedures to identify if the patient is a good candidate for surgery.
Initial physical examination can sometimes be misleading. Therefore, it must be supported by- the clinical decision must be supported from complimentary modalities.
Circulation may be sufficient at times. However, other factors such as infection, pressure necrosis, neuropathy, metabolism, or pharmacological derangements must be considered as they can affect the intended clinical outcome- that is ulcer healing and would closure.
This calls for a multi-disciplinary team approach to focus care on high risk patients.
We have learned from the restratification studies from Dr. Laveryâs group to focus care on high risk patients. Each test modality may have specific benefits depending on specific problem and desired clinical outcome, which include diagnosis, prevention, ulcer healing, and surgical planning.
Itâs important to identify the context and justify the device used based on evidence, cost, sensitivity, and its overall capabilities.
This slide summarizes the important device characteristics for all the modalities that we discussed today. There is a growing body of evidence emerging from these assessment techniques on modality in diabetic foot care. This knowledge base coupled with the contextual approach that we discussed today can enhance our ability to employee effective prevention strategies.
Production of this present lecture was made possible by a generous grant from HyperMed, the leader in hyperspectral medical imaging.
Ladies and gentleman, thank you very much for your attention today.