Diabetic Foot Amputation: The Need for an Objective Assessment Tool

Introduction Diabetic foot disease is a sequel of diabetes mellitus that is a growing problem worldwide. It is estimated that 221 million people will be affected with diabetes globally in 2010.[1] Patients with diabetes suffer blindness, kidney complications, and peripheral arterial disease.[2] Peripheral arterial disease is a significant risk factor for diabetic foot disease that may lead to critical limb ischemia, limb loss, and lower-extremity wounds in many patients.[2] Surgical management includes bypass grafting, angioplasty, wound debridement, and when inevitable, amputation.[3] The Saint Vincent declaration[4] called for a multidisciplinary team approach with an objective to reduce the amputation rate in diabetics by 50 percent; this target has not been achieved in the United Kingdom. Typically, a patient may have critical ischemia of the lower limbs with digital gangrene, loss of diabetic control, and underlying complications of the heart and kidneys. Another patient may have significant loss of nerve function in the feet and nonhealing wounds in the plantar surfaces that are complicated by infection that has tracked to the deeper foot tissues. There could be bone and joint deformities in the foot serving to complicate the condition. Multiple organ disease may also complicate the patient’s condition. In both instances, the viability of the foot must be evaluated in order to determine the best option for management. A surgical decision to amputate is based on assessment of the extent of necrotic digit/organ, the extent of tissue perfusion to sustain healing, and the general condition of the patient. The aim of this paper is to examine the objective measures of tissue viability available for clinical management. Viability Assessment Methods A patient presenting with symptoms of peripheral arterial disease, diminished or absent pedal pulses, will have Duplex ultrasonography followed by contrast angiography when indicated. These investigations that are widely available will yield reliable diagnostic information to manage the macrocirculation. In order to assess tissue viability objectively, there is a need to measure tissue perfusion and determine its functional status. Measurements of toe pressure and ankle-brachial pressure index (ABI) are direct and simple, though these merely indicate the need for surgical intervention without offering any guidance toward the management of tissue viability.[5] Tissue viability has been assessed by such direct methods as fluorescein angiography, thermography, radionuclide clearance techniques, as well as indirect methods, such as transcutaneous measurements of tissue oxygen and direct measurement of oxygen saturation.[6] The three former methods are imaging techniques. An injection of sodium fluorescein (NaF) permits the microcirculation to be visualized using an ultraviolet lamp in which perfused tissue appears green. This technique is qualitative and difficult to repeat, though it was widely used in clinical and animal studies. Thermography or imaging heat generated by tissues using highly sophisticated infrared camera systems can be used to assess tissue viability. Such systems generate thermal maps with a color to indicate “hot” spots and are incredibly helpful in managing soft tissue disease. The clearance of radioisotopes injected into the blood stream causes specific energy levels of radioactivity to be emitted hence permitting tissue perfusion measurements. This methodology was used in clinical and animal experiments using xenon (Xe 133) and iodine (I 125) using bolus injections and intradermal transfer of the radioactive tracers, respectively. Thermography and later the clearance of a radioisotope 4-iodoantipyrine (I 125) were successfully used to determine the viability of skin flaps for below-knee amputations. The combination of these techniques yielded a success rate of 93 percent for transfibial amputations. The ratio for transtibial to transfibial was 75 percent in Ninewells Hospital, Dundee, Scotland.[7] A healed stump fitted with a suitable prosthetic would offer good quality of life for that patient. Despite this success in Scotland, these techniques either singly or in combination are not reported in use in current literature, which may be because both methods need specialist skills and techniques. Such a study would be worthwhile to the literature; the demonstrated potential suggests the need for it. Direct measurement of tissue oxygen using an optical method is reported to have a sensitivity and specificity of 1.0 for transtibial amputations from a study in Ninewells Hospital, Dundee, Scotland.[8] Tissue oxygen saturation measurements rely on measuring reflected light from an organ; the principle is that at different wavelengths the oxygen content of hemoglobin would affect the level of light reflected by tissues. Simply, darker blood reflects less than well oxygenated redder blood. Both wavelength specific light using lasers or white light may be used as sources. When white light is used as the source, the reflected radiation may be detected selectively using optical filters in order to extract data about oxygen saturation. An advantage of using white light as a source is that it permits higher intensity to be used. As laser light is highly collimated and therefore focused, the maximum permissible power of laser sources is limited by regulations. For a class B laser this level is 2 mW. Direct measurement of tissue oxygen saturation using white light is used in the management of critically ill neonates where it is used to measure changes in cytochrome metabolism.[8] From a theoretical standpoint, this method could be well suited to studying problems in the diabetic patient, though the use of this method to measure tissue viability in the foot or digits has yet to be reported. The single major advantage of this method is that it is a direct measure of oxygen saturation compared with the transcutaneous technique. In the latter, a chemical sensor is used, which requires the sample volume to be heated beyond skin temperature. Transcutaneous measurement of tissue oxygen (TcpO2) is based on measuring the partial-pressure oxygen driving oxygen molecules through the dermal and epidermal layers and a membrane covering the sensor. The method has been well described in wound literature.[9] It relies on heating tissues to 43–45 degrees C, which permits patients to be discriminated from controls.[10] There are two commercial systems available for use in Europe. Both were reported to be well correlated with each other (r2 = 0.91) over the range of tissue oxygen values that may be expected in peripheral arterial disease.[11] The Roche oxygen sensor has a co-efficient of variation of 6.6 percent in skin over the malleolus.[12] On adults with peripheral arterial disease, there is a gradient in TcpO2. Values are higher on the chest and lowest on the foot. TcpO2 measurements have been used to predict foot, below-knee amputations in the diabetic patient, and above knee in the nondiabetic patient having traumatic amputation.[13] There have been numerous reports associating healing following TcpO2 values above 40mmHg, while values of 20mmHg and below are associated with poor healing. This gap of 20mmHg is wide and suggests that absolute values may be subject to large variability. Kram proposed the concept of using the ratio of 0.2 (site to control), which is more satisfactory based on recent reports.[14-16] Despite this, the technique is not widely used to manage diabetic foot disease. Should its use be encouraged or should we develop other methods of assessment? What is TcpO2? From theoretical considerations, it was proposed that TcpO2 is directly dependent blood flow, indirectly on local oxygen consumption as well as resistance to oxygen diffusion.[12] Edema and skin thickness will resist oxygen diffusion as evidenced from clinical studies in leg ulcers and patients with scleroderma, respectively.[12,17] Oxygen consumption for cellular control of repair must be expected to be high with increased macrophage activity in a healing wound. Oxygen consumption must also be expected to be high in an infected wound. In a diabetic foot, this is a common complication. Recent studies have reported that controlling postoperative pain increases tissue oxygen both preoperatively and postoperatively while reducing levels of infection.[18] It is difficult to extrapolate from this study, but it may be that a clear relationship of the effect of infection needs to be identified before we can model the effects of all relevant clinical variables on tissue oxygen values. The sensors should be calibrated in air regularly before use. The site should be degreased and dried before the sensors are placed in situ using double-sided adhesive rings. Avoid using adhesive tape to keep the sensor in place to avoid blanching dermal capillaries. Control values may be derived from the antecubital fossa or the chest where the electrode is best positioned in the fifth intercostal space along the mid-clavicular line. Sensor temperatures 43–45 degrees C may be used, but an electrode should be moved within two hours to avoid inducing burns. In Southampton, we measure TcpO2 with patients resting in the supine position in an environment free from noise and draughts and where the temperature is maintained between 21–24 degrees C. In addition to physiological variables, Hunt[19] has proposed that smoking, hydration status, and current medication should be also documented when measuring TcpO2. Wound infection and type, if known, as well as surgical variables, such as length of operation, should be well documented. Postoperative complications should also be well documented in deriving a predictive model for TcpO2 use in diabetic foot disease management. Discussion Ideally, rates of surgical amputation should be reduced to improve quality of life for patients and to augment resources available for wound management. To achieve this objective, effective management of risks in peripheral arterial disease in diabetic patients by advice, education, and therapeutic modification is the starting point. The UK National Service Framework for diabetes has clearly indicated the modes of managing foot disease by offering regular easy-to-access podiatric care and assessment of sensation threshold using the Semmes Wienstein 10g monofilament.[20] The next step may be to address the needs of those diabetic patients who develop peripheral arterial disease. So far as risk management is concerned, Burns[21] reported that “lowering total cholesterol and low-density lipoprotein cholesterol by 25 percent with statins reduces cardiovascular mortality and morbidity in patients with peripheral arterial disease by around a quarter, irrespective of age, sex, or baseline cholesterol concentration.” This augers well for the future. It also begs studies to examine the long-term effects of such therapeutic modifications. Amputation is an inevitable option of managing diabetic foot disease. Usually, this is part of care in an acute hospital setting where the support of vascular laboratory support is often available. The skin sensor to measure TcpO2 is simple to use and offers a reliable indication of the level of amputation. Undoubtedly, advances in science will usher in improved techniques. However, in order to improve the diagnostic accuracy, we need to understand and control variables in addition to local perfusion status of the patient.