Fig. 55.1
Image demonstrating placement of the sensor over the right foot to measure the tcpO2 skin level
TcpO2 monitoring is completely noninvasive and atraumatic. It is recommended that sensor placement at one skin site be limited to a maximum of 4 h. The test can be accomplished with the patient comfortably supine at ambient room temperature in an outpatient setting. Oxygen inhalation, change in limb position, and chest wall movements can normalize tcpO2 values that can also be used to increase the specificity and sensitivity of the test [4, 5]. Current monitors, such as the TCM400 from Radiometer (Radiometer, Copenhagen, Denmark), are not only portable but also provide up to six simultaneous measurements of transcutaneous oxygen tension (Fig. 55.2). Under normal conditions, the modified Clark sensors are usually placed on the dorsal aspect of the forefoot between the great and second toe, roughly 5 cm proximal to the second toe tip (forefoot measurement) on the medial aspect of the hindfoot in front of or behind the malleolus (hindfoot measurement) and 10 cm below the patella on the medial aspect of the calf (below-knee measurement) for basic lower extremity tcpO2 mapping. The sensor can also be placed 10 cm above the patella on the medial aspect of the thigh for an above-knee measurement (Fig. 55.3). Inaccurate readings may occur if the sensor is placed over a tendon or exposed bone. For optimal results, the sensor should be placed on skin that is free of edema, ulceration [6], hyperkeratosis, or cellulitis [7]. Also, the level can be compromised with an increased body mass index [8].
Fig. 55.2
Portable monitor for tcpO2 , which can provide up to six simultaneous measurements of tcpO2 and target location
Fig. 55.3
Diagram of location of sensor to measure tcpO2 at different locations
Tissue ischemia or inadequate perfusion to support major wound healing is presumed when the absolute tcpO2 value is less than 30 mmHg. For practical purposes, a low tcpO2 value can be interpreted as either reduced generalized arterial pO2, as in the case of patients suffering from cardiopulmonary diseases, or reduced regional blood flow due to impaired arterial pO2 supply from atherosclerosis. Many investigators have reported that wound healing can occur in some patients with a low tcpO2 value [9–19]. This can be partially explained by the nonlinear relationship between tcpO2 and cutaneous blood flow. Matsen et al. [15] reported that tcpO2 measurements are mostly dependent on arterial-venous gradients and cutaneous vascular resistance. In essence, there can be nutritive blood flow to the skin, even with a tcpO2 level of 0 mmHg.
The authors believe that the most beneficial adjunctive innovation of tcpO2 techniques used to improve the accuracy of tcpO2 measurements is the sensor probe heating (44 °C), which minimizes local vascular resistance. This makes transcutaneous oxygen tension more linear with respect to cutaneous blood flow. Additional techniques used to improve tcpO2 accuracy include measurements performed before and after oxygen inhalation or a change in limb position, oxygen isobar extremity mapping, and transcutaneous oxygen recovery halftime (Fig. 55.4).
Fig. 55.4
(a) Demonstrated utilization of tcpO2 before and (b) after revascularization to augment wound healing of the right big toe
Clinical Applications of TcpO2
Assessing Wound Healing Potential
Many respected authorities [20–27] believe that tcpO2 is a simple, accurate, noninvasive method to determine the appropriate level of amputation. In one study, a level of 50 mmHg predicted success for levels of amputation and for wound healing without reconstructive procedures, and values of 40 mmHg or less were associated with persistent wound problems and complications after amputation. As expected, increased tcpO2 values after vascular reconstruction of the legs predicted improved clinical status tcpO2 [28, 29].
TcpO2 can screen for peripheral vascular disease and assess the success of revascularization. Patients with critical limb ischemia (ABI <0.4) will almost always have a tcpO2 less than 30 mmHg. Still, it is intuitive that low-pressure values can also have many other causes. One of the clinical indications that prove the underlying peripheral arterial disease is the use of an oxygen challenge test, which involves having the patient breath 100% oxygen via a non-rebreathing face mask. Interval improvement will suggest that low values are due to a reversible oxygen barrier such as edema, inflammation, or macrovascular disease.
TcpO2 was found to augment other diagnostic tools in chronic limb ischemia, such as toe blood pressure (TBP), to help estimate the degree of macro- and microvascular dysfunction and to classify lower limb ulcers [30].
Another recent review [31] showed that tcpO2 is predictor of ischemic ulcer healing for cutoff tcpO2 values of less than 20 mmHg and more than 40 mmHg. In addition, the leg elevation method for tcpO2 may provide an important adjunct in the assessment of patients with borderline values.
Screening for Chronic Vascular Disease and Assessing the Success of Revascularization
Much the same as described above for the treatment of diabetic patients, tcpO2 measurements can be useful in the management of ill-defined leg/foot complaints, particularly in elderly patients with multiple medical problems. For instance, an adequate tcpO2 level (>30 mmHg) at the forefoot and hindfoot may obviate the need for an arteriogram and support nonoperative management. On the other hand, a low tcpO2 level likely indicates a situation that requires a higher level of care. The potential need for arteriography and arterial revascularization can be thoroughly discussed with the patient and family prior to treatment, which ensures reasonable expectations. Finally, a dramatic improvement in tcpO2 measurements following treatment is not only gratifying, but at least 90% of patients will experience a successful outcome.
TcpO2 measurements can also be used to determine if endovascular treatment has been successful at the skin level of nondiabetic as well as diabetic patients with peripheral arterial occlusive disease (PAD). Wagner et al. [32] previously demonstrated that percutaneous transluminal angioplasty (PTA) has a positive effect on oxygen supply to the skin in patients with PAD. In this study, 34 patients with PAD had tcpO2 measurements obtained at the dorsum of the foot 1 day prior to PTA, during PTA, 1 day after PTA, and 6 weeks after PTA. A significant increase in tcpO2 was noted immediately following PTA, at 1 day, and 6 weeks later.
Pardo et al. [33] evaluated the efficacy of endovascular revascularization in diabetic patients with critical limb ischemia after treatment with PTA by comparing tcpO2 measurements with the post-PTA ankle-brachial index (ABI). They prospectively studied 151 consecutive diabetic patients with posterior tibial and dorsalis pedis Doppler, ABI, tcpO2, and duplex scan results. If two of the four mentioned examinations were abnormal, arteriography was done and, if appropriate, PTA was performed concomitantly. At least 64 patients were considered suitable candidates for PTA. After PTA, the ABI increased from 0.67 ± 0.25 to 0.84 ± 0.25 (P < 0.001), and the tcpO2 increased from 27.20 ± 11.10 to 40 ± 12.10 mmHg (P < 0.001). While the tcpO2 could be measured in all patients, the ABI was not measurable in up to 25% of patients due to non-compressible vessels.
Statistical analysis revealed a meager correlation between the techniques used: tcpO2 and ABI (P = 0.20). Their study suggests that an increase in tcpO2 values in diabetic patients following PTA is a marker of improved microvascular revascularization and that simply obtaining a posttreatment ABI may not be as informative.
Arroyo and colleagues [34] used tcpO2 measurements to determine when previously ischemic tissue had adequate perfusion to support major wound healing. Eleven patients with severe chronic limb ischemia, defined as a forefoot tcpO2 <30 mmHg, were entered into this prospective study. TcpO2 measurements were recorded prior to lower extremity bypass and on postoperative days 1, 2, and 3. A statistically significant increase in mean transcutaneous oxygen pressure was observed between the preoperative and the day 3 postoperative measurements. Despite this finding, some bypass patients still had low tcpO2 values (<30 mmHg), even on postoperative day 3. Nevertheless, this small clinical series suggests that unless urgent, adjunctive minor foot amputation or major debridement should wait until at least 3 days after successful lower extremity bypass. This will ensure that there is adequate perfusion at the level of the foot to support major wound healing. This clinical recommendation could also be used to select the appropriate timing of minor foot amputations or major debridements that are performed after endovascular procedures.
Predicting the Amputation Level
Many studies have reported successful use of transcutaneous oxygen measurements to determine the appropriate lower extremity amputation level (Fig. 55.5) [9–19]. Wyss et al. [16] evaluated the results of tcpO2 measurements used as a predictor of successful wound healing following amputation. The study analyzed 162 patients who had 206 lower extremity amputations. The authors concluded that transcutaneous oxygen is a reliable indicator of local tissue ischemia and that can be used to predict failure of amputation healing due to tissue ischemia. However, there are two theoretical inadequacies that must be considered when using tcpO2 measurements. First, the measurement is quite localized and one value may not represent the overall degree of limb ischemia (Fig. 55.6). Second, as previously mentioned, there may still be some nutritive flow to the skin, despite a tcpO2 level of 0 mmHg.
Fig. 55.5
Demonstrates utilization of tcpO2 to confirm healing after amputation
Fig. 55.6
TcpO2 measured oxygen tension at level of non-healing wound after amputation
Despite the fact that in theory a tcpO2 value of 0 mmHg at a proposed site of amputation does not always indicate ischemia that precludes healing, a tcpO2 level of 20 mmHg or less clearly indicates severe limb ischemia. In the Wyss study [16], a tcpO2 measurement of 20 mmHg or less was associated with a rate of failure of amputations distal to the knee that was more than ten times the 4% rate of failure in patients that had a tcpO2 level of more than 20 mmHg.
One of the initial reports on this topic was by Franzeck et al. [10]. Mean tcpO2 levels in patients who experienced primary healing of a lower extremity amputation were compared to those of patients who failed to heal. The respective values for healing and non-healing were 36.5 ± 17.5 and less than 30 mmHg. However, three of nine patients whose tcpO2 level was less than 10 mmHg healed primarily.
In a study of below-knee amputations, Burgess et al. [9] noted that all 15 amputations that were associated with a tcpO2 level greater than 40 mmHg healed. Primary wound healing was noted in 17 of 19 below-knee amputations with a tcpO2 measurement between 1 and 40 mmHg, but none of the amputations in the three patients with a below-knee level of 0 mmHg healed. Katsamouris et al. [12] reported that lower extremity amputations healed in all 17 patients with a tcpO2 level greater than 38 mmHg or a pO2 index (chest wall control site) greater than 0.59. Ratliff et al. [14] reported that below-knee amputations healed in 18 patients with a tcpO2 measurement greater than 35 mmHg, whereas healing failed in 10 of 15 with a tcpO2 value less than 35 mmHg. In a study of 42 lower extremity amputations (28 below-knee and 14 above-knee), Christiansen and Klarke [17] found that 27 of 31 patients with a tcpO2 level greater than 30 mmHg healed primarily. Seven patients with values between 20 and 30 mmHg healed, although four patients had delayed healing. The amputation stumps of all four patients with a value below 20 mmHg failed to heal due to skin necrosis.
Malone et al. [13] and Christensen et al. [17] concluded that a transcutaneous oxygen tension of 20 mmHg or more accurately predicted amputation site healing, and they found no difference in the healing rate between diabetics and nondiabetics. A computerized analysis of various transcutaneous metabolic parameters by Malone et al. [13] demonstrated a high association with primary amputation site healing with the following values: transcutaneous oxygen tension greater than 20 mmHg, transcutaneous carbon dioxide value less than 40.5 mmHg, transcutaneous oxygen-to-transcutaneous carbon dioxide index greater than 0.472, and foot-to-chest transcutaneous oxygen index greater than 0.442.
The above data reinforced the fact that elective lower extremity amputations should not be performed without objective testing to ensure selection of the most distal amputation site that will heal primarily, yet allow for removal of infected, painful, or ischemic tissue [18, 19]. A variety of techniques are available to achieve this, depending on available equipment, the amputation level under consideration, and the accuracy of the chosen modality [19]. TcpO2 measurements continue to be a reliable technique; however, it is not suitable for whole limb mapping.
The ultimate role of any method used for amputation level determination is to inform the surgeon of the quantitative risk of non-healing at the proposed site of surgery. The level of amputation can then be decided on the basis of this objective finding, in conjunction with surgeon’s clinical judgment and patient physical findings. For example, a surgeon might perform an amputation distal to the knee through a site with a very low tcpO2 level in a patient who is well motivated, relatively young, and otherwise healthy. Such an amputation would almost certainly be ruled out in a fragile elderly person who faces a limited prospect for successful rehabilitation.
Patients with advanced critical limb ischemia may require an amputation and tcpO2 may objectively define the optimal level of amputation (level that will be more likely to heal). TcpO2 less than 40 mmHg is associated with a lower than normal likelihood of amputation healing. Many respected authorities recommend a tcpO2 value greater than 40 mmHg to increase the likelihood of healing at the amputation site. An oxygen challenge can be of some help here. If the baseline tcpO2 increases less than 10 mmHg while breathing HBO, this is highly predictive of failure of amputations at that level. In fact, this test is at least 68% accurate in predicting failure of healing after an amputation in patients for whom revascularization is not possible. So, if possible, recommend amputation at a level where the oxygen challenge increase is greater than 10 mmHg, but remember that an increase of 10 mm of mercury represents a really poor oxygen response and confirms that the patient has severe peripheral vascular disease. Still, tcpO2 should not be the sole diagnostic test to define the level of amputation [35].
Transcutaneous values obtained while breathing normobaric oxygen cannot be used to predict benefit from hyperbaric oxygen therapy. The sea level oxygen challenge test is not an accurate predictor of hyperbaric benefit. The best way to predict benefit from hyperbaric therapy is using a chamber tcpO2. If the in-chamber tcpO2 value is greater than 200 mmHg, this indicates that the patient is likely to heal with hyperbaric therapy, while an in-chamber tcpO2 value less than a 100 mmHg is likely associated with failure of hyperbaric therapy. Studies have shown that an in-chamber tcpO2 test is 75% accurate in predicting benefits from hyperbaric therapy. Although these data come from diabetic foot ulcer studies, they seem to apply to other types of wounds.
In addition, tcpO2 measurements may provide better prognostic values than ankle-brachial indices for healing after partial foot amputation [36].
Prospective Treatment of Diabetic Foot Problems
Successful treatment of the patient with diabetes and limb-threatening ischemia requires an accurate assessment of limb perfusion. Presenting clinical symptoms may be misleading. Physical examination of pedal pulses or ankle/brachial index (ABI) may not be accurate due to the non-compressible nature of a diabetic patient’s peripheral arteries (medial calcinosis of the arterial wall) will yield a falsely elevated ABI. Often, the cause of the presenting foot problem is multifactorial, and commonly used noninvasive lower extremity hemodynamic studies lack discriminative accuracy. On the other hand, arteriography is ultimately accurate. However, it is invasive and expensive and carries a small but well-defined set of associated complications [37, 38]. In this setting, tcpO2 measurements can be extremely useful as they are noninvasive, inexpensive, and reproducible [39–44].
In a clinical experience reported by Ballard et al. [45], tcpO2 measurements were prospectively demonstrated to accurately predict severity of foot ischemia in patients with diabetes. Based on clinical experience and previously published amputation level determination data, an absolute transmetatarsal tcpO2 measurement of 30 mmHg was used as the threshold value for selection of a treatment option. If the level was 30 mmHg or greater, the patient’s foot problem was managed conservatively with local wound care, wound debridement, or minor foot amputation. If the level was less than 30 mmHg, arteriography of the involved limb was performed to plan arterial reconstruction or to perform percutaneous intervention to improve foot perfusion.
Thirty-one of thirty-six (86%) limbs in the conservatively managed group were treated successfully including 73% (11/15 feet) of limbs without a palpable pedal pulse. The mean time to wound healing was 6.85 weeks and there were five treatment failures. In the operative/endovascular group, 83.3% of limbs achieved a TM tcpO2 level >30 mmHg after treatment. Twenty-two of twenty-six (85%) limbs in this group had complete resolution of their presenting foot problem. The mean time to wound healing was 9.52 weeks. Treatment failures eventually led to three BKA’s (one failed necessitating revision to the AK level) and one above-knee amputation.
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