TcPO2 values in limb ischemia: effects of blood flow and arterial oxygen tension

Calculation of Tissue Oxygenation via an Inverse Boundary Problem for Transcutaneous Oxygenation Wearable Applications

In this article, we present a toolset to fully leverage a previously developed transcutaneous oxygenation monitor (TCOM) wearable technology to accurately measure skin oxygenation values. We describe numerical models and experimental characterization techniques that allow for the extraction of precise tissue oxygenation measurements. The numerical model is based on an inverse boundary problem of the parabolic equation with Dirichlet boundary conditions. To validate this model and characterize the diffusion of oxygen through the oxygen sensing materials, we designed a series of control/calibration experiments modeled after the device's clinical application using oxygenation values in the physiological range expected for healthy tissue. Our results demonstrate that it is possible to obtain accurate tissue pO2 measurements without the need for long equilibration times with a small wearable device.

Analysis of biomonitoring data after full-thickness skin grafting

Skin graft vascularization is investigated mainly by histological evaluation. Immunohistochemical analysis has been conducted only in mice. Transcutaneous oxygen tension (TcPO₂ ), which is an index of blood flow, has not been evaluated in skin grafts and only a few studies have reported biologic monitoring data using color tone evaluation and surface temperature. In humans, these tests can be performed non-invasively. To evaluate human skin graft vascularization, we analyzed biomonitoring data after skin grafting. We evaluated 14 patients who underwent skin grafting surgery at Nagoya City University Hospital. The TcPO₂ , color tone, surface temperature, and dermoscopic observations at recipient sites were measured at postoperative day (POD) 4, 6, and 11. Mean TcPO₂ levels at POD4, 6, and 11 were 12.7, 15.2, and 33.5 mmHg, respectively, and significantly higher at POD11 than at POD4 (p = 0.003, Steel-Dwass test). Dermoscopic observation revealed gradually increasing redness and yellowness. Color tone evaluation measured by spectrophotometry supported the appearance. The a*(redness) value at POD4, 6, and 11 was 6.19, 9.20, and 11.27, respectively, and significantly higher at POD11 than at POD4 (p < 0.001, Steel-Dwass test). The b*(yellowness) value at POD4, 6, and 11 was 8.83, 9.24, and 13.02, respectively, and significantly higher at POD11 than at POD4 (p = 0.020, Steel-Dwass test). The surface temperature did not significantly differ between graft and control sites. These findings suggest that skin graft vascularization started by POD6 and stabilized by POD11. Because TcPO₂ increases after POD4, skin grafts should remain undisturbed until at least POD11.

Wearable device for remote monitoring of transcutaneous tissue oxygenation

Wearable devices have found widespread applications in recent years as both medical devices as well as consumer electronics for sports and health tracking. A metric of health that is often overlooked in currently available technology is the direct measurement of molecular oxygen in living tissue, a key component in cellular energy production. Here, we report on the development of a wireless wearable prototype for transcutaneous oxygenation monitoring based on quantifying the oxygen-dependent phosphorescence of a metalloporphyrin embedded within a highly breathable oxygen sensing film. The device is completely self-contained, weighs under 30 grams, performs on-board signal analysis, and can communicate with computers or smartphones. The wearable measures tissue oxygenation at the skin surface by detecting the lifetime and intensity of phosphorescence, which undergoes quenching in the presence of oxygen. As well as being insensitive to motion artifacts, it offers robust and reliable measurements even in variable atmospheric conditions related to temperature and humidity. Preliminary in vivo testing in a porcine ischemia model shows that the wearable is highly sensitive to changes in tissue oxygenation in the physiological range upon inducing a decrease in limb perfusion.

Resting TcPO2 levels decrease during liner wear in persons with a transtibial amputation

Background

In our clinic, a substantial number of patients present with transtibial residual limb pain of no specific somatic origin. Silicone liner induced tissue compression may reduce blood flow, possibly causing residual limb pain. Thus, as a first step we investigated if the liner itself has an effect on transcutaneous oxygen pressure (TcPO2).

Methods

Persons with unilateral transtibial amputation and residual limb pain of unknown origin were included. Medical history, including residual limb pain, was recorded, and the SF-36 administered. Resting TcPO2 levels were measured in the supine position and without a liner at 0, 10, 20 and 30 minutes using two sensors: one placed in the Transverse plane over the tip of the Tibia End (= TTE), the other placed in the Sagittal plane, distally over the Peroneal Compartment (= SPC). Measurements were repeated with specially prepared liners avoiding additional pressure due to sensor placement. Statistical analyses were performed using SPSS.

Results

Twenty persons (9 women, 11 men) with a mean age of 68.65 years (range 47–86 years) participated. The transtibial amputation occurred on average 43 months prior to study entry (range 3–119 months). With liner wear, both sensors measured TcPO2 levels that were significantly lower than those measured without a liner (TTE: p < 0.001; SPC: p = 0.002) after 10, 20 and 30 minutes. No significant differences were found between TcPO2 levels over time between the sensors. There were no significant associations between TcPO2 levels and pain, smoking status, age, duration of daily liner use, mobility level, and revision history.

Conclusion

Resting TcPO2 levels decreased significantly while wearing a liner alone, without a prosthetic socket. Further studies are required to investigate the effect of liner wear on exercise TcPO2 levels.

Non-invasive transdermal two-dimensional mapping of cutaneous oxygenation with a rapid-drying liquid bandage

Oxygen plays an important role in wound healing, as it is essential to biological functions such as cell proliferation, immune responses and collagen synthesis. Poor oxygenation is directly associated with the development of chronic ischemic wounds, which affect more than 6 million people each year in the United States alone at an estimated cost of $25 billion. Knowledge of oxygenation status is also important in the management of burns and skin grafts, as well as in a wide range of skin conditions. Despite the importance of the clinical determination of tissue oxygenation, there is a lack of rapid, user-friendly and quantitative diagnostic tools that allow for non-disruptive, continuous monitoring of oxygen content across large areas of skin and wounds to guide care and therapeutic decisions. In this work, we describe a sensitive, colorimetric, oxygen-sensing paint-on bandage for two-dimensional mapping of tissue oxygenation in skin, burns, and skin grafts. By embedding both an oxygen-sensing porphyrin-dendrimer phosphor and a reference dye in a liquid bandage matrix, we have created a liquid bandage that can be painted onto the skin surface and dries into a thin film that adheres tightly to the skin or wound topology. When captured by a camera-based imaging device, the oxygen-dependent phosphorescence emission of the bandage can be used to quantify and map both the pO2 and oxygen consumption of the underlying tissue. In this proof-of-principle study, we first demonstrate our system on a rat ischemic limb model to show its capabilities in sensing tissue ischemia. It is then tested on both ex vivo and in vivo porcine burn models to monitor the progression of burn injuries. Lastly, the bandage is applied to an in vivo porcine graft model for monitoring the integration of full- and partial-thickness skin grafts.

Perflubron emulsion increases subcutaneous tissue oxygen tension in rats

Oxygen plays a central role in wound healing. The hypothesis of this study was that the administration of a perfluorocarbon emulsion, which dissolves oxygen will increase subcutaneous tissue oxygen tension (PsqO2) in normovolemic and hemorrhaged animals. In the first set of experiments, PsqO2 was measured with a polarographic oxygen electrode along the dorsum of normovolemic-anesthetized rats (n=20) breathing supplemental oxygen. After baseline equilibration of tissue oxygen tension, perflubron emulsion (n=12) or saline control (n=8) was administered. Perflubron administration increased PsqO2 by 32.0 +/- 7.2 mmHg, whereas saline administration had no effect (-1.0 +/- 6.6 mmHg). In a second set of experiments, PsqO2 (n=12) was measured in rats breathing 100 percent oxygen after being bled 20 percent of their blood volume. Creating hypovolemic rats allowed for the study of the effect of perflubron emulsion administration on subcutaneous oxygen tension in tissues with compromised blood flow. Perflubron emulsion was only effective at increasing oxygen tension in tissues with oxygen extractions less then 2.0 vol%. These findings agree with those results analytically predicted based on the oxygen solubility of the perflubron emulsion.

Effects of oxygen inhalation on skin microcirculation in patients with peripheral arterial occlusive disease

BACKGROUND

Oxygen administration is currently used in clinical medicine to improve peripheral oxygen delivery to tissues threatened by ischemia. However, conflicting results have been reported on the effects of oxygen in ischemic areas. This study was aimed at investigating the effects of 40% oxygen inhalation on the skin microcirculation in the feet of patients with peripheral arterial occlusive disease (PAOD).

METHODS AND RESULTS

Transcutaneous oxygen tension (tc PO2) was measured on the dorsal skin of the foot, and the nailfold microcirculation was investigated by a combination of laser Doppler flowmetry (LDF) and dynamic capillaroscopy (CBV) in the great toes of 17 legs of 11 patients, with 13 legs of eight normal subjects as a control group. Inhalation of oxygen induced a significant decrease of both the total (delta LDF, -307%, p less than 0.02) and nutritional (delta CBV, -17%, p less 0.002) skin microcirculation in normal legs compared with baseline values. A similar response was observed in 10 legs of patients who showed a significant increase of the tc PO2 (greater than or equal to 10 mm Hg) (delta LDF, -14%, NS; delta CBV, -13%, p less than 0.005). By contrast, both the total (+21%, p less than 0.03) and nutritional (+52%, p less than 0.05) circulation significantly increased in the seven legs without significant tc PO2 increase. In addition, the flow motion, which was impaired in the patients, was significantly (p less than 0.05) improved by oxygen inhalation.

CONCLUSIONS

Inhalation of 40% oxygen induces a vasoconstriction in the skin microcirculation of toes of the normal subjects and patients with moderate PAOD but induces an increase of the skin microcirculation in patients with severe PAOD.

Intraoperative transcutaneous oxygen tension criteria for completion arteriography

During the past three years, we studied the value of transcutaneous oxygen monitoring in 28 lower extremity vascular bypass procedures. In 21 reconstructions, a rapid rise in the transcutaneous oxygen tension following reperfusion was indicative of a patent graft and patent runoff vessels. Inadequate revascularization was identified in three of four patients in whom transcutaneous oxygen tension failed to rise following femorodistal arterial bypass (positive predictive value 75%). A normal intraoperative transcutaneous oxygen tension study following femorodistal bypass had a negative predictive value of 95%. The overall accuracy was 91%. Transcutaneous oxygen tension monitoring during lower extremity vascular bypass procedures is useful in assessing the success of revascularization and may be used to select which patients should undergo completion arteriography as opposed to those in whom an arteriogram is not essential.

Experimental methods in the pathogenesis of limb ischemia

Ischemia of extremities is responsible for considerable morbidity and mortality and the pathophysiology of this condition warrants further study. The purpose of this review is to discuss techniques used in the evaluation of limb ischemia and reperfusion. It is of critical importance to study limb blood flow distribution to the microcirculation where nutritive exchange occurs. Skeletal muscle ischemia progresses to infarction when critical deficits of cellular metabolites develop, which mandates that studies be focused at the cellular level. It is clear that the adverse effects of ischemia can be exacerbated by a reperfusion injury to the endothelium of the microvasculature. Investigators wishing to study limb ischemia have a wide spectrum of methodology and established models available to use in improving the understanding of the complex events of ischemic injury.