Introduction

Transcutaneous oximetry (TcPO) is a non-invasive, simple and reliable diagnostic method used to assess peri-wound oxygenation and microcirculatory blood flow objectively. The term “peri-wound“ should be emphasized because the measurement is taken at the edge of the wound and not at the center, where it would most likely be lower than the measured levels. Hypoxia is recognized as one of the most significant risk factors for non-healing wounds. It is, therefore, important to identify wound hypoxia early to provide an effective and accurate treatment plan.[1][2][3]

Data indicate that hyperbaric oxygen therapy (HBOT) improves hypoxia and is frequently used in the management of non-healing wounds. TcPO measurement has helped our understanding of HBOT mechanisms in wound healing. One of the most significant observations related to HBOT was that, with TcPO use, although cutaneous blood flow decreased in the areas where oxygen pressure increased beyond normal levels, it did not decrease in areas where oxygen pressure remained subnormal. This evidence supported the view that unless oxygen pressure does not increase beyond normal values, the vasoconstrictive effect will not occur. Because HBOT is not readily available in all wound care centers and because it poses a significant burden to the healthcare system, the identification of patients most likely to benefit from HBOT is critical. TcPO, in this regard, is currently the most objective method for patient selection for HBOT.[4][5][6]

Procedures

The method relies on the placement of a Clark-type polarographic oxygen electrode on the skin where it heats up, usually to 43.5 C, causes vasodilatation in the underlying arterioles and capillaries, improves oxygen diffusion towards the electrode, and eventually provides the reading of the partial oxygen pressure at that specific site. An adhesive ring filled with a specific solution provides the attachment of the electrode to the skin. Following the attachment of the electrode, the application requires a strict calibration prior to the measurement. The system incorporates a thermistor which keeps the temperature between 42 C to 45 C and avoids thermal trauma.[7][8][9]

Indications

TcPO was first used in neonatology, followed by pediatric intensive care units and then spread to other disciplines including plastic surgery, vascular surgery, anesthesiology, orthopedics and hyperbaric medicine. [10][11][12]Today, it is frequently used in wound care centers. Specifically, TcPO measurement can be used for:

• The serial assessment of peri-wound oxygenation
• The classification of peripheral arterial disease
• The diagnosis of critical limb ischemia
• The prediction of the outcome of non-healing wounds
• The prediction of amputation
• The suggesting of an optimal amputation level
• The selection of patients for hyperbaric oxygen therapy (HBOT).

Potential Diagnosis

Wound hypoxia is defined as a TcPO level less than 40 mmHg. A more pronounced hypoxia, i.e., less than 20 mmHg, is termed critical limb ischemia. Patients with a TcPOlevel less than 40 mmHg are potential candidates for HBOT. Not all patients with hypoxia, however, will benefit from HBOT and identifying those who are most likely to benefit from HBOT requires additional assessment, again using TcPO measurement, either at sea level or preferably inside the hyperbaric chamber.

Normal and Critical Findings

Normal and Critical Findings

Normal TcPO values measured in the feet of healthy adults are as follows:

• Between 50 mmHg to 90 mmHg at sea level, i.e., 1 atmosphere absolute (ATA), while breathing air
• Between 250 mmHg to 450 mmHg at 1 ATA while breathing 100% oxygen
• Between 700 mmHg to 900 mmHg at 2 ATA while breathing 100% oxygen
• Between 900 mmHg to 1300 mmHg at 3 ATA while breathing 100% oxygen.

Interfering Factors

There can be an average of about 10% variability in TcPO values in healthy individuals. TcPO measurement is sensitive to positioning, e.g., results may be erroneously elevated in the depending and decreased in the elevated position. A supine position is, therefore, suggested for the most accurate assessment. Measurement shortly after exercising may also lead to decreased values of TcPO particularly in patients with peripheral arterial occlusive disease (PAOD). Finally, a variety of pathological conditions at the site of measurements such as edema, acute infection, inflammation, scar tissue, irradiated tissue, and sclerosis can result in erroneous readings because they may affect the dynamics of oxygen diffusion. The accuracy of TcPO values may further be supported by using a reference electrode placed on the upper front part of the thorax. 

For the most reliable and comparable results, the ambient temperature should be between 21 C to 23 C, smoking and caffeine consumption should be avoided before the measurement, the temperature of the electrode, the duration of the measurement, as and the site of measurement, should be documented alongside the measured TcPO values.

Complications

No serious complications have been reported regarding TcPO measurement.

Patient Safety and Education

The procedure is safe and does not pose the patient at risk for severe complications. The most significant drawback of the treatment is the duration of the measurement which takes between 15 to 20 minutes. 

Clinical Significance

TcPO has recently become increasingly popular for wound assessment and patient selection for HBOT. Notably, logistic limitations such as the absence of an HBOT center in the vicinity, plus the fact that committing to HBOT schedules is physically demanding for patients and finally the high cost of treatment renders patient selection a critical step before the start of HBOT. TcPO measurement at sea level or higher pressures, before or during HBOT, while breathing room air or 100% oxygen may provide clues for the effectiveness of HBOT. Among these, measuring TcPO while the patient is inside the chamber breathing 100% oxygen at 2 ATA to 2.5 ATA (in-chamber TcPO measurement) is accepted as the most reliable method of identifying patients most likely to benefit from HBOT. 

Studies have shown that if TcPO levels increase above 200 mmHg inside the chamber, there will be a 74 to 88% success rate with HBOT. If, however, TcPO remains under 100 mmHg, the failure rate with HBOT might be as high as 90%. In-chamber TcPO2 levels under 50 mmHg are almost always associated with HBOT failure. If in-chamber TcPO measurement is not available, taking a TcPO measurement at sea level while the patient inhales 100% oxygen may also provide some clues on the potential benefit of HBOT in wound healing. This is known as the normobaric oxygen challenge test. Accordingly, if TcPO levels remain under 35 mmHg while the patients inhale 100% oxygen, or if the increase in TcPO levels remains under ten mmHg, almost 89% of the patients will fail to heal with HBOT. One study reported an 88% success rate with HBOT if TcPO values increased above 100 mmHg during the normobaric oxygen challenge test.