Transcutaneous pressure of Oximetry (tcpO2）
Transcutaneous pressure of Oximetry (tcpO2) is a non-invasive way of detecting the microcirculation of the skin and subcutaneous tissues, which can reflect the oxygen supply status of the capillary blood flow.
1. What is being tested?
What is tested is the amount of dissolved oxygen in the tissue: it refers to the oxygen dissolved in the blood in the tissue capillary network in a physical state, expressed in millimeters of mercury.
2. What is the principle of tcpO2?
There are many kinds of oxygen partial pressure sensors, and the electrochemical oxygen sensor is usually used. Insert two metal electrodes into the aqueous solution containing electrolyte, and apply an appropriate DC voltage to the electrodes. When detecting, fix the fixed ring of the electrode filled with electrolyte to the detecting part, heat the electrode to a certain temperature, then the electrode can continuously measure the oxygen pressure in the electrolyte. After the oxygen enters the human body, it finally reaches the capillaries, where the combined oxygen becomes free oxygen, which is used by the human body. Under normal circumstances, the amount of oxygen diffused to the surface of the skin is difficult to detect. However, when the skin is heated to 43-45°C, the capillaries reach the maximum expansion, the blood flow is the largest, and the flow velocity is the fastest. Most of the blood oxygen diffused to the skin surface can be detected.
3. Influencing factors
Influencing factors include environmental factors: temperature, humidity, atmospheric pressure, etc.; oxygen sensor sensitivity; own factors: smoking, body position, cleanliness of the detection part, thickness of the keratinized layer, degree of extension and more. Actual operating experience also has a greater impact on the test results. The decisive factor of tcpO2 is the capillary diffusion function and blood oxygen content.
4. The difference between TCPO2 and SO2
SO2 is the percentage of the volume of oxygenated hemoglobin in the blood to the total volume of hemoglobin, which can provide a non-invasive continuous blood oxygen monitoring for the clinic. But it has certain limitations. Anemia, local hypoxia and high oxygen saturation cannot truly reflect the body’s oxygen status.
TCPO2 can non-invasively continuously monitor the oxygen dynamic changes of the tissue capillary network, and will not be affected by the above limitations.
5. The difference between TCPO2 and blood gas analysis
Blood gas analysis is an invasive blood oxygen detection method, which cannot be continuously monitored, nor can it detect the oxygen supply of local tissues. Compared with blood gas analysis, transcutaneous oxygen partial pressure monitoring is a non-invasive, continuous, and real-time detection method.
6. The scope of application of transcutaneous oxygen partial pressure measurement
Transcutaneous partial pressure of oxygen has more and more applications, and it has been approved for hyperbaric oxygen therapy, non-healing wounds, peripheral vascular disease/limb ischemia, amputation range determination, vascular reconstruction evaluation, burns, Raynaud’s/vibration syndrome, Skin flap monitoring and other clinical application fields. It has broad application prospects in the field of critically ill rescue.
7. The clinical value of TCPO2
In clinical applications, in addition to the absolute value of the monitoring results, it is more important to observe the dynamic changes in order to better guide the clinical use of drugs, the definition of surgical sites, and the evaluation of clinical efficacy.
8. The significance of TCPO2 in the subject of hyperbaric oxygen
The core of hyperbaric oxygen therapy is to increase the oxygen partial pressure and oxygen content of the tissue. Transcutaneous oxygen partial pressure measurement can monitor the exact value of tissue oxygen partial pressure at any time during hyperbaric oxygen treatment in real time.As a basic research, transcutaneous oxygen partial pressure measurement can help formulate a more reasonable hyperbaric oxygen treatment plan.
9. Preparation before the test
The indoor temperature should be normal; the body surface temperature should be tested when the body temperature is comfortable；clean the skin before testing.；drinking alcohol, smoking, drinking caffeine beverages, and taking certain drugs are prohibited.
10. Selection of test site
The percutaneous oxygen sensor must avoid large arteriovenous blood vessels, usually at the bend of the upper arm elbow joint, under the clavicle and lower abdomen (depending on the situation in special cases).
11. The main points in the test
The electrochemical characteristics of the sensor are relative values and must be calibrated before use.
The sensor must be in complete contact with the skin and isolated from air, otherwise it will affect the measured value.
After the sensor is attached to the skin, the reading value generally enters a stable state after 10-20 minutes.
Generally, the normal value change law is that the value decreases first after being isolated from the air, which can be used as a criterion for judging whether the sensor is fixed.
Although TCPO2 is a non-invasive, quantitative and continuous method, but because the skin is continuously heated during the test, the continuous time is generally not more than 4 hours, and it is recommended to change the position in 2 hours.
article: Beijing Qiumanshi Medical Technology Co., Ltd.
He Ruifeng, Li Qinglong, Bi Kexu (technical guidance);
Department of Hyperbaric Oxygen, Beijing Haidian Hospital