Final answer:
TcPO2 and TcPCO2 measurements differ from PaO2 and PaCO2 values due to factors like skin perfusion, temperature, and thickness that affect gas passage.
Step-by-step explanation:
Transcutaneous oxygen tension (TcPO2) and carbon dioxide tension (TcPCO2) values can differ from arterial blood gas (ABG) values such as PaO2 and PaCO2 due to several factors. The TcPO2 and TcPCO2 measure the partial pressure of oxygen and carbon dioxide through the skin, while PaO2 and PaCO2 values are measured directly from arterial blood.
When blood leaves the lungs through the pulmonary veins, it has a venous Po2 of 100 mm Hg and a venous PCO2 of 40 mm Hg. As blood enters the systemic capillaries, Po2 equates to 100 mm Hg, but the tissue cells have a Po2 of 40 mm Hg, encouraging diffusion of oxygen out of the capillaries and into tissue. Conversely, blood PCO2 equals 40 mm Hg, while systemic tissue PCO2 is 45 mm Hg, driving CO2 out of tissue and into the blood.
Transcutaneous measurements can be affected by the perfusion of the skin site, local temperature, skin thickness, and other factors that can alter the passage of gases through the skin compared to direct arterial measurement. Therefore, while noninvasive and useful for continuous monitoring, TcPO2 and TcPCO2 are generally less precise than ABG measurements and serve as adjuncts rather than replacements for blood gas analyses. These transcutaneous readings are less precise than arterial blood gases but useful for continuous, noninvasive monitoring.