Transcutaneous means measurement across the depth of skin. These devices are able to measure PO2 and PCO2 (referred to as TcPO2 and TcPCO2) through an electrode on the skin surface. The electrode is heated to 40-45°C, which causes local dilatation of blood vessels and increased skin permeability, so gases diffuse through the skin and are picked up by the sensor. This allows for a non-invasive measurement of the PaCO2. Due to the slight underestimation of the PO2, TcPO2 and TcPCO2 are more often used for monitoring trends than precise measurements.