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.