The present invention relates to a sampling unit, a measurement system and method for transcutaneous blood gas measurements. In particular the invention relates to a sampling unit and system adapted for rapid measuring and monitoring of blood gases in a continuous gas flow. The sampling unit is provided with an ambient air inlet and a blood gas extraction and mixing chamber wherein air is mixed with extracted blood gases. The method of continuous transcutaneous measurement of carbon dioxide in the blood utilizes a pulsed heating to minimize the detrimental effects of the heating.