A probe comprising a tubular member arranged substantially in a plane within a perimeter fitting into the sublingual or postlabial area of a human patient. The tubular member is made preferably of silicone rubber or a material permeable for gases, particularly for carbon dioxide, and forming a barrier against body fluids. The tubular member has consecutive curved sections communicating with each other and form a continuous flow pass. Ends of the tubular member have connection tubes adapted for delivering the fluid inside the lumen of the tubular member directly or indirectly to a measuring apparatus. The connection tubes are made preferably of a material substantially impermeable for carbon dioxide.
