Devices for treating a patient by measuring a condition, such as the partial pressure of CO2, at a location on a mucosal membrane surface in the mouth region of the patient, includes a sensor (14, 16) with an end (44, 46) that lies against the mucosal surface, and a seal (20) that extends 360° around the sensor end and presses against the mucosal surface. The sensor end and the seal lie on the first end portion (24) of a holder (22) which has a second end portion (26) that presses against the outside of the patient at a location opposite the sensor and seal. The holder is a clasp which can be formed as a single piece of resilient material that extends in a loop, or which can be formed in the manner of a clothespin with a spring that pivots two bars to urge their end portions towards each other.