A method and apparatus are provided for assessing impairment of blood circulation of a patient by measurement of PCO.sub.2 (partial pressure of carbon dioxide) in the upper digestive/respiratory tract of the patient. The method includes introducing a CO.sub.2 sensor into the mouth-nose area and against a mucosal surface. In one example, the sensor is placed under the tongue, in the manner of an oral thermometer, and sublingual (under the tongue) measurements of CO.sub.2 are taken. This allows for the triage of patients or victims in emergency of disaster settings. The measurement involves minimal invasion while avoiding false readings. For monitoring of more than about one or two minutes, holders are used to hold the CO.sub.2 sensor instrument stabily in the mouth or nose, and to isolate the mucosal area immediately around the CO.sub.2 sensor from air flow that could carry away CO.sub.2, while maintaining high humidity.