A device is provided for assessing impairment of blood circulation in a patient, such as that in perfusion failure, by measurement of blood flow adjacent a mucosal surface accessible by a mouth or nose and connecting with the gastrointestinal tract or upper respiratory/digestive tract of a patient. The device includes a blood-flow sensor adapted to be positioned adjacent a mucosal surface with a patient's body and measuring blood flow in adjacent tissue and a PCO.sub.2 sensor adapted to be positioned adjacent the mucosal surface and measuring PCO.sub.2. In addition a pH sensor may be used in combination with the blood flow determination. A method of detecting perfusion failure is also disclosed. The method includes utilizing blood-flow measurements in conjunction with a surface perfusion pressure index and/or an optical plethysmography index to more accurately assess perfusion failure. These measurements may also be supplement by taking measurements of pH, sublingual PCO.sub.2, and Sa O.sub.2. The invention affords rapid measurement and detection of perfusion failure.