The present invention is directed to a sensor system for use with a blood characteristic measurement device such as a pulse oximeter, on areas of the body having low normal cutaneous blood flow and for monitoring a blood characteristic such as oxygen saturation and pulse rate of patients, preferably over an extended period of time. The sensor system includes (a) a transdermal device containing a blood perfusion-enhancing agent that is administered in a controlled amount to the skin of a human patient and (b) a skin surface sensor.