an apparatus and method for measuring and monitoring blood oxygen content and local perfusion factor . a transcutaneous gas measurement probe is heated by application of an electric current to a semi - conductor device housed therein . the heat developed in a semi - conductor device heats the probe to a constant temperature under the control of a temperature monitoring and regulating circuit and the amount of power applied to the semi - conductor device is measured by multiplying the current conducted through the semiconductor device by the voltage across it to provide a measurement of local perfusion factor . an alerting device is provided to signal when blood oxygen content and local perfusion factor are outside a predetermined acceptable range .