This is Phase I application for the development of a new catheter with two thermistors and a heating element for the continuous (actually frequently intermittent) measurement of cardiac output. During a control period without heating, any temperature difference between the two thermistors is measured and integrated for subsequent correction of the temperature difference recorded during the period of heating of an element placed in between the two thermistors. The specific aims of the project are (1) to develop and fabricate special catheters, microprocessor controlled instrumentation and software (2) to analyze and validate the system on the bench and (3) to test the system in 4 pigs. Specific advantages claimed for the new system are that it will respond much faster to changes in cardiac output than the current Baxter or Abbott systems and that it will be able to ignore the effects of relatively fast changes in deep body temperature, such as occur during recovery from hypothermia. Measures of success consist of accuracy and speed of repetition.