The overall objective of this proposed research is the theoretical and clinical evaluation of the electrical impedance technique as a simple, harmless, non-invasive, and continuous means of quantitating and monitoring intrathoracic fluids, cardiac function, and respired volume in man and animals. Previous research in our laboratory (and others) has shown the feasibility of rising the impedance technique for assessing respired volume, cardiac output, and intrathoracic fluid accumulation (pleural effusion, pericardial effusion, and pulmonary edema). In the course of our investigation of intrathoracic fluids in the dog, it was found that the presence of intrathoracic fluid may modify the impedance waveform such that values of cardiac output calculated from impedance were significantly lower than values measured simultaneously by means of an electromagnetic flow probe on the ascending aorta. The proposed work is directed toward elucidating the extent to which interactions of intrathoracic fluids, tissue edema, and PEEP modify the thoracic impedance waveform and thereby the measurement of respired volume, stroke volume, and cardiac output using the impedance technique.