Many different algorithms have been developed for the conversion of thoracic impedance signals into a calculation of cardiac output including Kubicek18 , Bernstein-Sramek19, Shmulewitz20, adaptive algorithms.20 Each of these algorithms requires some sort of empirically derived factor to convert from measurements of resistance into measurements of volume. The ECOM system uses an adaptive multi-parameter algorithm which allows for the reduction of between subject variability. We have derived the empiric fit for the porcine model but have not done so for patients. This study will be therefore divided into two components. The first data collection of 15 patients will provide the data for the empiric fit between the ECOM system and true cardiac output. The second set of 300 patients will be the test group for the algorithm.