Impedance cardiography is a non-invasive technique purported to measure cardiac output and stroke volume. Correlations obtained between cardiac output using impedance cardiography and using classical invasive techniques have varied from about 0.6 to more than 0.9. This study will assess the validity of the impedance cardiography technique by comparison with the Swan-Ganz thermodilution technique. To date no clinical work has been done, awaiting a suitable patient population.