One of the limitations of TEB is the signal to noise ratio. Commonly TEB systems use an alternating current (1 - 4 ma at 20-100 Khz) applied to the skin and have little control over the percentage of current passing through the vascular structures in the chest. Changes in the electrical impedance of the lungs with respiration change the percentage of the total current passing through blood containing structures. Finally, treating all of the blood containing structures in the chest as a single impedance to be measured, does not allow separation of the signal into its various components. A technique which stabilizes the percentage of current delivered to the target structure and records the electrical impedance signals directly from that target structure improves the accuracy and reliability of impedance based measurement of cardiac output.