Despite a plethora of developments over recent decades, there still exists a clear need for highly accurate measures of CO that can be applied in various clinical settings in a cost-effective and scalable fashion. Thermodilution has been the most extensively utilized approach and is considered highly accurate, but inadequacies with this method have been widely reported.^5-8 Thermodilution is invasive, time-consuming, and relatively expensive, and requires the attention of a trained physician. Finally, due to its invasive nature— the technique requires the invasive placement of a right heart/pulmonary artery catheter (PAC)—thermodilution comes with a degree of risk and complications. Its use is therefore limited to intensive care units (ICUs), operating rooms and the cardiac catheterization laboratory. As a result, thermodilution is utilized for only a small percentage of patients in whom the measurement of CO could prove useful. With such evident flaws, numerous less invasive methods have been proposed to measure CO. However, it is well documented that these methods—which have included echocardiography, bioimpedence, arterial pulse wave contour analysis, and CO2 rebreathing—are also beset with limitations, specifically regarding their reliability and reproducibility.^9-13

More recently, bioimpedence has come to the forefront in the non-invasive arena. This technique was founded on the concept that electrical conducting properties of the thoracic space vary with the amount of blood contained therein. The method entails the measurement of charges in the electrical impedance of the chest cavity, which are subsequently highly related to changes in the amount of blood contained within the aorta. Analysis of the rate of change of aortic blood volume can readily be related to CO. Bioimpedence offers an easy-to-use, cost-effective CO measurement modality that can be scaled to sites of less intensive care, including the emergency room, regular hospital wards, and physician offices. Provisional studies for this method displayed highly promising results. More recently, however, concerns have surfaced over the levels of accuracy of CO achieved by bioimpedance, particularly in complex hemodynamic patients.¹⁰ Furthermore, available bioimpedence devices are not intended for continuous CO monitoring in any setting; rather, they are intended to provide spot measurements with the patient sitting in a chair.