Significance: A simple, inexpensive, continuous, non-invasive, accurate, reproducible method of measuring cardiac output in patients would reduce the need for pulmonary artery catheter measurements of cardiac output. Such a technique would allow the rapid diagnosis and treatment of hemodynamic instability in the critically ill patient. The maintenance of cardiac output is essential for the prevention of end organ injury which results in morbidity and mortality. A technique that accurately measured cardiac output would greatly assist the management of the critically ill patient.