Cardiac output is a key parameter in the assessment of cardiac function, and its measurement is fundamental to the diagnosis, treatment, and prognostic evaluation of all heart diseases. Until recently, cardiac output determination during exercise had been only possible through invasive methods, which were not practical in the clinical setting. Because Symbolo2 is cardiac output times arteriovenous content difference, evaluation of cardiac output is usually included in its measurement. Because of the difficulty of directly measuring peak exercise cardiac output, indirect surrogate parameters have been proposed, but with only modest clinical usefulness. Direct measurement of cardiac output can now be made by several noninvasive techniques, such as rebreathing inert gases, impedance cardiology, thoracic bioreactance, estimated continuous cardiac output technology, and transthoracic echocardiography coupled to cardiopulmonary exercise testing, which allow more definitive results and better understanding of the underlying physiopathology.