Cardiac output (CO) is an important cardiovascular system function parameter. Changes in cardiac function are commonly observed as a response to physical training and pharmacological interventions. Unfortunately, the methods for assessing CO are invasive, leading to well-known complications and considered inconvenient in daily practice. For this reason, the search for new noninvasive methods that can accurately detect CO at rest, at physical exertion or as a response to a clinical intervention has become desirable in academic and non-academic circles. The ideal method for measuring CO at rest and during exercise should be noninvasive, safe, reproducible and inexpensive.

The Cardiopulmonary Exercise Testing (CPT) is recommended in the evaluation of cardiorespiratory fitness and exercise tolerance in athletes, the general population and in patients. Briefly, CO and systolic volume can be estimated during CPT through measured VO₂. In 2001, Williams et al. were the first ones to integrate CPT with non-invasive measures of CO using rebreathing (RB) of carbon dioxide, but the technique was quickly abandoned due to its difficulty and inaccuracy. Another non-invasive method is thoracic electrical bioimpedance (TEB), first described in 1966 by Kubicek et al., which measures thoracic resistance as a result of changes in blood velocity during the cardiac cycle and uses an algorithm to calculate the CO.

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