Thoracic bioimpedance estimates cardiac output based on the principle that each heartbeat induces a decrease in the electrical impedance of an electrical current applied through the thorax and that this decrease is proportional to stroke volume. It is a completely noninvasive and easy-to-use technique. However, it suffers from many limitations which reduce its reliability in measuring cardiac output under many circumstances. This probably explains why its reliability is rather consensually considered insufficient. Bioreactance is based on the same principle but analyses the signal of electrical reactance rather than the impedance. This considerably improves the signal-to-noise ratio. Validation studies, less numerous than for bioimpedance, have shown variable results, again undoubtedly less good in critical care patients. The most recent version of the system improves its ability to track changes in cardiac output, especially during a passive leg-raising test. Bioreactance could be indicated outside of the intensive care unit, for the evaluation and monitoring of patients in the perioperative, prehospital or emergency medicine department settings.