The last of the techniques we will concern ourselves with here is the one perhaps most related to my own proposed (improved!) technique. Bioreactance is the name of the parameter used in non-invasive cardiac monitoring (NICOM) systems utilizing the same passive electrical properties described in the Bioimpedance Vector Analysis section. Beyond being an initiative to rebrand the term bioimpedance, bioreactance as a metric distinguishes itself from typical bioimpedance measurements by being a measure of frequency modulation rather than amplitude modulation. That is, in traditional bioimpedance systems, the magnitude of the output signal with respect to the input signal is of primary concern (as indeed it is for impedimetric intravascular volume evaluation technique developed in 2014). Amplitude variations stem directly from the amount of conductor being measured. Thus, given a constant control volume, the amplitude of a bioimpedance signal will only change if the conductivity of the volume changes. Bioreactance, on the other hand, is the difference in the frequency of the output signal compared to the input signal. These frequency varia- tions stem from phase shifts from the input signal within the system and can be caused by the movement of a conductor or the increased presence of time-dependent elements, such as cellular membranes. Because the phenomenon we are dealing with here is a phasor (that is, a phase vector), the actual physical distinction between the two methods is not as great as some of the literature may suggest.
Bioreactance, as it is currently employed, is used to assess cardiac output by applying at least four electrodes to the chest and continuously measuring the thoracic impedance. Phase shifts in the impedance signal arise from large bolus of blood ejected into the aorta. The combined effects of introducing a large volume of electrically conductive material with a significant reactive component (blood) introduce time delays in the signal traversing the chest, shifting the phase, and giving rise to bioreactance.
There is evidence to suggest that the phase shifts of the bioreactance signal are well correlated with stroke volume [80, 81] and therefore holds promise for the kind of non-invasive cardiac monitoring it underlies. In fact, several studies have shown some effectiveness in accurately tracking cardiac output and volume responsiveness [82, 83, 84, 85]. There is also evidence to the contrary . Furthermore, one of the shortcomings of the technique, especially in regards to fluid responsiveness, is its requirement of including a fluid challenge of some sort to make its measurement. Most studies couple a passive leg raise (previously discussed) with a change in cardiac output to determine the volume status and responsiveness of a patient. As such it suffers from many of the same drawbacks as a traditional passive leg raise.