Related to a technique proposed elsewhere, bioimpedance vector analysis for analyz- ing fluid status has come into prominence over the past two decades because of its relative simplicity and its usefulness in key areas, such as measuring whole body fluid volumes. To use the bioimpedance vector as a metric of volume status, a functional relationship was empirically established, by normalizing both resistance and reactance to a patient specific metric, usually by dividing these values by their height. Having performed many experiments, researchers and clinicians have been able to establish the bounds where a typical vector sought to land given a patients hydration status. Where the tip of the normalized vector lands, can therefore indicate whether a patient has more or less fluids and/or more or less soft tissues (see Figure 3.7).
A number of studies have shown that the vector length is proportional to whole-body fluid volume with a high degree of correlation and accuracy, having been validated in many patient populations (kidney, liver, and heart failure) to assess volume status . Bioimpedance vector analysis has proven especially effective evaluating the static volume status in both heart failure patients [58, 59] and for those patients undergoing dialysis.