Bioelectric impedance measurement for fluid status assessment

Adequacy of body fluid volume improves short- and long-term outcomes inpatients with heart and kidney disorders. Bioelectrical impedance vector analysis (BIVA) has the potential to be used as a routine method at the bedside for assessment and management of body fluids.

Impedance (Z vector) is a combination of resistance, R (function of intra- and extracellular fluid volume) and reactance, Xc (function of the dielectric material of tissue cells), with the best signal to noise ratio at 50 kHz. BIVA allows a direct assessment of body fluid volume through patterns of vector distribution on the R-Xc plane without the knowledge of the body weight. Reference tolerance ellipses (50, 75 and 95%) for the individual vector were previously calculated in the healthy population.

We determined the optimal vector distribution in patients undergoing hemodialysis without hypotension or intradialytic symptoms. Most vectors lay within the reference 75% tolerance ellipse of the healthy population indicating full electrical restoration of tissues. We also determined the optimal vector distribution of patients undergoing continuous ambulatory peritoneal dialysis without edema and with a residual urine output. The vector distribution was close to the distribution of both healthy subjects and pre-session distribution of hemodialysis patients. We established the relationship between central venous pressure and BIVA in critically ill patients. Shorter vectors (overhydration) were associated with increasing venous pressure, whereas longer vectors were associated with decreasing venous pressure. The association between BIVA and NT-proBNP has been evaluated in patients with acute cardiac-related dyspnea. In the 'gray zone' of NT-proBNP values between 'ruling out' and 'ruling in' acute heart failure, BIVA detected latent peripheral congestion.

Simple patterns of BIVA allow detection, monitoring, and control of hydration status using vector displacement for the feedback on treatment.