Tetrapolar body impedance is influenced by body posture and plasma sodium concentration.

Tetrapolar body impedance measures body resistance, which is highly correlated to total body water and fat-free mass. We investigated the change in resistance measured with tetrapolar impedance, resulting from fluid shifts, following an altered body posture and changes in resistance following an altered electrolyte composition of the extracellular fluid during hypertonic saline and mannitol infusion. An increase in body resistance was measured during recumbency, which was positively correlated with the calculated shift of interstitial fluid to the intravascular space. This effect was completely abolished after 5 min in the upright position. During infusion of 5% saline or 20% mannitol the change in resistance and the change in plasma sodium concentration were inversely correlated. Therefore the measurement of resistance is essentially influenced by alterations in the amount and composition of extracellular fluids. Using the resistance measurements standardization is obligatory for posture and plasma sodium concentration. This is particularly important when repeated measurements are performed.