Non-invasive conductivity method for detection of dynamic body fluid changes: in vitro and in vivo validation

Since intracellular and extracellular fluid volume (ICV, ECV) cannot be measured under dynamic circumstances, a non-invasive conductivity technique was developed for this purpose. To validate the technique, experiments in vitro and in vivo were performed. In vitro, dilution of blood led to a variation in haematocrit, which could be calculated accurately by means of the low-frequency conductivity value combined with the plasma conductivity value. Combination of high- and low-frequency conductivity values made calculation of haematocrit possible without measuring plasma conductivity. Changes in mean cellular volume, caused by addition of osmotically active substances, were detectable in the same way. Haemolysis of blood cells was performed to validate the intracellular conductivity. For in vivo validation the effects of position change (erect to supine) and of 40 mg frusemide i.v. were investigated. Position change caused a significant decrease in ECV and tended to increase blood volume (BV). Frusemide caused a mean iso-osmotic urine production of 1.8 +/- 0.2 litres. ECV decreased 12.3 +/- 2.0% (P < 0.05), while ICV increased 5.0 +/- 3.0% (P < 0.05). BV decreased by 7.0 +/- 5.4% (P < 0.05), while mean blood pressure increased (P < 0.05). Changes in both ECV and in ICV were correlated with diuresis (r = 0.88 and r = 0.85 respectively; P < 0.01). The ICV increase was unexpected and might be caused by an aldosterone-induced transcellular sodium influx. From both studies it can be concluded that non-invasive conductivity measurements are reliable for detecting changes in ECV and ICV under dynamic circumstances.