[Critical evaluation of changes in circulating blood volume, hematocrit, hemoglobin and plasma protein concentration for the calculation of normotonic fluid losses]. 
Normotonic fluid losses always lead to intravascular hypovolemia and subsequently to hypovolemic shock.  Unfortunately, there are no diagnostic methods sufficient to calculate the extent of normotonic losses for fluid therapy.  Therefore, indirect parameters such as changes in hemoglobin concentration, hematocrit, or plasma protein concentration are usually used for monitoring fluid therapy.  This paper presents the results of our study in ten healthy volunteers.  We investigated whether changes in circulating blood volume or hematocrit, hemoglobin, and plasma protein concentration indicate the amount of normotonic fluid loss in dehydrated humans.  Normotonic fluid losses were induced using furosemide in repeated doses of 20 mg.  Control measurements were carried out before furosemide administration (ZDM1 = time of sample taken) and after 2000 (ZDM2) and 3000 ml urinary output (ZDM3).  We measured hematocrit as well as concentrations of hemoglobin, plasma protein, sodium, potassium, and chloride using standard laboratory methods.  Hematocrit values were corrected for trapped plasma (3%) and the body/venous hematocrit ratio (0.91).  Circulating blood volume was measured using J-131 RIHSA and the "volemetron" introduced by Williams and Fine.  Plasma volume and plasma water volume were calculated.  After checking the data for normal distribution, statistics were calculated using the paired t test (P less than 0.01).  The results are listed in Table 2.  We calculated normotonic fluid losses on the basis of plasma and urinary sodium concentrations and found them to be 1720 +/- 155 ml (ZDM2) and 2392 +/- 262 ml (ZDM3).(ABSTRACT TRUNCATED AT 250 WORDS)