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Solute-free versus electrolyte-free water clearance in the analysis of osmoregulation
Kurakazu Shimizu1, Takeshi Kurosawa, Teizo Sanjo
1Sumiyoshi Clinic Hospital, Mito, Fourth Department of Internal Medicine, Tokyo Women's Medical University, Tokyo, Japan. aquamoon@mui.bioglobe.ne.jp
Background:
Although attention has recently focused on electrolyte-free water clearance (E-CH2O) as a replacement for solute-free water clearance (CH2O), especially from the viewpoint of plasma sodium regulation, a thorough comparison of the two has yet to be conducted.
Methods:
CH2O and E-CH2O were systematically compared in normal subjects in different diuretic stages, including furosemide-induced solute diuresis, and in patients with renal disease.
Results:
The normal renal ability to conserve free water based on E-CH2O was only 41% of that based on CH2O. E-CH2O remained positive until the urinary osmolality exceeded 500 mosm/kg H2O, markedly different from the 300 mosm/kg H2O for CH2O. The difference between E-CH2O and CH2O could ultimately be attributed to urea osmolar clearance, i.e., urea excretion rate/plasma osmolality, which accounted for about 40% of the osmolar clearance. CH2O underestimated the free water clearance by about 1 ml/min on average at all diuretic stages.
Conclusions:
E-CH2O is a more correct parameter than CH2O with regard to the regulation of both plasma sodium and plasma osmolality. However, there is the opinion that the concept of E-CH2O is difficult to understand and that E-CH2O is still not a generally accepted parameter. It is expected that the results of the present study will lead to more general acceptance.