title: Evaporation of free water causes concentrational alkalosis in vitro.
abstract: BACKGROUND
  The development of metabolic alkalosis was described recently in patients with hypernatremia. However, the causes for this remain unknown. The current study serves to clarify whether metabolic alkalosis develops in vitro after removal of free water from plasma and whether this can be predicted by a mathematical model.
  
  
  METHODS
  Ten serum samples of healthy humans were dehydrated by 29 % by vacuum centrifugation corresponding to an increase of the contained concentrations by 41 %. Constant partial pressure of carbon dioxide at 40 mmHg was simulated by mathematical correction of pH [pH(40)]. Metabolic acid-base state was assessed by Gilfix' base excess subsets. Changes of acid-base state were predicted by the physical-chemical model according to Watson.
  
  
  RESULTS
  Evaporation increased serum sodium from 141 (140-142) to 200 (197-203) mmol/L, i.e., severe hypernatremia developed. Acid-base analyses before and after serum concentration showed metabolic alkalosis with alkalemia: pH(40): 7.43 (7.41 to 7.45) vs 7.53 (7.51 to 7.55), p = 0.0051; base excess: 1.9 (0.7 to 3.6) vs 10.0 (8.2 to 11.8), p = 0.0051; base excess of free water: 0.0 (- 0.2 to 0.3) vs 17.7 (16.8 to 18.6), p = 0.0051. The acidifying effects of evaporation, including hyperalbuminemic acidosis, were beneath the alkalinizing ones. Measured and predicted acid-base changes due to serum evaporation agreed well.
  
  
  CONCLUSIONS
  Evaporation of water from serum causes concentrational alkalosis in vitro, with good agreement between measured and predicted acid-base values. At least part of the metabolic alkalosis accompanying hypernatremia is independent of renal function.
authors.affiliation: Department of Internal Medicine, Inselspital,University Hospital Bern, Bern, Switzerland.
authors.forename: Gregor
authors.initials: G
authors.lastname: Lindner
authors.forename: Daniel
authors.initials: D
authors.lastname: Doberer
authors.forename: Christoph
authors.initials: C
authors.lastname: Schwarz
authors.forename: Bruno
authors.initials: B
authors.lastname: Schneeweiss
authors.forename: Georg-Christian
authors.initials: GC
authors.lastname: Funk
mesh_terms: D000136:Acid-Base Equilibrium; D000471:Alkalosis / Q000097:blood*; D001769:Blood / Q000378:metabolism*; D001834:Body Water / Q000378:metabolism*; D003890:Desiccation; D006801:Humans; D006863:Hydrogen-Ion Concentration; D006955:Hypernatremia / Q000097:blood*; D066298:In Vitro Techniques; D012964:Sodium / Q000097:blood*
chemical_list: D012964:Sodium