Basic Concepts of Fluid and Electrolyte Therapy

Mixed acid-base disorders
These are defined as the presence of more than one acid-base disorder.
The patient’s history or a lesser or greater than predicted compensatory
respiratory or renal response may raise suspicions of mixed
acid-base disorder.
A normal pH in the setting of substantial changes in both serum
HCO 3 – or arterial Pco 2
indicates a mixed-acid base disorder is present.
Stewart approach to acid-base disorders
The Stewart approach, termed the Strong Ion Difference (SID), is
based upon the central tenet that serum bicarbonate does not alter
blood pH. Stewart defined acids as ions that shift the dissociation
equilibrium of water to a higher concentration of H + and a lower concentration
of OH - .
The SID is the difference between the completely dissociated cations
and anions in the plasma. It is defined as the difference between the
sum of the strong cations, Na + , K + , Ca 2+ and Mg 2+ and the sum of the
net charge of the major strong cations, Cl – and lactate.
SID = (Na + + K + + Ca 2+ + Mg 2+ ) – (Cl – + lactate) = 38-46 mmol/l
An increase in the SID is associated with an increase in blood pH, an
alkalosis, e.g. vomiting leads to a loss of chloride and a decrease in
serum chloride levels resulting in an increase in SID and alkalosis. The
Stewart approach therefore explains the alkalosis associated with
vomiting as excessive loss of chloride.
A decrease in SID is associated with a decrease in blood pH, an acidosis,
e.g. the excessive infusion of saline results in an increase in chloride
levels and therefore a decrease in SID and an acidosis. The Stewart
approach therefore explains the hyperchloraemic metabolic
acidosis associated with excessive saline infusion by the gain of chloride.
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