Basic Concepts of Fluid and Electrolyte Therapy

dividing the extracellular (ECF) from the intracellular fluid (ICF) space,
and the intravascular from the interstitial space. Osmotic or oncotic
shifts occur across these membranes, modified by physiological as
well as pathological mechanisms.
Anion gap – the difference between the plasma concentration of the
major cation Na + (135-145 mmol/l) and the major anions Cl – (95-105
mmol/l) and HCO 3–
(22-30 mmol/l), giving a normal anion gap of 5-11
mmol/l.
It is enlarged in metabolic acidosis due to organic acids as in diabetic
ketoacidosis, lactic acidosis, renal failure, and ingested drugs and toxins.
Anion gap (mmol/l) = [Na + ] – ([Cl – ] + [HCO 3 – ])
The anion gap is normal in hyperchloraemic acidosis (e.g. after excess
0.9% saline administration). It is, therefore, useful in the differential
diagnosis of metabolic acidosis, although specific measurement of
organic acids such as -hydroxy butyrate or lactate may also be necessary
to define the problem.
Strong ion difference (SID) – Stewart has described a mathematical
approach to acid-base balance in which the strong ion difference
([Na + ]+[K + ]-[Cl – ]) in the body is the major determinant of the H + ion
concentration. A decrease in the strong ion difference is associated
with a metabolic acidosis, and an increase with a metabolic alkalosis.
A change in the chloride concentration is the major anionic contributor
to the change in H + homoeostasis. Hyperchloraemia caused by a
saline infusion, therefore, will decrease the strong ion difference and
result in a metabolic acidosis.
Strong ion difference (mmol/l) = [Na + ] + [K + ] – [Cl – ]
e.g. If Na + is 140 mmol/l, K + is 4 mmol/l and Cl – is 100 mmol/l, the SID
is 44 mmol/l. The normal range is 38-46 mmol/l.
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