Basic Concepts of Fluid and Electrolyte Therapy

Base excess - Base excess is defined as the amount of strong acid that
must be added to each litre of fully oxygenated blood to return the pH
to 7.40 at a temperature of 37°C and a pCO 2
of 40 mmHg
(5.3 kPa). A base deficit (i.e., a negative base excess) can be correspondingly
defined in terms of the amount of strong base that must
be added.
Acidaemia and Alkalaemia – An increase in the H + ion concentration
or a decrease in the pH is called acidaemia; a decrease in the H + ion
concentration or an increase in the pH is called alkalaemia.
Acidosis and Alkalosis – Processes that tend to raise or lower the H +
ion concentration are called acidosis and alkalosis respectively. These
may be respiratory, metabolic or a combination of both. CO 2
retention
causing a rise in pCO 2
in respiratory failure leads to respiratory acidosis
and hyperventilation with a consequent lowering of pCO 2
leads to
respiratory alkalosis. Accumulation of organic acids such as lactate or
-hydroxybutyrate or of mineral acidic ions such as chloride cause a
metabolic acidosis in which arterial pH falls below 7.4, bicarbonate is
reduced and pCO 2
falls as the lungs attempt to compensate by blowing
off more CO 2
. This is called a compensated metabolic acidosis.
Similarly, ingestion of alkalis such as bicarbonate or loss of gastric
acid cause a rise in pH and a metabolic alkalosis.
Maintenance
requirements.
- Provide daily physiological fluid and electrolyte
Replacement - Provide maintenance requirements and add like for
like replacement for on going fluid and electrolyte losses (e.g. intestinal
fistulae).
Resuscitation - Administration of fluid and electrolytes to restore
intravascular volume.
28