As an example, consider the infusion of an 8.4%
NaHCO3 solution. This
is a molar solution of NaHCO3.
Dissociation into two particles in solution results in a solution
with an osmolality of 2,000 mOsm/kg. This is about 7 times the
Infusion of this solution has effects because
with a high [Na+]
The high sodium concentration restricts the
distribution of the solution to the ECF. The hypertonic nature of
the solution draws water out of cells until the ECF and ICF
tonicities are equal. The increase in ECF volume will be greater
than the volume of solution administered into it.
The ECF [Na+]
will increase dependent on the amount of solution administered but
the water drawn out of the cells will tend to minimise this
increase. Sodium bicarbonate solution has occasionally been
recommended for emergency treatment of acute hyponatraemia
particularly where there was also a perceived benefit of the
The ECF [HCO3]
will increase and this exogenous administration of base will cause
a metabolic alkalosis. This causes intracellular movement of K+
and ECF [K+] will
decrease. This is the basis of the use of NaHCO3
solution for the emergency treatment of hyperkalaemia.
Under normal circumstances, if the plasma
bicarbonate rises above about 27 mmol/l then HCO3-
is rapidly excreted in the urine. A metabolic alkalosis will
rapidly correct unless there is some additional factor which
maintains it. Because of the brief nature of the alkalosis, the
compensatory hypoventilation is minimal.
There are conflicting influences on ADH levels:
• A rise in extracellular
tonicity of 1 to 2% or more will increase ADH levels
(effect via hypothalamic osmoreceptors)
• An increase in blood volume
of 7 to 10% or more will decrease ADH levels (effect via
low pressure baroreceptors).
A decrease in ADH will increase water
excretion. An increase in blood volume due to NaHCO3
infusion will cause a fall in plasma oncotic pressure and water
reabsorption in the proximal tubule will decrease slightly due to
The increases in tonicity and blood volume can be estimated
from a knowledge of the volume of solution administered.
similar calculations in Section 8.2.]
- All material © Copyright - Kerry Brandis, 2003