Basic Concepts of Fluid and Electrolyte Therapy

Fluid balance charts
These provide useful information about changes in urine output and
abnormal losses, e.g. gastric aspirate, but they have inherent inaccuracies.
With great care in measurement and recording, they may be
helpful in assessing balance over 24 hours. However, an assumption
has to be made concerning insensible loss, and errors in measurement
and recording are common. The cumulative error over several days
can, therefore, be considerable.
Weight
There is no substitute for daily weighing in order to monitor external
water balance accurately, yet outside renal units, it is seldom practised.
As it is a major safeguard against clinically important errors in
fluid volume administration, it is well worth the extra effort and
resources required, particularly in complex post-operative cases. It
does, of course, only measure external balance, which may conceal
significant changes in internal balance between fluid compartments.
For example, in the presence of ileus or intestinal obstruction, large
volumes of extracellular fluid may be pooled in the gut and therefore
be functionally inert. Weight is, therefore, unchanged despite this
clinically important fluid shift, which reduces effective ECF volume
and necessitates salt and water replacement. Valuable as weighing is,
therefore, it cannot be followed blindly. Like any other parameter, it
requires intelligent interpretation in its clinical context and in the
light of all the other information available.
Invasive monitoring
Invasive techniques such as insertion of central venous catheters,
arterial lines and catheters to measure pulmonary artery wedge pressure
are useful to help direct fluid therapy in more complex patients.
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