Basic Concepts of Fluid and Electrolyte Therapy

volume at all costs. It also explains why sick patients can be so easily
overloaded with excessive salt and water administration during the
flow phase. Since water as well as salt is retained, it is also easy to
cause hyponatraemia by giving excess water or hypotonic fluid. It is
important, therefore, to administer crystalloids, not only in the correct
volume but also in the appropriate concentration. In the presence of
the response to injury, the kidneys are unable to correct for errors in
prescribing.
The convalescent phase of injury is not only characterised by the
return of anabolism but also by a returning capacity to excrete any
excess salt and water load that has been accumulated. These periods
have been termed the ‘sodium retention phase’ and the ‘sodium diuresis
phase’ of injury.
Transcapillary escape rate of albumin
The response to injury, stress and sepsis also results in an increase in
the size of the pores in the capillary membrane and the transcapillary
escape rate of albumin increases from about 5%/h in health to 13-
15%/h. This phenomenon can last from several hours to days. Albumin
leaks out from the intravascular compartment into the interstitial
space and along with it, water and sodium are also drawn into the
interstitial space. This results in a net contraction of the intravascular
compartment and expansion of the interstitial space (Fig. 4). As the
return of albumin to the circulation via the lymphatics is unchanged,
the net result is an intravascular hypovolaemia with oedema.
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