Basic Concepts of Fluid and Electrolyte Therapy

the need for colloid to expand the intravascular volume, improve renal
blood flow and allow the excretion of the salt and water overload. If,
on the other hand, the jugular venous pressure (JVP) is elevated, then
immediate cessation of crystalloid administration, with or without
diuretics, will correct the underlying imbalance.
Measurements and Investigations
Urine
As described above, the volume and concentration of urine are important
indicators of renal function. Oliguria may be physiological postoperatively,
or indicative of intravascular or ECF deficit. If this is
accompanied by a concentrated urine and a rising blood urea, it is
termed pre-renal acute kidney injury (AKI), correctable by appropriate
fluid replacement. A persisting low volume and concentration combined
with a rising blood urea and creatinine suggest AKI due to
intrinsic damage has now developed, necessitating some form of renal
replacement therapy (e.g. haemofiltration or haemodialysis). Changes
in urine volume must, therefore, be interpreted in the light of accompanying
features and circumstances.
Nurses are often instructed to call junior doctors if the postoperative
urine output falls below 30 ml/h. As a consequence, the doctor often
prescribes extra saline “just to be on the safe side”. This commonly
results in salt and water overload. In fact, such “oliguria” is usually a
physiological response to surgery. While it is important to identify the
patient who has become hypovolaemic and to resuscitate adequately,
it is unlikely that a patient who appears well with warm pink peripheries
and no tachycardia or tachypnoea has need of volume expansion.
Urine output in such patients should be averaged over four hours
and interpreted in combination with serial trends in vital signs of circulatory
adequacy.
33