Page 13 - Medicine and Surgery

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Chapter 1: Fluid and electrolyte balance 9

The highest rate of administration of potassium recom- clinical examination as well as monitoring of serum elec-
mended in severe hypokalaemia is 20 mmol/h: this is trolytes by serial blood tests.
higher than the usual rate of 10 mmol/h recommended Fluid preparations: Intravenous fluid has to be iso-
in mild to moderate hypokalaemia. In asymptomatic pa- tonic to lysis of red blood cells. The administration of
tients with mild-to-moderate hypokalaemia oral or in- wateralonewouldleadtowatermovingacrosscellmem-
travenous potassium supplements are given. The serum branes by osmosis, such that the cells would swell up and
potassium must be rechecked frequently, e.g. every 4 burst. Giving hypertonic fluid is equally dangerous, as it
hours. Any underlying cause should be looked for and causes water to move out of cells.
managed appropriately. Most intravenous fluids used are crystalloids (saline,
dextrose,combineddextrose/saline,Hartmann’ssolu-
tion).Itshouldberememberedthatdextroseisrapidly
Intravenous fluids
metabolised by the liver; hence giving dextrose solu-
Intravenous fluids may be necessary for rapid fluid re- tion is the equivalent of giving water to the extra-
placement, e.g. in a shocked patient, or for maintenance cellular fluid compartment. If insufficient sodium is
in patients who are unable to eat and drink or who giveninconjunction, or the kidneys do not excrete the
are unable to maintain adequate intake in the face of free water, hyponatraemia results. This is a common
large losses, e.g. due to diarrhoea. When prescribing in- problem, often because of inappropriate use of dex-
travenous fluids certain points should be remembered: trose or dextrosaline and because stress from trauma
Are intravenous fluids the best form of fluid replace- or surgery as well as diseases such as cardiac failure

ment? If possible, oral fluids are preferable or if swal- promote antidiuretic hormone (ADH) release. This
low is impaired consider nasogastric administration, leads to a mild form of syndrome of inappropriate
which has the advantage of allowing nasogastric feed antidiuretic hormone (SIADH; see page 444) where
to be given to provide nutrition. there is water retention by the kidneys with resulting
Which intravenous fluid should be given? Ideally this
hyponatraemia.
should be the one that matches any fluid and elec- Colloids (albumin, dextran or gelatin-based fluids)
trolyte deficit or losses most closely. For example, containhigh-molecular-weightcomponentsthattend
blood loss should be replaced with a blood transfusion to be retained in the intravascular compartment. This
and salt and water loss (e.g. vomiting, diarrhoea) with increases the colloid osmotic pressure (oncotic pres-
normal saline. Additional potassium replacement is sure) of the circulation and draws fluid back into the
often needed in bowel obstruction, but may be dan- vascular compartment from the extracellular space. A
gerous in renal failure. smaller volume of colloid compared to crystalloid is
In calculating the volume required for maintenance needed to have the same haemodynamic effect. Theo-

check if is there increased insensible loss, e.g. due to retically they are of benefit for rapid expansion of the
sweating in pyrexial patients, or are there other fluids intravascular compartment; however, they have anti-
being administered which need to be taken into ac- coagulant, antiplatelet and fibrinolytic effects, which
count? For example, some patients are on intravenous may be undesirable. There has been no consistent
drugs or intravenous nutritional supplements (total demonstrable benefit of using colloid over crystalloid
parenteral nutrition). inmostcircumstances.Inaddition,theuseofalbumin
Patients at risk of cardiac failure (elderly, cardiac solution in hypoalbuminaemic patients (which seems

disease, liver or renal impairment) require special logical)hasbeenassociatedwithincreasedpulmonary
caution as they are more prone to develop fluid oedema,possiblyduetorapidhaemodynamicchanges
overload. or capillary leakage of albumin.
There is no universally applicable fluid regimen. The Fluid regimens: These should consist of maintenance
choice of fluid given and the rate of administration fluids (which covers normal urinary, stool and insensible
depend on the patient, any continued losses and all losses) and replacement fluids for additional losses and
patients must have continued assessment of their fluid to correct any pre-existing dehydration. Fluid regimens
balance using fluid balance charts, observations and must also take into account that patients of differing