3. Fluid therapy Flashcards
Intro
optimum choice of fluids for many different clinical circumstances remains
confusing and contentious, and you will not be expected to resolve the various
controversies. Volume restoration, however, is such an important part of anaesthetic
practice that you will be expected to demonstrate both an understanding of the fluid
compartments of the body and a logical appreciation of the characteristics of the
different replacement fluids.
Normal body fluid compartments
of the total body weight in men, 60% is water. In
women, who have a higher proportion of body fat, it is 50–55%.
These proportions change with age;
total body water (TBW) as a percentage of body weight may be 80% in the neonate
and 50% in the elderly
Division of water
Two-thirds of TBW is intracellular water (ICW);
the remaining third is extracellular fluid (ECF), which
can be divided further into interstitial fluid (ISF) and the intravascular volume.
There is a small volume of residual transcellular fluid, which has been secreted, but which remains separated
from plasma, for example as cerebrospinal or intraocular fluid
Fluid Loss from Body Compartments
Blood loss:
this is straightforward. Intravascular volume may be depleted directly by
trauma or during surgery. It may occur pre-operatively, for example following the
rupture of a varicose venous ulcer or an arterial aneurysm, or postoperatively.
Pure dehydration:
Pure dehydration:
this implies a loss of water alone, without electrolytes.
This may be caused by prolonged lack of fluid intake,
protracted pre-operative fasting and as
a result of any condition that may prevent swallowing.
Dehydration depletes all the fluid compartments and is corrected by a solution that equilibrates across all three, namely glucose 5%.
Even in these situations there are always some electrolyte losses.
Dehydration
Dehydration:
in the context of clinical medicine,
most water deficits are also accompanied by electrolyte losses.
The causes are numerous and include
inappropriate diuretic therapy,
diarrhoea and vomiting,
intestinal obstruction,
pre-operative bowel preparation,
diabetes mellitus (and insipidus)
pyrexia.
Insensible losses in a healthy individual in a temperate climate are of the order of 0.5 ml kg/h
Perioperative fluid losses
Perioperative fluid losses:
these include the fluid deficits accrued as a result of
preoperative fasting,
pre-operative pathology,
intraoperative haemorrhage and
what are termed ‘third space’ losses.
This refers to fluid that is sequestered at the site of injury.
Losses are variable, but, during the course of a long laparotomy through a large
abdominal incision, fluid replacement may be needed by a balanced salt solution at a
rate of up to 15 ml kgh
Restoration of volaemic status.
Crystalloids
— A crystalloid solution is defined chemically as one containing a water-soluble
crystalline substance capable of diffusion through a semi-permeable membrane.
— Crystalloids can be infused rapidly in large volumes, are readily available and are
cheap. Disadvantages include their short duration in the circulation, with only
about 50% of the infused volume remaining in the intravascular compartment at
20 minutes. This increases the potential for overinfusion, circulatory overload and
pulmonary oedema. Crystalloids have no oxygen-carrying capacity.
Normal saline (NaCl 0.9%):
Normal saline (NaCl 0.9%): this contains 154 mmol l1 each of sodium and
chloride ions and is isotonic. The excess of chloride ions means that if large
volumes are infused, a hyperchloraemic acidosis may supervene. This can be a
particular problem in children
Hartmann’s
Hartmann’s (compound sodium lactate):
this is a balanced salt solution whose composition approximates that of ECF.
The lactate in Hartmann’s is gluconeogenic,
and so it has been recommended that the solution should not be used in
diabetics.
Given that basal lactate production is around 1,300 mmol 24–1 (0.8
mmol kg–1 hr–1), however, it seems unlikely that the 29 mmol of lactate in a litre
of Hartmann’s solution is going to make a substantial difference to any diabetic’s
glycaemic status, and so this is a prohibition that many anaesthetists are happy to
ignore, viewing the infusion of normal saline as the greater problem
Glucose 5%
Glucose 5%:
this is effectively a means of giving free water. Isotonic glucose
solutions are appropriate for resuscitation of the intracellular compartment, but
will have minimal impact on intravascular volumes because they will equilibrate
throughout the 42 litres of water in the body’s fluid compartments. Fluids which
contain glucose have no place in acute fluid resuscitation.
Colloids
Colloids
— A colloid is defined chemically as a dispersion, or suspension, of finely divided
particles in a continuous medium. It is not therefore a solution
Colloids are theoretically more effective than crystalloids in resuscitation, but the
evidence to support their superiority is equivocal. All contain NaCl 0.9%, and
Haemaccel contains small amounts of potassium and calcium. Blood is also a
colloid, but by convention it is treated separately.
Gelatins
Gelatins: gelatins (Gelofusine and Haemaccel) contain modified gelatin of
molecular weight between 30,000 and 35,000 Da
Their use in severe sepsis is also associated with a higher
incidence of kidney injury, and they carry the additional risk of allergic reactions
CRISTAL trial randomized critically ill patients to
fluid resuscitation with any colloid or any crystalloid and found no difference in
mortality at 28 day
Human albumin solution (HAS)
Human albumin solution (HAS): this was previously supplied as plasma protein
fraction (PPF) and has an intravascular half-life of 24 hours. It is derived from
pooled human plasma but is sterile.
useful volume expander that has been shown
in other
Starches:
Starches: these, however, did turn out to be the killer fluid that human albumin
never was.
identified greater risks of kidney injury associated with the use of starches,
and a higher mortality in the critically ill.
