Page 14 - Medicine and Surgery

P1: JYS
BLUK007-01 BLUK007-Kendall May 12, 2005 17:17 Char Count= 0

10 Chapter 1: Principles and practice of medicine and surgery

Table 1.4 Maintenance fluid requirements Bicarbonate is a very important buffer, as it has both
agaseous and an aqueous phase:
Fluid requirement 30–35 mL/kg/day
Sodium requirement 1.5–2 mmol/kg/day + −
CO 2 ↔ CO 2 + H 2 O ↔ H 2 CO 3 ↔ H + HCO 3
Potassium requirement 1 mmol/kg/day
This means that bicarbonate buffering is a very powerful
way of maintaining the body’s pH through both rapid
and slow compensation:
weight have different fluid and electrolyte requirements
Rapid compensation takes place at the lungs, where
(see Table 1.4). Potassium is added to intravenous flu-
CO 2 can be blown off in response to acidosis. This
ids in patients who are not being fed, although this
reduces the amount of H 2 CO 3 (carbonic acid) in the
shouldbedonewithcare.Bothhypokalaemiaandhyper-
blood as shown by the equation and so acutely com-
kalaemia (see page 7) are potentially life-threatening and
pensates for acidosis. Conversely, if pCO 2 concentra-
serum potassium must be checked daily in patients who
tions rise, e.g. in hypoventilation, then acidosis can
are given potassium replacement. Patients with acute or
result (respiratory acidosis).
chronic renal failure should not have potassium added
In long-term abnormalities of pH balance, this mech-
routinely to fluid replacement (although hypokalaemia
anism is inadequate because the body’s stores of
should of course be treated). Rapid administration of
bicarbonate become depleted. The kidney is able to
potassium is dangerous, so even in hypokalaemia no
compensate for this, by increasing its reabsorption of
more than 10 mmol/h is recommended (except in se-
bicarbonate in the proximal tubule.
vere hypokalaemia within an intensive care setting) and
The arterial blood gas is used to assess acid–base status.
the potassium must be uniformly mixed in the bag.
The pH is first examined to see if the patient is acidotic or
Atypical daily maintenance regime for a 70 kg man with
alkalotic. The pCO 2 and bicarbonate are then examined
normal cardiac and renal function consists of 8 hourly
to identify the cause of any acid–base disturbance and
bags of:
any compensation that may have occurred. Most arterial
1L of 0.9% saline with 20 mmol KCl added,

blood gas machines also provide the base excess. This
1L of 5% dextrose with 20 mmol KCl added and

is a calculated figure, which provides an estimate of the
1L of 5% dextrose with 20 mmol KCl added.

metabolic component of the acid–base balance. The base
In general, dextrosaline is not suitable for mainte-
+
excess is defined as the amount of H ions that would be
nance, as it provides insufficient sodium and tends
to cause hyponatraemia. Postoperative patients are also requiredtoreturnthepHofthebloodto7.35,ifthepCO 2
wereadjustedtonormal.Anormalbaseexcessis–2to+2.
more prone to hyponatraemia due to mild SIADH,
Amorenegativebaseexcesssignifiesametabolicacidosis
so may require proportionally more sodium, e.g. 2 L
(hydrogen ions need to be removed) and a more positive
of 0.9% saline to1Lof5%dextrose. Replacement fluids
base excess signifies a metabolic alkalosis (hydrogen ions
generally need to be 0.9% saline, as losses tend to have a
need to be added). The pO 2 is examined separately to
high sodium concentration, e.g. drain fluid, blood, vom-
determine if there is respiratory failure.
itus and diarrhoea.
There are four main patterns:
Fluids should not be prescribed without taking into
Acidosis with high pCO 2 defines a respiratory acido-
account the patient’s current fluid balance, continued
sis. If this is acute, there is no compensation (i.e. the
losses and underlying coexistent diseases. It should also
bicarbonate levels are normal). In chronic respiratory
be remembered that intravenous fluids do not provide
acidosis renal reabsorption of bicarbonate will reduce
any significant nutrition.
the acidosis (partial metabolic compensation) or re-
turn the pH to a normal level (complete metabolic
compensation).Causesincluderespiratoryfailure(see
Acid–base balance
page 127).
The normal pH of arterial blood is 7.35–7.45. Normally Acidosiswithlowbicarbonateandnegativebaseexcess
+
hydrogen (H )ions are buffered by two main systems: defines a metabolic acidosis. If the patient is able the
Proteins including haemoglobin comprise a fixed respiration will increase to reduce carbon dioxide and

buffering system. hence return the pH to normal (partial or complete