Page 11 - Medicine and Surgery

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Chapter 1: Fluid and electrolyte balance 7

Insulin and activation of β 2 receptors tend to drive may be a cardiac arrhythmia or sudden cardiac arrest.
potassium into cells, lowering the serum potassium con- Uncommonly there may be reduced tendon reflexes and
centration. muscle power.

Hyperkalaemia Investigations
U&Es, calcium, magnesium to look for evidence of renal
Definition impairment and any associated abnormality in sodium,
Aserumpotassiumlevelof>5.5mmol/Lisdefinedashy- calcium and magnesium. Low calcium can increase the
perkalaemia. Hyperkalaemia of >6.0 mmol/L can cause risk of arrhythmia. An arterial blood gas to look for aci-
cardiac arrhythmias and sudden death without warning. dosis may be indicated and diabetics should have their
glucose checked.
Incidence AnECGshouldbeperformedimmediatelyinallcases.
This is a common problem, affecting as many as 1 in 10 Abnormalities occur in the following order: tall, tented
inpatients. T-waves, small P-wave and a widened, abnormal QRS
complex. Patients may develop bradycardia or complete
Aetiology heartblock,andifleftuntreatedmaydiefromventricular
The causes are given in Table 1.2. standstill or fibrillation. Continuous ECG monitoring
should occur until the hyperkalaemia is treated and ECG
Pathophysiology abnormalities resolve.
Hyperkalaemia lowers the resting potential, shortens the
cardiac action potential and speeds up repolarisation, Management
therefore predisposing to cardiac arrhythmias. The ra- Ideally hyperkalaemia should be prevented in at-risk pa-
pidity of onset of hyperkalaemia often influences the risk tientsbyregularmonitoringofserumlevelsandcarewith
of cardiac arrhythmias, such that patients with a chron- medication and intravenous supplements. Once hyper-
ically high potassium level are asymptomatic at much kalaemia is diagnosed, withdraw any potassium supple-
greater levels. ments or causative drugs.
If the hyperkalaemia is mild (<6.0 mmol/L) and the
Clinical features ECG is normal, withdrawal of causative drugs or treat-
Hyperkalaemia is almost always asymptomatic and only mentoftheunderlyingcausemaybesufficient.Moderate
diagnosed on blood testing. There may be a history of asymptomatic hyperkalaemia (6.0–6.9 mmol/L) needs
conditions that predispose to hyperkalaemia and it is early specific treatment. If there are ECG changes, severe
important to take a careful drug history. Foods high in muscle weakness or the potassium level is >7 mmol/L,
potassium include bananas, citrus fruits, tomatoes and it is a medical emergency:
salt substitutes. The first indication of hyperkalaemia Calciumgluconateisgivenintravenously.Thecalcium
providessomeimmediatecardio-protectionbyreduc-
Table 1.2 Causes of hyperkalaemia* ing myocardial excitability, even in a patient with nor-
mal serum calcium levels. It can be repeated after a
Increased Transcellular Decreased few minutes if the abnormalities on ECG persist.
intake movement output
Until treatment of the underlying cause can take place,

+
Excess K therapy: Acidosis Renal failure aglucose and insulin infusion promotes intracellular
(oral or i.v.) Insulin deficiency Drugs e.g. K +
+
Diet β-blockers sparing K uptake. Salbutamol nebulisers have a similar effect
Massive Haemolysis diuretics, ACE throughβ receptorstimulation.Thesecanberepeated
transfusion of Rhabdomyolysis inhibitors whilst the underlying cause is addressed, but have only
stored blood Digoxin toxicity Addison’s disease
atemporary effect.
Diuretics, e.g. loop diuretics can be used to increase
*Artefactual hyperkalaemia may occur in old or haemolysed blood
samples. renal excretion. Oral ion-exchange resins or enemas