Page 12 - Medicine and Surgery

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8 Chapter 1: Principles and practice of medicine and surgery

may be used to increase gastrointestinal elimination of repolarisation. This causes muscle weakness including
potassium. Oral resins can cause severe constipation, the respiratory muscles and ECG changes (see below)
so these should be given with laxatives and are not a predisposing to atrial and ventricular arrhythmias. In
long-term solution. severe hypokalaemia sudden cardiac or respiratory ar-
Anyacidosis should be corrected. rest may occur. Other effects include the following:

Refer to a renal physician or intensive care unit for Metabolic alkalosis: In hypokalaemia there is reduced

haemofiltrationorhaemodialysisifthehyperkalaemia potassium secretion into the renal tubules and in-
is refractory to treatment or if there is severe renal creased reabsorption at the H /K + ATPase pump
+
failure. so more H ions are lost. Hypokalaemia can both
+
cause and maintain a metabolic alkalosis. Alkalosis
+
Hypokalaemia also tends to promote the movement of K into cells,
worsening the effective hypokalaemia.
Definition Increased digoxin toxicity: Digoxin acts by inhibition
Aserum potassium level of <3.5 mmol/L. Moderate hy- of the Na /K ATPase pump. In low-potassium states
+
+
pokalemia is defined as a level of 2.5–3 mmol/L and the effect of digoxin is increased, thereby increasing
severe as <2.5 mmol/L. the risk of toxicity even at normal digoxin levels.
Incidence
Clinical features
This is a very common problem, occurring in up to 20%
Hypokalaemia is often asymptomatic even when se-
of inpatients.
vere and is therefore frequently diagnosed on incidental
blood testing. Symptoms include skeletal muscle weak-
Aetiology
ness, muscle cramps, constipation, nausea or vomiting
The most common cause is diuretics. Other causes are
and polyuria. Neuropsychiatric symptoms include con-
giveninTable 1.3.
fusion, hallucinations, depression and even psychotic
features. It is important to take a careful drug history.
Pathophysiology On examination the patient may be hypotensive and
Hypokalaemia causes disturbance of neuromuscular there may be evidence of cardiac arrhythmias such as
function by altering the resting potential and slowing bradycardia, tachycardia or ectopic beats. There may be
reduced muscle strength, fasciculations or tetany. The
Table 1.3 Causes of hypokalaemia first sign may be cardiorespiratory arrest.
Decreased Transcellular Increased
intake movement output Investigations
Apart from checking the serum potassium, U&Es, cal-
Usually Alkalosis Renal losses: diuretics,
iatrogenic: Insulin low serum cium and magnesium should be sent to look for other
lack of oral treatment magnesium, renal electrolyte abnormalities. An arterial blood gas may be
intake or i.v. tubular acidosis indicated to look for alkalosis. The ECG shows pro-
replacement GI losses: vomiting,
diarrhoea, purgative longed PR interval, depressed ST segment, flattened or
abuse, intestinal inverted T-wave and rarely a prominent U-wave (which
fistula, ileal loop appears as a long QT interval). Ventricular/atrial prema-
Malnutrition Conn’s/Cushing’s ture contractions or fibrillation may be seen or torsades
syndrome and 2 ◦ de pointes.
hyperaldosteronism
Ectopic ACTH (e.g. small
lung carcinoma) Management
Liquorice abuse, If severe hypokalaemia or cardiac arrhythmias are
carbenoxolone present, urgent treatment is required. Treat any life-
Drugs: β agonists, threatening arrhythmias appropriately and give intra-
steroids, theophylline
venous potassium with continuous cardiac monitoring.