U waves

The following are recognised causes of a U wave on an electrocardiogram EXCEPT

which one?

a) Congenital long QT syndrome

b) Hypercalcaemia

c) Flecainide

d) Thyrotoxicosis

e) Hypokalaemia

Answer: c

Explanation

A U wave is a small wave on the ECG, which begins after the T wave terminates. The U

wave usually has the same polarity as the T wave. A number of hypotheses explain

their presence on an ECG. These include U waves representing repolarisation of the

Purkinje fibres or that they may represent delayed after-potentials. They occur with the

disturbances outlined above, on exposure to adrenaline or as part of digitalis toxicity.

They are sometimes seen following an intracranial haemorrhage or mitral valve prolapse.

They can be found with Class Ia and III antiarrhythmic agents. Flecainide is a Ic

agent and does not cause U waves. Inverted U waves are sometimes seen following

myocardial infarction or fluid overload of the left ventricle and have a high specificity

for demonstrating significant underlying disease.