Conditions not primarily affecting the heart

doi:10.1136/bmj.324.7349.1320

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Corey Slovis,
Richard Jenkins

To function correctly, individual myocardial cells rely on normal concentrations of biochemical parameters such as electrolytes, oxygen, hydrogen, glucose, and thyroid hormones, as well as a normal body temperature. Abnormalities of these and other factors affect the electrical activity of each myocardial cell and thus the surface electrocardiogram. Characteristic electrocardiographic changes may provide useful diagnostic clues to the presence of metabolic abnormalities, the prompt recognition of which can be life saving.

It is important to recognise that some electrocardiographic changes are due to conditions other than cardiac disease so that appropriate treatment can be given and unnecessary cardiac investigation avoided

Hyperkalaemia

Increases in total body potassium may have dramatic effects on the electrocardiogram. The most common changes associated with hyperkalaemia are tall, peaked T waves, reduced amplitude and eventually loss of the P wave, and marked widening of the QRS complex.

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Electrocardiographic features of hyperkalaemia

The earliest changes associated with hyperkalaemia are tall T waves, best seen in leads II, III, and V2 to V4. Tall T waves are usually seen when the potassium concentration rises above 5.5-6.5 mmol/l. However, only about one in five hyperkalaemic patients will have the classic tall, symmetrically narrow and peaked T waves; the rest will merely have large amplitude T waves. Hyperkalaemia should always be suspected when the amplitude of the T wave is greater than or equal to that of the R wave in more than one lead.

Serial changes in hyperkalaemia

As the potassium concentration rises above 6.5-7.5 mmol/l, changes are seen in the PR interval and the P wave: the P wave widens and flattens and the PR segment lengthens. As the concentration rises, the P waves may disappear.

The QRS complex will begin to widen with a potassium concentration of 7.0-8.0 mmol/l. Unlike right or left bundle branch blocks, the QRS …