Hyperkalaemia: ECG changes
Hyperkalaemia (a high plasma potassium concentration) first shows on the ECG as tall, narrow, peaked T waves, the part of the trace that represents repolarisation.
Remember
Raised potassium speeds up ventricular repolarisation, so tall, narrow, peaked T waves come first; if it keeps rising, impulses travel more slowly through the heart and the ECG changes in stages. A normal ECG does not exclude hyperkalaemia.
Memory aid
low potassium pushes the T wave down, and high potassium pulls it up
Use it to remember which way the T wave moves when the potassium level changes.
In hyperkalaemia, a high plasma potassium concentration, the ECG changes in a recognisable order as the level climbs.
The first change is in the T wave, which represents repolarisation, the return of the ventricular muscle cells to their resting electrical state. When the potassium outside the heart cells is raised, repolarisation is completed faster, and the T wave grows tall, narrow and peaked (often described as tented).
If the potassium keeps rising, fewer sodium channels remain available, because the ones that drive each beat's rapid upstroke become inactivated, so impulses travel more slowly. The atria show this as a flattened P wave, and the AV node (the electrical gateway between the atria and ventricles) shows it as a longer PR interval. These are changes of depolarisation rather than repolarisation.
At higher levels the P wave disappears and the QRS complex widens, because impulses travel slowly through the ventricles as well. Last comes a sine-wave pattern, in which the wide QRS has merged with the T wave into one smooth wave. It comes just before ventricular fibrillation (the ventricles quivering chaotically, with no pumping) or asystole, which means no electrical activity.
A normal ECG does not exclude hyperkalaemia, and the potassium level does not match the ECG closely. A lost P wave or a widening QRS calls for intravenous calcium to protect the heart, with insulin and glucose to move potassium back into the cells.
Key facts
- Earliest sign
- tall, narrow, symmetrical peaked T waves, often described as tented.
- Why the T wave
- raised potassium outside the heart cells speeds up repolarisation.
- Next stage
- a flattened P wave and a longer PR interval, as impulses travel more slowly through the atria and the AV node.
- Later stage
- the P wave disappears and the QRS widens.
- Final pattern
- a sine-wave pattern, which comes just before ventricular fibrillation or asystole.
- Level and ECG
- the potassium level does not correlate closely with the ECG, and a normal ECG does not exclude hyperkalaemia.
Exam traps
- Hypokalaemia (low potassium) gives the opposite of peaked T waves: the T wave is lowered and flattened, and a prominent U wave appears (a small extra wave following the T wave).
- A prolonged PR interval is a later change in hyperkalaemia, caused by impulses travelling more slowly through the AV node, not by faster repolarisation.
- When peaked T waves are the only abnormality, the picture is typical of mild hyperkalaemia; at a moderate level the P wave and the PR interval have usually been affected as well.
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