Hypokalemia ECG changes: flat T waves and U waves
Hypokalemia sags the ST segment, flattens the T wave and brings out a U wave. See the arrhythmias that follow, the digoxin risk and the IV potassium limits.
- Below 3.5 mmol/L: sagging ST segment, flat T wave, U wave.
- Digoxin makes even mild hypokalemia dangerous.
- Check magnesium when potassium is low.
- IV potassium: no more than 10 mmol an hour routinely. Never a push.
Hypokalemia makes the ST segment sag, flattens the T wave and brings out a U wave after it. As potassium falls further, the T wave gets smaller and the U wave gets larger.
Sources checked 6 October 2026.
The ECG changes
Hypokalemia is a serum potassium below 3.5 mmol/L. The Merck Manual lists three changes on the ECG.
- The ST segment sags.
- The T wave is depressed. It looks flat or small.
- The U wave rises. It is a small extra wave after the T wave.
Potassium varies widely between patients with the same ECG picture, so the ECG does not tell you the level. It tells you the heart is affected.
The arrhythmias that follow
- Premature atrial and premature ventricular beats.
- Supraventricular and ventricular tachyarrhythmias.
- Second-degree or third-degree atrioventricular block.
- With increasingly severe hypokalemia, ventricular fibrillation.
Who is at risk
- Clients taking loop or thiazide diuretics. Diuretics are by far the most common drug cause.
- Clients with chronic diarrhea or laxative overuse.
- Clients with protracted vomiting or gastric suction.
The usual symptoms are muscle weakness and passing a lot of urine.
Digoxin and magnesium
A client who takes digoxin or has significant heart disease can develop conduction problems with even mild hypokalemia. Low potassium also increases the effect of digoxin, so it raises the risk of digoxin toxicity.
What the nurse does
- Put the client on a cardiac monitor and get an ECG.
- Report the result. Ask whether the client takes digoxin or a diuretic.
- Give oral potassium when it is prescribed and the client can take it.
- Give IV potassium diluted and on a pump. Never give it as a push.
IV potassium limits
IV replacement is used when hypokalemia is severe, when oral treatment has not worked, or when a hospitalised client takes digoxin, has significant heart disease or has ongoing losses. The Merck Manual gives these limits. Follow your facility's policy where it is stricter.
| Limit | Value |
|---|---|
| Routine infusion rate | No more than 10 mmol an hour |
| Rate for a client with arrhythmias | Up to 40 mmol an hour, only with continuous cardiac monitoring |
| Concentration in a peripheral vein | No more than 40 mmol/L, because potassium irritates the vein |
See hyperkalemia ECG changes for the high side, and potassium and the ECG for why potassium moves the T wave. To practise, try the potassium question in our free REx-PN practice questions.
These changes are easier to remember once you have watched them happen. Medibara's lesson on potassium and the ECG is a short video that shows the trace as potassium falls: the T wave flattens, the U wave appears and extra beats slip in. It also covers the digoxin risk and the rule against giving IV potassium as a push. You can play it in the demo on the Medibara home page. In the app, questions in your exam's format follow the lesson, and Ask Cappy explains any rationale you are unsure of. Start learning on Medibara.