Potassium and the ECG: what low and high levels look like
Learn why potassium changes the T wave, what low and high potassium look like on the ECG, and what the nurse does first.
- Normal potassium is 3.5 to 5.0 mmol/L.
- Low potassium flattens the T wave and adds a U wave.
- High potassium makes the T wave tall and peaked.
- Never give IV potassium as a push.
Potassium questions appear on every nursing licensing exam. They are easier when you know why the ECG changes. This guide follows the Medibara lesson on potassium and the ECG.
Why potassium changes the ECG
Each heart cell holds a lot of potassium inside and only a little outside. That gap sets the cell's resting charge. The normal blood level is narrow: 3.5 to 5.0 mmol/L. On the ECG, the T wave shows the cells resetting as potassium flows back out.
Low potassium: below 3.5 mmol/L
When potassium falls, the cells reset slowly. The heart turns irritable, and extra beats slip in.
- The T wave flattens.
- A U wave appears after the T wave.
- Digoxin toxicity arrives faster.
High potassium: above 5.0 mmol/L
When potassium rises, the cells reset too fast. As the level climbs, the signal itself slows.
| Potassium | T wave | Other changes |
|---|---|---|
| Below 3.5 mmol/L | Flat | U wave, extra beats |
| 3.5 to 5.0 mmol/L | Normal | None |
| Above 5.0 mmol/L | Tall and peaked | The P wave fades. The QRS widens. |
What the nurse does
For low potassium, replace it slowly.
For high potassium, stop every source of potassium, get an ECG and call the provider. Calcium gluconate protects the heart. Insulin with dextrose moves potassium back into the cells.
How to remember it
- Low and flat: look for the U wave.
- High and tight: peaked T wave.
- Read the potassium. Then read the rhythm.
For each change in order and the treatment to expect, see hyperkalemia ECG changes and hypokalemia ECG changes.