Hypokalemia

Hypokalemia classically causes flattened T waves, large U waves and/or a prolonged QTc.ref ST depression is often (but not always) present.

How do you recognise a U wave?

One of the most difficult skills in interpreting ECGs with hypokalemia is recognising the difference between T waves and U waves. One clue is that if the QT interval is extremely long it might actually be measuring a QU interval. This happens when the U wave is mistaken for a T wave. Look out for negative T waves that are almost hidden in the ST depression. Negative T waves that are followed by positive U waves can also look like biphasic T waves. Or, sometimes U waves can be joined to the T wave so that the T wave appears to have a ‘double hump’. Lastly, U waves can be mistaken for P waves! If in doubt, consider the clinical context, repeat the ECG and/or look across all of the leads of the 12-lead ECG.

How good are these signs?

U waves are sensitive but not specific for hypokalemia.ref Prominent U waves are more specific (but less sensitive). None of these signs are perfect! Overall, hypokalemia is most likely when the clinical context fits, the QT is prolonged and there are prominent U waves.

See also: Electrolytes

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References

ECG Library (34)

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In the ECGquest archives, this ECG has been tagged with: - 12-Lead Long QT Tall U waves Hypokalemia Dr Smith's ECG Blog CC BY-NC
“Long QT” after droperidol

This ECG is from a patient who presented wuth altered mental status and falls soon after treatment for intractable hiccups.

This ECG shows large U waves and long QU interval due to hypokalemia (1.7).

In the ECGquest archives, this ECG has been tagged with: - 12-Lead Long QT Hypokalemia Dr Smith's ECG Blog CC BY-NC
Potassium and Magnesium: how low is too low?

This ECG is from a woman in her 30s who presented with vomiting, diarrhoea and cough for 2-3 days.

This ECG shows long QTc due to hypokalemia. She was found unresponsive in VF shortly after and required defibrillation.

In the ECGquest archives, this ECG has been tagged with: - 12-Lead Long QT ST elevation in aVR Tall U waves Widespread ST depression Hypokalemia Dr Smith's ECG Blog CC BY-NC
Diffuse ST depression, and ST elevation in aVR. Left main, right?

This ECG is from a middle aged male who presented with diffuse pain on a background of sickle cell disease.

This ECG shows LVH, diffuse ST depression with ST elevation in aVR and apparrent prolonged QTc due to U waves. The cause was hypokalemia (2.6).

In the ECGquest archives, this ECG has been tagged with: - Chest pain 12-Lead Widespread ST depression Hypokalemia Dr Smith's ECG Blog CC BY-NC
Chest pain and Diffuse ST depression, with STE in aVR. You probably think it is left main…..

This ECG is from a woman in her 70s who presented with very atypical chest pain lasting days that was worse with movement and palpation.

This ECG shows diffuse ST depression likely due to hypokalemia as it normalised after replacement.

In the ECGquest archives, this ECG has been tagged with: - 12-Lead Irregular Normal rate Tall U waves Wide QRS Atrial fibrillation Hypokalemia Dr Smith's ECG Blog CC BY-NC
Pre-existing Left Bundle Branch Block and Atrial Fib: what is alarming on this routine pre-procedure ECG?

This ECG is from an elderly patient who had a routine pre-procedure ECG.

This ECG shows atrial fibrillation with LBBB (old) and large U waves. The cause was hypokalemia.

In the ECGquest archives, this ECG has been tagged with: - Weakness 12-Lead Long QT T-U waves Tall U waves Hypokalemia Dr Smith's ECG Blog CC BY-NC
A young man with back spasms

This ECG is from a young man who presented with weakness and back pain on a background of an eating disorder and chronic kidney disease.

In the ECGquest archives, this ECG has been tagged with: - Weakness 12-Lead Long QT Tall U waves Hypokalemia Dr Smith's ECG Blog CC BY-NC
A pathognomonic ECG you should recognize instantly

This ECG is from a patient of unknown age who presented with generalised weakness and malaise.

In the ECGquest archives, this ECG has been tagged with: - Diarrhoea Vomiting 12-Lead Nonspecific baseline artefact Long QT Tall U waves Hypokalemia ECG of the Week CC-BY-NC-SA
ECG of the Week – 16th July 2018 – Interpretation

This ECG is from a woman in her 50s who presented with diarrhoea and vomiting on a background of paroxysmal AF and anxiety.

In the ECGquest archives, this ECG has been tagged with: - 12-Lead Tall U waves Hypokalemia Toxicology / Overdose Dr Smith's ECG Blog CC BY-NC
Insulin Overdose: Does this ECG help to Guide your Management?

This ECG is from a patient of unknown age who presented after an insulin overdose.