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.
- Hampton, J and Hampton, J (2019) - The ECG Made Easy, 9th edn, Elsevier
- Rowlands, A and Sargent, A (2019) - The ECG Workbook, 4th edn, M&K Publishing
- Thaler, MS (2018) - The Only EKG Book You'll Ever Need, 9th edn, Lippincott Williams and Wilkins
- Grauer, K (2014) - ECG 2014 Pocket Brain, 6th edn, KG/EKG Press
- Houghton, AR and Gray, D (2014) - Making Sense of the ECG: A Hands-on Guide, 4th edn, Taylor and Francis
- Wagner, GS and Strauss, DG (2013) - Marriott's Practical Electrocardiography, 12th edn, LWW
- Goldberger, A (2006) - Clinical Electrocardiography: A Simplified Approach, 9th edn, Elsevier
Groups of Signs
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).
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.
Where did they come from?
These ECGs were collected from Free Open Access Medical Education (#FOAMed) blogs, with the permission of their authors. You can find out more about each ECG's source by clicking on it.
Why are they here?
This is an experiment in digital curation. The idea is to collect resources to increase awareness and accessibility. Over time, more ECGs in the collection will be tagged to make it easier to find them and reused in new interactive quizzes.
How can I use these ECGs?
You can use these ECGs for your own learning, teaching or research - as long as you abide by the terms of each ECG's copyright licence as stipulated by the original author.