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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