Metabolic and drug effects
Hypokalaemia
Flattened T waves with prominent U waves and a long apparent QT — a substrate for torsades.
82 bpmSinus
What you see
- T wave flattening and then inversion.
- Prominent U waves, best seen in V2–V3, sometimes exceeding the T wave in height.
- ST depression.
- An apparently long QT which is really a fused QU interval.
- Increasing atrial and ventricular ectopy as the level falls.
Why it looks like that
A low extracellular potassium hyperpolarises the cell and prolongs repolarisation, separating ventricular from Purkinje repolarisation. That separation is what makes the U wave visible, and the dispersion of refractoriness is what makes the heart irritable.
What to do
- Replace potassium, and check magnesium at the same time — hypomagnesaemia makes potassium impossible to correct.
- IV replacement with cardiac monitoring if there are ECG changes or the level is below 2.5 mmol/l.
- Look for the cause: diuretics, GI losses, hyperaldosteronism, renal tubular acidosis, refeeding.
The trap
Potassium will not come up while the magnesium is low, no matter how much you give — magnesium is a cofactor for the Na⁺/K⁺-ATPase and low levels increase renal potassium wasting. In a patient with recurrent unexplained hypokalaemia, check the magnesium before escalating replacement.
U wavemagnesiumQU intervaltorsades risk