Hypokalemia ECG progression: from flattened T waves to torsades risk
Hypokalemia produces characteristic ECG changes that correlate with increasing potassium deficiency. The progression: (1) T-wave flattening: the earliest finding. T waves flatten or invert as repolarization is affected. (2) Prominent U waves: a U wave is a positive deflection after the T wave, best seen in leads V2–V4. Normally small or absent, hypokalemia produces prominent U waves that may exceed the T-wave amplitude. (3) T-U fusion: the T wave and U wave merge, creating an apparent single broad 'T wave' — this is actually a QU interval, not QT. Measuring this as the QT interval falsely identifies QT prolongation. (4) ST depression: mild baseline ST depression may accompany hypokalemia. (5) QU prolongation and torsades de pointes: true torsades de pointes risk exists even when the measured 'QTc' reflects QU fusion rather than true QT.
Hypokalemia clinical significance: arrhythmia risk and replacement priorities
Hypokalemia potentiates arrhythmias from two mechanisms: (1) increased automaticity (ectopic beats, atrial and ventricular arrhythmias), and (2) prolonged repolarization (QU prolongation increases torsades risk). The combination of hypokalemia + QT-prolonging medications (sotalol, amiodarone, azithromycin, haloperidol) is particularly dangerous.
Nursing priorities: Monitor potassium in all patients on diuretics (especially loop diuretics and thiazides), with diarrhea/vomiting, or receiving QT-prolonging medications. Target serum K+ ≥ 4.0 mEq/L in patients with arrhythmia risk or QT-prolonging medications. IV potassium replacement: maximum 10–20 mEq/hr via peripheral IV (concentrations > 20 mEq/100 mL require central access and continuous cardiac monitoring).
Hypokalemia + hypomagnesemia: magnesium is required for intracellular potassium retention. Replacing potassium without correcting magnesium fails — the K+ is lost again immediately. Always check and replace magnesium concurrently in hypokalemic patients.