Expansion
Hyperkalaemia gives tall T waves and a wide QRS; hypokalaemia gives flat T waves and U waves
Expansion
Hyperkalaemia raises (makes less negative) the resting membrane potential, inactivating sodium channels and slowing conduction. Progressive changes:
- Tall, tented T waves
- Flattened or absent P waves, prolonged PR
- Widened QRS
- Sine wave pattern, then asystole or ventricular fibrillation
Hypokalaemia hyperpolarises the cell and prolongs repolarisation:
- Flattened T waves
- ST depression
- Prominent U waves
- Prolonged QT and a risk of torsades de pointes
Calcium is given in hyperkalaemia because it restores the gap between the resting and threshold potentials, stabilising the membrane without altering the potassium level, which is why insulin and dextrose or salbutamol must follow to shift potassium into cells.