Q: IV magnesium is usually not indicated when hyperkalemia exists except in one condition?

Answer: Magnesium can potentiate myocardial conduction abnormalities and therefore is probably contraindicated in patients with hyperkalaemia. However, in hyperkalaemia  associated with digoxin poisoning, intravenous magnesium sulphate may eliminate refractory ventricular tachycardia and decrease the serum potassium.