Digoxin × Hydrochlorothiazide

The hydrochlorothiazide label gives digitalis glycosides their own entry: thiazide-induced hypokalaemia or hypomagnesaemia can favour digitalis-induced arrhythmia. The mechanism matches the one with loop diuretics – competition between digoxin and potassium at the Na+/K+-ATPase – and the digoxin label lists diuretics among agents that raise sensitivity to the glycoside. The thiazide label also asks for hypokalaemia and hypomagnesaemia to be corrected before treatment starts.

Nausea, vomiting, poor appetite, yellow-haloed vision, confusion. On ECG, ventricular ectopy, bradycardia and AV block. Muscle weakness and cramps point to low potassium in their own right, before any digitalis picture appears.

Potassium and magnesium before the thiazide starts and during follow-up. The label asks for hypokalaemia and hypomagnesaemia to be corrected first and for hydrochlorothiazide to be stopped if hypokalaemia comes with muscle weakness, paresis or ECG changes. In a patient on digoxin the potassium target stays above 4 mmol/L; potassium supplementation, a potassium-sparing agent or a change of diuretic all serve.