Name two electrolyte disturbances that predispose to torsades de pointes.

• A. Hypomagnesemia and hypokalemia
• B. Hypermagnesemia and hyperkalemia
• C. Hyponatremia and hypercalcemia
• D. Hypernatremia and hypocalcemia

Answer: (A)

Torsades de pointes arises when the QT interval is prolonged, creating a substrate for early afterdepolarizations that can trigger a polymorphic ventricular tachycardia. The two electrolyte disturbances that most reliably tilt the balance toward this situation are low magnesium and low potassium.

Low magnesium disrupts the normal regulation of cardiac ion channels and membrane stability, increasing susceptibility to afterdepolarizations and prolonging repolarization, especially when other QT-prolonging factors are present. Magnesium sulfate is a well-known treatment for torsades because it helps stabilize the myocardium and suppress these abnormal depolarizations.

Low potassium slows repolarization by reducing outward potassium currents during the action potential, which lengthens the QT interval and favors afterdepolarizations. Together, hypomagnesemia and hypokalemia create the classic scenario for torsades de pointes.

While hypocalcemia can also prolong the QT, the strongest and most textbook associations with torsades are low magnesium and low potassium; hypermagnesemia or hyperkalemia tend not to predispose to this arrhythmia.