Question
Which of the following does NOT cause prolongation of QT interval?
| A. | Hypomagnesaemia |
| B. |
Hyperkalaemia |
| C. |
Erythromycin |
| D. |
Amiodarone |
Show Answer
|
Correct Answer � B Explanation |
 |
The QT interval represents the time from ventricular depolarization to repolarization on an ECG. Prolonged QT interval increases the risk of torsades de pointes (a life-threatening ventricular arrhythmia).
- Hyperkalemia shortens the QT interval, not prolongs it.
- It accelerates ventricular repolarization, leading to:
- Peaked T waves (early sign).
- QRS widening and sine-wave pattern in severe cases.
- Bradycardia or asystole in extreme hyperkalemia.
Why Other Options Cause QT Prolongation:
A. Hypomagnesemia
- Low magnesium levels impair repolarization, leading to QT prolongation.
- Hypomagnesemia is a major risk factor for torsades de pointes.
- Often associated with hypokalemia, which worsens the effect.
C. Erythromycin
- A macrolide antibiotic that blocks potassium channels, delaying repolarization and prolonging the QT interval.
- Can increase the risk of torsades de pointes, especially when combined with other QT-prolonging drugs.
D. Amiodarone
- A class III antiarrhythmic drug that prolongs the QT interval by blocking potassium channels.
- However, unlike other QT-prolonging drugs, amiodarone has a lower risk of causing torsades de pointes due to its rate-dependent effects.