Topic Reviews A-Z
QT Interval
The QT interval is the time from the beginning of the QRS complex (ventricular depolarization) to the end of the T wave (ventricular repolarization).
The normal QT interval is controversial and multiple normal durations have been reported. In general, the normal QT interval is less than 400-440 ms (0.4 to 0.44 seconds). Females have a longer QT interval than males and lower heart rates result in a longer QT interval.
Due to the heart rates effects, the corrected QT interval (QTc) is frequently used. The QTc is considered prolonged if greater than 450 ms in males and 470 in females and is calculated using the Bazett’s formula as below:
Prolongation of the QT interval can occur from multiple medications, electrolyte abnormalities (hypocalcemia, hypomagnesemia and hypokalemia) and certain disease states (i.e. intracranial hemorrhage). The serious complication of QT interval prolongation is polymorphic ventricular tachycardia (Torsades de Pointes) which can be life threatening. Treatment is removing the cause. Polymorphic ventricular tachycardia is best treated with intravenous magnesium.
Quinidine, disopyramide, procainamide, sotalol, dofetilide, ibutilide and amiodarone can potentially prolong the QT interval.
The antiarrhythmic drugs that do not prolong the QT interval include propafenone, flecainide, lidocaine and mexiletine.