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Toxicology – QT Interval Prolongation on ECG
Overview
The QT interval represents the total time for ventricular depolarization and repolarization, measured from the beginning of the Q wave to the end of the T wave. Because it varies with heart rate, the corrected QT (QTc) is used for accuracy. Normal QTc is generally less than 420 milliseconds. Prolongation occurs when potassium channel activity is impaired, delaying repolarization. When the QTc exceeds 500 milliseconds, there is a significant risk of torsades de pointes, a potentially life-threatening ventricular arrhythmia.
High-Risk Medications
Macrolide Antibiotics
Agents such as erythromycin and clarithromycin are well-known to prolong the QT interval.
Butyrophenones
Drugs like haloperidol, droperidol, and domperidone can significantly increase QT duration.
Class Ia Antiarrhythmics
Quinidine, procainamide, and disopyramide prolong cardiac repolarization and increase arrhythmia risk.
Class Ic Antiarrhythmics
Flecainide, propafenone, and moricizine can also contribute to QT prolongation.
Class III Antiarrhythmics
Medications such as amiodarone, sotalol, ibutilide, dofetilide, and dronedarone are strongly associated with QT prolongation.
Methadone
This opioid is a notable cause of QT prolongation and increases risk of torsades de pointes.
Moderate-Risk Medications
Fluoroquinolone Antibiotics
Ciprofloxacin, levofloxacin, moxifloxacin, and gatifloxacin may prolong the QT interval.
Selective Serotonin Reuptake Inhibitors (SSRIs)
Fluoxetine, paroxetine, sertraline, and particularly citalopram (and less commonly escitalopram) are associated with QT prolongation.
Atypical Antipsychotics
Quetiapine, risperidone, and ziprasidone may increase QT duration.
Other Agents
Azithromycin, lithium, octreotide, and tizanidine have also been implicated in QT prolongation.
Management
Intravenous magnesium sulfate is the treatment of choice. For QTc prolongation (>500 ms), 2 g IV may be administered and repeated every 6 hours until normalization. In cases of torsades de pointes, 2 g IV should be given rapidly (over about 60 seconds) and repeated every 5–15 minutes as needed. Continuous infusion may be required for persistent arrhythmias.
Mechanism of Treatment
Magnesium stabilizes cardiac membranes and suppresses abnormal electrical activity without significantly affecting heart rate or directly shortening the QT interval.
Key Points
Overview
The QT interval represents the total time for ventricular depolarization and repolarization, measured from the beginning of the Q wave to the end of the T wave. Because it varies with heart rate, the corrected QT (QTc) is used for accuracy. Normal QTc is generally less than 420 milliseconds. Prolongation occurs when potassium channel activity is impaired, delaying repolarization. When the QTc exceeds 500 milliseconds, there is a significant risk of torsades de pointes, a potentially life-threatening ventricular arrhythmia.
High-Risk Medications
Macrolide Antibiotics
Agents such as erythromycin and clarithromycin are well-known to prolong the QT interval.
Butyrophenones
Drugs like haloperidol, droperidol, and domperidone can significantly increase QT duration.
Class Ia Antiarrhythmics
Quinidine, procainamide, and disopyramide prolong cardiac repolarization and increase arrhythmia risk.
Class Ic Antiarrhythmics
Flecainide, propafenone, and moricizine can also contribute to QT prolongation.
Class III Antiarrhythmics
Medications such as amiodarone, sotalol, ibutilide, dofetilide, and dronedarone are strongly associated with QT prolongation.
Methadone
This opioid is a notable cause of QT prolongation and increases risk of torsades de pointes.
Moderate-Risk Medications
Fluoroquinolone Antibiotics
Ciprofloxacin, levofloxacin, moxifloxacin, and gatifloxacin may prolong the QT interval.
Selective Serotonin Reuptake Inhibitors (SSRIs)
Fluoxetine, paroxetine, sertraline, and particularly citalopram (and less commonly escitalopram) are associated with QT prolongation.
Atypical Antipsychotics
Quetiapine, risperidone, and ziprasidone may increase QT duration.
Other Agents
Azithromycin, lithium, octreotide, and tizanidine have also been implicated in QT prolongation.
Management
Intravenous magnesium sulfate is the treatment of choice. For QTc prolongation (>500 ms), 2 g IV may be administered and repeated every 6 hours until normalization. In cases of torsades de pointes, 2 g IV should be given rapidly (over about 60 seconds) and repeated every 5–15 minutes as needed. Continuous infusion may be required for persistent arrhythmias.
Mechanism of Treatment
Magnesium stabilizes cardiac membranes and suppresses abnormal electrical activity without significantly affecting heart rate or directly shortening the QT interval.
Key Points
- QT prolongation increases the risk of torsades de pointes, especially when QTc exceeds 500 ms.
- Among SSRIs, citalopram is most strongly associated with QT prolongation.
- Octreotide has also been recognized as a potential contributor to QT interval prolongation.
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