Drugs Causing Arrhythmia
Multiple drug classes can cause cardiac arrhythmias, with antiarrhythmic agents carrying the highest risk (1-10% incidence of Torsade de Pointes), followed by psychotropic medications, antimicrobials, and anticancer drugs, all primarily through QT interval prolongation and IKr channel blockade. 1
High-Risk Drug Categories
Antiarrhythmic Agents (Highest Risk)
The antiarrhythmic drugs represent the highest-risk category with TdP incidence of 1-10% 1:
Class IA agents:
- Quinidine (unique: causes TdP at low concentrations due to potent IKr blockade)
- Disopyramide
- Procainamide
Class III agents:
- Sotalol
- Dofetilide
- Ibutilide
These drugs block sodium and/or potassium channels, directly prolonging the QT interval 1.
Psychotropic Medications
Antipsychotics (both typical and atypical) are strongly associated with arrhythmias 2:
- Haloperidol - high-profile risk for TdP
- Thioridazine - high-profile risk for TdP
- Tricyclic antidepressants (TCAs) prolong QT and delay AV-node conduction, causing AV block
- Amitriptyline and maprotiline have documented TdP cases
- TCA use associated with 69% increased risk of cardiac arrest (OR=1.69) 2
Antimicrobial Agents
Macrolide antibiotics and antifungals pose significant risk 1:
- Erythromycin - especially with IV route or high doses, often with other risk factors
- Ketoconazole - potent CYP3A4 inhibitor that increases levels of other QT-prolonging drugs
Opioids
- Methadone - high-profile example of non-antiarrhythmic drug causing TdP 1
Anticancer Drugs
Anthracyclines carry substantial arrhythmic risk 3:
- Associated with 90% increased risk of arrhythmia (OR=1.90)
- 114% increased risk of supraventricular arrhythmia (OR=2.14)
- Epirubicin ranks highest probability for arrhythmia among anthracyclines (OR=43.07)
- Overall arrhythmia incidence: 20% (95% CI: 15-25%)
Other High-Risk Agents
- Droperidol (antiemetic)
- Terfenadine (antihistamine, now withdrawn) - risk primarily with CYP3A4 inhibition
Mechanism of Drug-Induced Arrhythmias
The primary mechanism is dose-dependent inhibition of IKr (rapid delayed rectifier potassium current) through hERG channel blockade 2, 4. This causes:
- QT interval prolongation
- Increased spatial and temporal dispersion of repolarization
- Triangular action potential configuration
- Substrate for Torsade de Pointes
Critical caveat: QT prolongation is NOT equivalent to arrhythmogenicity - it serves as a proxy marker but doesn't guarantee TdP will occur 1.
Risk Stratification Algorithm
High-risk drugs (require other risk factors less often):
- Antiarrhythmic agents
- Methadone
- Haloperidol
Low-risk drugs (typically require additional risk factors):
- Moxifloxacin
- Most other antimicrobials
Dose-Response Relationship
For virtually all QT-prolonging drugs, risk increases with plasma drug concentration 1, with the notable exception of quinidine (paradoxically causes TdP at low concentrations).
Drug Interactions
CYP3A4 inhibition dramatically increases risk by elevating plasma levels of substrate drugs 1:
- Erythromycin + terfenadine
- Ketoconazole + terfenadine
- Advanced liver disease impairing metabolism
Beyond QT Prolongation
Drugs can trigger other arrhythmias 5:
- Bradyarrhythmias (TCAs causing AV block)
- Atrial fibrillation/flutter
- Atrial tachycardia
- AV nodal reentrant tachycardia
- Monomorphic ventricular tachycardia
- Brugada syndrome
Clinical Decision Framework
When alternatives exist with equivalent efficacy, choose the non-QT-prolonging agent 1. However, where benefit clearly outweighs risk, QT prolongation should not limit necessary therapy.
For patients requiring high-risk drugs:
- Check baseline ECG for QTc
- Monitor electrolytes (potassium, magnesium)
- Review all concomitant medications for interactions
- Consider enhanced ECG monitoring during initiation
- Cardiac rhythm monitoring is mandatory for anthracyclines 3
Common Pitfall
The absolute incidence of TdP with many non-antiarrhythmic drugs is exceedingly small (terfenadine: nearly 100 million prescriptions before small risk recognized) 1, but risk is concentrated in patients with specific risk factors - particularly drug interactions and electrolyte abnormalities.