QT Prolongation by Phenothiazines and Related Antipsychotics

Drug-Specific Effects

• The American Academy of Pediatrics recommends that droperidol carries an FDA black box warning for QT prolongation, torsades de pointes, and sudden death, with a median QTc prolongation of 37, 44, and 59 msec at doses of 0.1, 0.175, and 0.25 mg/kg, respectively 1
• The European Heart Journal suggests that metoclopramide, while having limited specific QT prolongation data, shares dopamine antagonist properties and potential for similar hERG channel effects as other phenothiazines 3

Arrhythmogenic Consequences

• The American Heart Association, as published in Pediatrics, notes that QT intervals generally exceed 500 msec before development of torsades de pointes, a polymorphic ventricular tachycardia that can degenerate into ventricular fibrillation and sudden death 1
• The American College of Cardiology, as published in Circulation, states that the risk of torsades de pointes is not uniform and high-risk drugs like haloperidol and droperidol may increase risk in individuals with genetic mutations, while the absolute incidence remains very low 4

Critical Risk Factors That Amplify the Mechanism

• The European Heart Journal recommends that electrolyte abnormalities, particularly hypokalemia and hypomagnesemia, should be corrected before initiating therapy, as they further impair repolarization 3
• The American Academy of Pediatrics suggests that concomitant use of other QT-prolonging medications creates additive hERG blockade and increases the risk of torsades de pointes 1
• The American College of Cardiology, as published in Circulation, notes that female gender is a risk factor, as women have inherently longer QT intervals and higher risk of torsades de pointes 4

Important Clinical Caveats

• The American Heart Association, as published in Pediatrics, recommends that intravenous haloperidol carries significantly higher risk than oral or intramuscular administration, and intramuscular dosing is the preferred parenteral route for antipsychotics in emergency settings 1
• The American College of Cardiology, as published in Circulation, states that not all QT prolongation carries equal arrhythmic risk, and promethazine prolongs QTc but does not affect transmural dispersion of repolarization, suggesting low torsadogenic potential despite measurable QT changes 4

Monitoring Recommendations

• The American Academy of Pediatrics recommends obtaining 12-lead ECG prior to administration to identify baseline QTc >440 ms (males) or >450 ms (females), and correcting electrolyte abnormalities before initiating therapy, targeting potassium >4.5 mEq/L 1
• The European Heart Journal suggests that co-administration with enzyme inhibitors (macrolide antibiotics, imidazole antifungals, fluoxetine, fluvoxamine) can cause plasma levels to rise unduly, increasing QT prolongation risk 3