Cardiac Arrhythmia Management

Patient Risk Factors

• Pre-existing conditions such as QT prolongation, female sex, age >65 years, electrolyte abnormalities, concurrent use of other QT-prolonging medications, bradycardia, heart failure, and recent conversion from atrial fibrillation are risk factors for QT prolongation, as noted by the European Heart Journal 1
• Female sex, advanced age (>65 years), underlying heart disease, bradyarrhythmias, electrolyte abnormalities, concurrent use of other QT-prolonging medications, and hepatic dysfunction are risk factors that compound QTc prolongation risk, as suggested by the American Heart Association 2

QT Interval Measurement and Interpretation

• The baseline QTc interval should be measured using the lead with the most well-defined T-wave end, and applying an appropriate correction formula, such as Bazett or Fridericia, as suggested by the American College of Cardiology 2
• Normal QTc interval is defined as <430 ms (males), <450 ms (females), with grade 1 prolongation: 450-480 ms, grade 2 prolongation: 481-500 ms, grade 3 prolongation: >501 ms, and grade 4 prolongation: ≥501 ms or >60 ms change from baseline with torsades de pointes or sudden death, as defined by the American Heart Association and the American College of Cardiology 2, 3
• A QTc interval of ≥500 ms is associated with a 2-3 fold higher risk for Torsades de Pointes, according to the American Heart Association 2

Medications that Prolong the QT Interval

Treatment Recommendations

• The American College of Cardiology, American Heart Association, and European Society of Cardiology guidelines recommend removal of offending agents in drug-induced long QT syndrome (Class I recommendation) 1
• If QT prolongation worsens or torsades de pointes develops, discontinue all QT-prolonging medications immediately, as recommended by the European Respiratory Society 6
• Administer intravenous magnesium sulfate (2g IV) as first-line therapy for torsades de pointes, according to the American Heart Association guidelines 6
• Maintain potassium in high-normal range (4.5-5 mmol/L) to prevent torsades de pointes, as suggested by the European Society of Cardiology 6
• Immediate electrical cardioversion is recommended for hemodynamic instability in patients with torsades de pointes, according to the American Heart Association, as published in Circulation 7
• Isoproterenol (30-150 ng/kg/min) may be considered for acquired long QT with bradycardia, but should be avoided in congenital LQTS, as recommended by the European Heart Journal and the American Heart Association, as published in Circulation 1, 7

Monitoring and Prevention

• Obtaining baseline ECG and checking electrolytes (particularly potassium and magnesium) before starting QT-prolonging medications, and monitoring ECG at 2 weeks, then monthly thereafter, with additional ECG after adding any new QT-prolonging medication is recommended by the American College of Cardiology and the American Heart Association 2, 8, 9, 6
• Ensuring magnesium levels are above 1.8 mg/dL is crucial, according to the American College of Cardiology 10
• The National Institute of Health and other medical organizations advise reviewing concurrent medications for potential QT-prolonging drug interactions to minimize the risk of QT prolongation when using ondansetron, as highlighted in the Cancer Treatment Reviews 5
• The FDA and other regulatory agencies have warned about the risks of combining ondansetron with other QT-prolonging medications, such as hydroxychloroquine and azithromycin combinations, as reported in the Cancer Treatment Reviews 5 and Anaesthesia 11

Antiemetic Therapy

• The American College of Cardiology recommends avoiding ondansetron in patients with a QTc interval of ≥500 ms, and considering alternative antiemetics 2
• First-line options for antiemetic therapy without QT prolongation risk include scopolamine, meclizine, dimenhydrinate, diphenhydramine, trimethobenzamide, and aprepitant, as recommended by the American Gastroenterological Association 12
• Dopamine antagonists such as metoclopramide, haloperidol, droperidol, prochlorperazine, and chlorpromazine are known QT-prolonging antiemetics to avoid due to the risk of potentially fatal arrhythmias like Torsades de Pointes, according to the American Heart Association 13
• 5-HT3 antagonists like ondansetron and granisetron should also be avoided due to their QT-prolonging effects, as suggested by the American Gastroenterological Association 12