Makindo Medical Notes

Related Subjects: |ECG-QT interval |Brugada Syndrome |Long QT syndrome (LQTS) Acquired |Long QT syndrome (LQTS) Congenital |Torsade de Pointes |Ventricular Fibrillation |Ventricular Tachycardia |Resuscitation - Adult Tachycardia Algorithm |Automatic Implantable Cardioverter Defibrillator (AICD)

Consider the diagnosis of Long QT syndrome in any patient presenting with polymorphic ventricular tachycardia (VT) or syncope, especially focusing on drug and electrolyte causes. Torsades de Pointes may occur due to inhibition of the HERG-type potassium channel. QT prolongation is generally considered when the QTc interval exceeds 440 ms, with arrhythmias most commonly associated at values of 500 ms or more.

About

• Long QT syndrome is a cardiac condition that prolongs the QT interval, increasing the risk of arrhythmias such as Torsades de Pointes.
• An updated list of drugs that may cause QT prolongation can be found at www.qtdrugs.org.
• Over 50 medications are known to lengthen the QT interval, with this list growing as new drugs are developed.
• Associated risks include ventricular tachycardia (VT), syncope, cardiac arrest, and sudden death.

Torsades de Pointes

Aetiology

• The QT interval reflects ventricular repolarization, and prolonged QT creates a substrate for arrhythmias.
• Prolongation can lead to life-threatening arrhythmias such as polymorphic VT (Torsades de Pointes) and ventricular fibrillation (VF).
• May result from a subclinical genetic predisposition affecting cardiac tissue or metabolism of certain drugs.

Causes

• Medications: Includes antiarrhythmics (sotalol, amiodarone), antibiotics (macrolides, fluoroquinolones), antipsychotics (haloperidol, ziprasidone), among others.
• Electrolyte Imbalances: Hypokalemia, hypomagnesemia, and hypocalcemia can contribute significantly.
• Bradycardia: Particularly in combination with QT-prolonging drugs.
• Structural Heart Disease: Conditions such as myocardial infarction or heart failure may predispose to QT prolongation.

ECG Appearance

Medications Known to Lengthen QT Interval (Not Comprehensive)

• Antihistamines, antibiotics, antipsychotics, and drugs like cisapride.
• Examples: Terfenadine, Astemizole, IV erythromycin, clarithromycin, phenytoin, lithium, chlorpheniramine, IV prochlorperazine, tricyclic antidepressants, phenothiazines, halofantrine, sotalol, amiodarone, and chloroquine.
• Miscellaneous factors include acute myocardial infarction, starvation, bradycardia, myocarditis, hypertrophic cardiomyopathy (HCM), hypothermia, right ventricular dysplasia, and methadone use.

Clinical Presentation

• More common in women; important to review drug history.
• Can present as asymptomatic, syncope, cardiac arrest, sudden death, or seizures.
• Symptoms may occur with exertion, rest, or emotional events.
• Always inquire about a family history of sudden cardiac death (SCD).

Investigations

• Blood Tests: U&E, magnesium, calcium to check for electrolyte abnormalities.
• ECG: Long QTc interval. Normal QTc ranges between 0.35 and 0.46 seconds (350-460 ms). Values above 450 ms in men and 460 ms in women are considered prolonged.
• Check for arrhythmias: Torsades de Pointes or ventricular fibrillation.

Management

• Torsades de Pointes: IV magnesium sulfate is the treatment of choice, regardless of serum magnesium level. Administer 2g bolus followed by infusion if needed.
• Identify and Address Causes: Discontinue QT-prolonging drugs, correct electrolyte imbalances.
• Monitoring: QTc > 500 ms requires monitoring and cardiology consultation.
• Avoid using amiodarone or other QT-prolonging antiarrhythmics if possible, as they may worsen QT prolongation.
• Potassium and Magnesium Correction: Ensure normal levels to minimize arrhythmia risk. Potassium supplementation may reduce QT interval in some cases.
• Overdrive Pacing: Consider pacing or isoprenaline if torsades is bradycardia-dependent, as it prevents long RR intervals.
• Defibrillation: Recommended for unstable tachyarrhythmias.

References

• Acquired, Drug-Induced Long QT Syndrome
• Mechanisms, Risk Factors, and Management of Acquired Long QT Syndrome: A Comprehensive Review