Cardiology · Arrhythmias
The facts most likely to be tested
Corrected QT interval (QTc) prolongation is defined as >440 ms in men and >460 ms in women, significantly increasing the risk of Torsades de Pointes.
Torsades de Pointes is a polymorphic ventricular tachycardia characterized by a twisting appearance of the QRS complexes around the isoelectric line.
The most common electrolyte disturbances causing QT prolongation are hypokalemia, hypomagnesemia, and hypocalcemia.
First-line treatment for hemodynamically unstable Torsades de Pointes is immediate unsynchronized defibrillation.
First-line treatment for hemodynamically stable Torsades de Pointes is intravenous magnesium sulfate.
Common offending medications include macrolides, fluoroquinolones, ondansetron, and antipsychotics like haloperidol.
Congenital Long QT Syndrome (e.g., Romano-Ward syndrome) is caused by mutations in cardiac ion channels and often presents with syncope or sudden cardiac death in young patients.
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A 68-year-old woman is brought to the emergency department after a syncopal episode. She has a history of depression and was recently started on levofloxacin for a urinary tract infection. Physical examination reveals a heart rate of 92 bpm and blood pressure of 110/70 mmHg. An ECG shows a prolonged QT interval and a polymorphic ventricular tachycardia with a twisting morphology. Her serum potassium is 3.4 mEq/L and magnesium is 1.6 mg/dL.
What is the most appropriate initial pharmacologic intervention for this patient?
Intravenous magnesium sulfate
The patient is presenting with stable Torsades de Pointes secondary to drug-induced QT prolongation and electrolyte abnormalities; intravenous magnesium is the treatment of choice to stabilize the cardiac membrane.
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Etiology / Epidemiology
Caused by electrolyte imbalances (hypokalemia, hypomagnesemia) or medication toxicity. Congenital forms involve Long QT Syndrome.
Clinical Manifestations
Often asymptomatic until syncope or Torsades de Pointes occurs. Sudden cardiac arrest is a common presentation.
Diagnosis
Diagnosed via 12-lead ECG with a corrected QT (QTc) > 470ms (men) or > 480ms (women).
Treatment
Discontinue offending agents; administer IV Magnesium Sulfate for acute arrhythmias. Avoid Class IA, IC, and III antiarrhythmics.
Prognosis
Risk of ventricular fibrillation and sudden cardiac death. Requires lifelong monitoring of QTc interval.
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Epidemiology & Etiology
Acquired cases are frequently secondary to polypharmacy or metabolic derangements. Congenital Long QT Syndrome is often linked to mutations in cardiac ion channels. Female gender and advanced age are independent risk factors for drug-induced prolongation.
Pertinent Anatomy
The QT interval represents the total time for ventricular depolarization and repolarization. Prolongation occurs primarily due to delayed potassium channel efflux during phase 3 of the action potential.
Pathophysiology
Delayed repolarization creates a window of early afterdepolarizations (EADs). These EADs trigger Torsades de Pointes, a polymorphic ventricular tachycardia characterized by a twisting of the QRS complex around the isoelectric line. If sustained, this rhythm degenerates into ventricular fibrillation.
Clinical Manifestations
Patients may present with palpitations, lightheadedness, or unexplained syncope. The most feared complication is Torsades de Pointes, which manifests as sudden cardiac arrest or seizure-like activity due to cerebral hypoperfusion. Always assess for a family history of sudden unexplained death.
Diagnosis
The 12-lead ECG is the diagnostic standard. Calculate the QTc using the Bazett formula. Diagnostic thresholds for high risk are QTc > 500ms, which significantly increases the risk of malignant arrhythmias.
Treatment
Immediate management of Torsades de Pointes requires IV Magnesium Sulfate even if serum levels are normal. Remove all QT-prolonging drugs immediately. Avoid Class IA (procainamide), IC (flecainide), and III (sotalol, amiodarone) antiarrhythmics as they exacerbate the condition.
Prognosis
Patients are at high risk for recurrent syncope and sudden death. Long-term management may require beta-blockers or an implantable cardioverter-defibrillator (ICD) in high-risk congenital cases.
Differential Diagnosis
Brugada Syndrome: ST-segment elevation in V1-V3
Hypertrophic Cardiomyopathy: systolic murmur increasing with Valsalva
Wolff-Parkinson-White: delta wave on ECG
Catecholaminergic Polymorphic VT: exercise-induced syncope
Vasovagal Syncope: prodromal symptoms with identifiable trigger