A 17-year-old girl collapses during swim practice, her wet hair plastered to her cheeks and teammates shouting from the pool deck. She wakes within seconds but is pale, nauseated, and embarrassed, insisting she “just stood up too fast.” Her ECG shows a sinus rhythm with a markedly prolonged repolarization interval; the monitor intermittently displays premature ventricular beats. Her mother mentions that a cousin died suddenly at 19, but the next move has not yet been made.

— What’s your move? Read on.

Before you read
  • What is the first treatment for polymorphic ventricular tachycardia in congenital long QT?
  • Which common ED medications can worsen the problem?

When to Think of It

Think of congenital long QT with unexplained syncope, seizure-like activity, palpitations, or sudden cardiac arrest—especially during exertion, swimming, emotional stress, or sudden auditory stimulation. Look for QTc prolongation, torsades, a family history of unexplained drowning or sudden death, and recurrent “seizures” with rapid recovery.

Sick or Not Sick

Sick vs. not sick: Is the patient having ventricular ectopy/torsades, hemodynamic instability, or recurrent syncope now? The key call is whether there is active electrical instability requiring immediate magnesium and defibrillation, rather than simply a prolonged QT in a stable patient.

The First Fifteen Minutes

  • Place on a defibrillator-capable monitor, obtain IV/IO access, repeat a 12-lead ECG, and check glucose, potassium, magnesium, calcium, and renal function.
  • QTc markedly prolonged with ventricular ectopy or torsades → magnesium sulfate 2 g IV over 1–2 minutes in adults; 25–50 mg/kg IV/IO in children, maximum 2 g. Magnesium suppresses early afterdepolarizations even when the serum magnesium is normal.
  • Pulseless or unstable polymorphic VT → immediate unsynchronized defibrillation: 200 J biphasic in adults; 2 J/kg in children, escalating to 4 J/kg. Electrical energy terminates the immediately lethal rhythm.
  • Hypokalemia → potassium chloride 10–20 mEq IV over at least 1 hour, with continuous ECG monitoring; use slower rates unless in a life-threatening arrhythmia. Raising potassium shortens repolarization and reduces triggered activity; check institutional maximum infusion rates.
  • Bradycardia-triggered recurrent torsades despite magnesium and electrolyte correction → urgent cardiology consultation for temporary overdrive pacing or isoproterenol only if the mechanism is acquired pause-dependent torsades. Isoproterenol can worsen congenital LQTS and should generally be avoided in congenital disease.
  • Stop QT-prolonging medications immediately; use a QT-risk reference such as CredibleMeds or the institutional formulary before prescribing.

Definitive Care & Disposition

Admit anyone with syncope plus significant QT prolongation, ventricular ectopy, torsades, cardiac arrest, or concerning family history to a monitored setting with pediatric/adult electrophysiology involvement as appropriate. Long-term therapy usually includes a nonselective beta-blocker—typically nadolol or propranolol—genetic counseling and family screening, avoidance of QT-prolonging drugs and electrolyte depletion, and an implantable cardioverter-defibrillator for selected survivors of cardiac arrest or patients with recurrent events despite therapy. Exercise and swimming restrictions depend on genotype, phenotype, and specialist assessment.

How This One Kills

The fatal error is labeling exertional syncope as vasovagal or epilepsy, discharging the patient, and then prescribing a QT-prolonging antiemetic, macrolide, fluoroquinolone, or psychotropic drug. A normal ECG between episodes does not exclude the syndrome.
The Differential — What Else Looks Like This
  • Vasovagal syncope — prodrome with prolonged standing and no exertional/family-risk pattern; confusing it with LQTS can miss sudden-death risk.
  • Catecholaminergic polymorphic VT — bidirectional or polymorphic VT during exercise with a usually normal resting QT; confusing them leads to the wrong genetic and medication strategy.
  • Brugada syndrome — coved ST elevation in V1–V3 and fever-triggered ventricular arrhythmia; missing it may lead to inappropriate QT-focused counseling.
  • Epilepsy — prolonged postictal confusion and stereotyped neurologic features; misdiagnosis delays cardiac prevention and exposes the patient to unnecessary antiseizure therapy.

The Second-Day Story

Older adults may present with an unexplained fall, palpitations, or a remote “seizure” history, while a partially treated patient may have a less dramatic QTc after electrolyte correction. The ECG can be borderline, especially when the heart rate is abnormal or the T-wave end is difficult to identify. Recalculate QTc manually, review prior ECGs and medications, and ask specifically about sudden death, drowning, exertional collapse, and family members labeled as epileptic.
Back to Our Patient
Back to the 17-year-old swimmer: her exertional collapse, family history of sudden death, prolonged QTc, and ventricular ectopy make congenital long QT with active electrical instability the leading diagnosis. She is placed on continuous defibrillator-capable monitoring, IV access is obtained, and electrolytes are urgently measured and corrected; if torsades appears or she becomes unstable, she receives IV magnesium and immediate unsynchronized defibrillation as indicated. She is not discharged after transient recovery: pediatric cardiology/electrophysiology evaluates her for beta-blockade, genetic testing, family screening, activity counseling, and possible ICD therapy, with admission to a monitored unit.
Patient Presentation to Attending
How you’d present this patient on the floor — tight, pertinent positives and negatives, no rambling
“This is a 17-year-old previously healthy girl with exertional syncope during swim practice, now awake but pale and nauseated. She had abrupt collapse with rapid return to baseline, no prolonged postictal confusion, no preceding illness, and a maternal-side cousin who died suddenly at 19. Her ECG shows marked QTc prolongation with ventricular ectopy; she is currently perfusing but remains high risk for torsades. I’m placing her on a defibrillator-capable monitor, obtaining IV access and electrolytes, stopping QT-prolonging drugs, and giving magnesium immediately if ectopy progresses or torsades occurs, with defibrillation for instability. I’m consulting pediatric electrophysiology and admitting her to a monitored setting for congenital long QT evaluation and prevention.”

Study Directive

  • Manually calculate QTc on three ECGs using Bazett and identify when Bazett is unreliable at extreme heart rates.
  • Memorize the emergency sequence: monitor → magnesium → correct potassium/calcium → defibrillate unstable torsades → electrophysiology.
  • Build a personal list of 10 common QT-prolonging ED medications using CredibleMeds or your institutional formulary.
  • Review genotype-specific triggers for LQT1, LQT2, and LQT3 and explain why swimming is particularly high risk in LQT1.
  • Practice a 30-second oral presentation for exertional syncope with a prolonged QTc.

Recent Literature