What is the diagnosis for a patient with a prolonged QT interval and sinus bradycardia on a 12‑lead electrocardiogram, blood pressure 103/60 mmHg, and heart rate 47‑53 bpm?

Long QT Syndrome (LQTS)

The diagnosis for a patient presenting with prolonged QT interval and sinus bradycardia on 12-lead ECG is most likely Long QT Syndrome (LQTS), which can be either congenital (cLQTS) or acquired (aLQTS), and requires immediate differentiation between these two etiologies to prevent life-threatening torsades de pointes and sudden cardiac death 1.

Diagnostic Framework

The combination of QT prolongation with sinus bradycardia is particularly characteristic of LQT3 (SCN5A mutation) and LQT4 (Ankyrin-B syndrome), as these are the only LQTS subtypes where both findings consistently occur together on a familial basis 2. The bradycardia in this patient (HR 47-53 bpm) is significant and not merely coincidental—it actually increases the risk of malignant arrhythmias by further prolonging ventricular repolarization 1.

Immediate Differential Considerations:

First, rule out acquired causes (aLQTS), as these are more common and immediately reversible:

• QT-prolonging medications (antidepressants, antipsychotics, antibiotics like moxifloxacin, antiarrhythmics)
• Electrolyte abnormalities (hypokalemia, hypomagnesemia, hypocalcemia)
• Severe bradycardia from AV block
• Thyroid dysfunction (hypo- or hyperthyroidism)
• Hypothermia
• Less common: Takotsubo cardiomyopathy, autoimmune conditions with anti-Ro/SSA antibodies 1

If no acquired causes are identified, suspect congenital LQTS, particularly given the sinus bradycardia which strongly suggests LQT3 2.

Critical Risk Assessment

The risk of torsades de pointes and sudden death increases dramatically when:

• QTc exceeds 500 ms
• QTc increases by >60-70 ms from baseline (especially if rapid)
• Female sex (well-established risk factor for aLQTS)
• Multiple risk factors coexist 1

This patient's hemodynamic stability (BP 103/60 mmHg) does not eliminate risk—many LQTS patients remain asymptomatic until a fatal arrhythmic event occurs 3.

Immediate Management Algorithm

Step 1: Verify QT Measurement

• Manually verify the QT interval—automated ECG measurements are frequently inaccurate, especially with abnormal baseline tracings 1
• Measure in leads II or V5/V6 where T-wave end is clearest
• Use Bazett's or Fridericia's formula for correction, but recognize limitations at extreme heart rates

Step 2: Identify and Remove Triggers (for aLQTS)

• Immediately discontinue all QT-prolonging medications
• Correct electrolyte abnormalities aggressively:
  • Target potassium >4.0 mEq/L (ideally 4.5-5.0)
  • Normalize magnesium and calcium
  • Consider supratherapeutic magnesium if high suspicion 1
• Check thyroid function

Step 3: Determine Congenital vs Acquired

For suspected cLQTS:

• Obtain detailed personal history: syncope, seizures (often misdiagnosed), cardiac arrest, exercise-related symptoms
• Obtain family history: sudden death <40 years, unexplained drowning, single-vehicle accidents, SIDS
• Perform exercise stress testing with 12-lead ECG at 1-minute intervals throughout exercise and recovery:
  • LQT1: QTc lengthens at peak exercise and remains prolonged in recovery
  • LQT2: QTc may normalize at peak exercise but becomes markedly prolonged in late recovery (>440 ms at 4 minutes post-exercise)
  • LQT3: minimal QT change with exercise 1
• Refer to inherited arrhythmia/cardiogenetic clinic (www.heartsinrhythm.ca in Canada, www.sads.org internationally) 1

Definitive Treatment Based on Etiology

For Congenital LQTS:

β-blockade is the cornerstone and dramatically reduces mortality 1:

• First-line: Nadolol (1-1.5 mg/kg once daily in adults) or propranolol—nonselective β-blockers provide superior protection compared to β-1 selective agents
• Avoid atenolol and metoprolol (inferior efficacy)
• Bisoprolol may be used in lower-risk patients (better tolerated, though β-1 selective)
• Start low dose and uptitrate gradually to minimize side effects (fatigue is common initially)

Critical caveat: β-blockers do NOT significantly shorten the absolute QT interval, especially at heart rates 60-100 bpm. Their benefit is through preventing adrenergic surges that trigger premature ventricular contractions leading to torsades de pointes 1.

Additional measures:

• Strict avoidance of QT-prolonging drugs (even with β-blockade)
• Maintain normal electrolytes
• Genetic testing for types 1-3 (strong evidence genes: KCNQ1, KCNH2, SCN5A) 1
• Cascade screening of first-degree relatives (genotype-based if mutation identified, clinical screening if genotype-negative proband)
• ICDs rarely required (only for high-risk patients with recurrent events despite β-blockade) 1

For Acquired LQTS:

• Remove offending agent(s)
• Correct electrolytes
• If marked bradycardia with aLQTS: consider temporary transvenous pacing or isoproterenol (NOT β-blockade, which would worsen bradycardia) 1

Management of Acute Torsades de Pointes

If torsades de pointes or arrhythmic storm occurs:

• For cLQTS: β-blockade, IV magnesium (even if levels normal), lidocaine, possibly transvenous pacing
• For aLQTS with bradycardia: transvenous pacing or isoproterenol to increase heart rate
• Correct electrolytes aggressively
• Stop all QT-prolonging drugs

NEVER use amiodarone or procainamide—these further prolong the QT interval and worsen bradycardia, potentially precipitating fatal arrhythmias 1.

Key Pitfalls to Avoid

1. Do not rely on automated QT measurements—verify manually 1
2. Do not dismiss bradycardia as benign—it synergistically increases arrhythmic risk with QT prolongation 1, 2
3. Do not assume hemodynamic stability equals low risk—many LQTS patients are asymptomatic until sudden death 3
4. Do not use β-1 selective agents (atenolol, metoprolol) for cLQTS—inferior protection 1
5. Do not treat LQTS-related torsades with amiodarone or procainamide—contraindicated 1
6. Do not overlook genetic testing and family screening—autosomal dominant inheritance means 50% risk to offspring 1