QT prolongation

Background

• Prolonged ventricular repolarisation → increased risk of ventricular arrythmias
• QT interval is from the beginning of the Q wave to the end of the T wave; it is rate dependent and should become proportionately small with increasing rate rate
• An abnormal QT is >440-450 ms (males) and >460-470 ms (females); >500 may result in torsades

Clinical Features

• Most are asymptomatic
• History may or may not include
  • Syncope, cardiac arrest, family history of long QT or sudden death
  • Medication history should always be obtained especially so to avoid interactions and further QT prolongation.

Differential Diagnosis

• Pause Dependent (Aquired)
  • Drug induced
    • Antidyrhythmics
    • Phenothiazines
    • TCAs
    • Organophosphates
    • Antihistamines
  • Electrolyte Abnormalities (hypoKalemia, hypoMag, hypoCa)
    • Hypokalemia triad
      • Long QT, ST depressions, PVCs
  • Hypothermia
  • Diet related (starvation, low protein)
  • Severe Bradycardia/AV Block
  • Hypothyroid
  • Contrast injection
  • CVA (intraparenchymal)
  • Elevated intracranial pressure and Intracranial hemorrhage
  • MI
• Adrenergic Dependent
  • Congenital
    • Jarvel/Lange-Nielsen
      • (+deafness; AR)
    • Romano-Ward synd
      • (nl hearing; AD)
    • Sporatic
    • Mitral valve prolapse
  • Acquired
    • CVA (subarachnoid)
    • Autonomic surg (catechol excess: neck dissection, carotid endarterect, truncal vagotomy)

Drug List

• Antiarrhythmics
  • Amiodarone, disopyramide, dofetilide, flecainide, ibutilide, mexiletine, procainamide, quinidine, sotalol
• Antibiotics
  • Macrolide
    • Azithromycin, erythromycin, clarithromycin
  • Fluoroquinolone
    • Ciprofloxacin, gatifloxacin (most common), gemifloxacin, levofloxacin, moxifloxacin, ofloxacin
  • Other
    • Pentamidine, telithromycin, trimethoprim-sulfamethoxazole
• Antidepressants
  • Amitriptyline, citalopram, doxepin, fluoxetine, nortriptyline, paroxetine, sertraline, venlafaxine
• Antiemetics
  • Dolasetron, droperidol, granisetron, ondansetron
• Antifungals
  • Fluconazole, itraconazole, ketoconazole, voriconazole
• Antihypertensives
  • Nicardipine
• Antineoplastics
  • Lapatinib, nilotinib, sunitinib, tamoxifen
• Antimalarials
  • Chloroquine, halofantrine
• Antipsychotics
  • Chlorpromazine, clozapine, galantamine, haloperidol, lithium, paliperidone, pimozide, quetiapine, risperidone, thioridazine, ziprasidone
• Antivirals
  • Amantadine, atazanavir, foscarnet
• Diuretics
  • Indapamide
• Immune suppressants
  • Tacrolimus
• Opiates
  • Methadone
• Phosphodiesterase inhibitors
  • Sildenafil, vardenafil
• Skeletal muscle relaxants
  • Tizanidine
• Urinary antispasmodics
  • Solifenacin

Evaluation

• ECG
  • quick/imprecise measure: QT takes up more than half the R-R distance
  • Measure QT interval in lead II or V5-6
  • QTc = QT /√R-R
  • Long QT: QTc >440 (male), >460 (female)
  • >500 = real concern (may result in torsades)

Management

Pause Dependent (precipitated by bradycardia)

• Unstable/sustained torsades→ defibrilation (unsynchronized)
• Stable
  • Treat underlying prob
  • Increase HR (isoproterenol or overdrive pacing)
  • Magnesium sulfate IV
  • Consider amiodarone

Adrenergic Dependent (precipited by tachycardia)

• Unstable/sustained torsades→ defibrilation (unsynchronized)
• Stable
  • Slow HR (beta-blockers)
  • May consider magnesium sulfate

Disposition

• Highly consider admission, especially for QT >500

See Also

• ECG (Main)
• Torsades de Pointes

External Links

• Amal Mattu's Case of the Week (5/20/2012)
• Amal Mattu's Case of the Week (10/13/2014)