QT prolongation

Background

• Prolonged ventricular repolarisation → increased risk of ventricular arrythmias
  • Males >440-450 ms
  • Females >500 ms
  • Rule of thumb: Normal QT interveal is less than half of preceding RR interval
• QT interval is from the beginning of the Q wave to the end of the T wave
  • Rate dependent and should become proportionately shorter with increasing heart rate

Clinical Features

• Most are asymptomatic
• History may include:
  • Syncope
  • Cardiac arrest
  • Family history of long QT or sudden death
• Medication history may include QT prolonging medications

Differential Diagnosis

• Pause Dependent (Acquired)
  • Drug induced
    • Antiarrhythmics
    • 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 syndrome
      • (normal hearing; AD)
    • Sporadic
    • Mitral valve prolapse
  • Acquired
    • CVA (subarachnoid)
    • Autonomic surgery (catechol excess: neck dissection, carotid endarterectomy, 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

Workup

• ECG
• CBC
• Chem 10

Diagnosis

Acquired QT prolongation

• ECG
  • On visual inspection, QT takes up more than half the R-R distance
  • Measure QT interval in lead II or V5-6
  • QTc = QT /√R-R

Management

Pause Dependent (precipitated by bradycardia)

• Unstable/sustained torsades→ defibrilation (unsynchronized)
• Stable
  • Treat underlying etiology
  • 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

• Consider admission, especially for QT >500 or if symptomatic
• May require consultation for discontinuation of QT prolonging medications
• Avoid prescribing medications that may contribute to prolonged QT

See Also

• ECG (Main)
• Torsades de Pointes
• Hypomagnesemia
• Hypermagnesemia

External Links

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