Long QT Syndrome

    Table of contents

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    Intro

    • Long QT (LQT) is a disorder caused by lengthening of the repolarization of the ventricular action potential.
      • QT interval represents entire depolarization and repolarization cycle (to the end of T)
    • The danger with long QT syndrome comes from the possiblity of initiating a form of polymorphic VT (Torsades De Pointes) which carries the potential risk of death.
      • Typically to diagnose torsades there must be polymorphic VT (esp if "turning of points" pattern) with preceeding beats having prolonged QT.
      • Torsades de pointes is initiated by a premature ventricular contraction that occurs in the setting of prolonged QT. This is termed the "R on T phenomenon."

    Causes

    • Congential
      • Heritable multation in genes for transmembrane depolarizaing sodium channel or repolarizing potassium channel.  There are 15 of them (LQT1 to LQT15).
      • Most common are LQT1 and LQT2
      • LQT2 involves hERG mutation, similar mechanism to drug-induced QT prolongation
      • Increased risk during different stressors which include sudden noise to exercise (specifically swimming).
      • Jervell and Lange-Nielson Syndrome - accompanied with bilateral sensorineural hearing loss (LQT1 variant)
      • Romano-Ward Syndrome
      • Timothy Syndrome (LQT8)
      • Andersen-Tawil Syndrome (LQT7)
    • Acquired
      • DRUGS!
      • Metabolic:
        • Hypokalemia
        • Hypomagnesemia
        • Hypocalcemia
        • Protein-sparing fasting (using liquid protein as only diet source).
      • CV
        • Myocardial ischemia
        • Stroke
        • Marked Bradycardia
      • Toxins:
        • Organophosphorus compounds.
        • Cocaine abuse.
      • Infectious
        • HIV
      • Neuro:
        • Subarachnoid hemorrhage
        • Autonomic neuropathy

    Diagnosis

    • QT interval may lengthen abnormally during recover phase of exercise in 100% of LQT1 and 89% of LQT2
    • QTc (Defined as QT interval / square root of RR interval)
      • ≥ 480 ms - 3 points
      • 460-470 ms - 2 points
      • 450 ms and male gender - 1 point
    • Torsades de pointes - 2 points
    • T wave alterans - 1 point
    • Notched T wave in at least 3 leads - 1 point
    • Low heart rate for age (children) - 0.5 points
    • Syncope (one cannot receive points both for syncope and torsades de pointes)
      • With stress - 2 points
      • Without stress - 1 point
    • Congenital deafness - 0.5 points
    • Family history (the same family member cannot be counted for LQTS and sudden death)
      • Other family members with definite LQTS - 1 point
      • Sudden death in immediate family members (before age 30) - 0.5 points
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