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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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