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1

Long QT Syndrome

The long QT syndrome (LQTS) is a disorder of myocardial repolarization characterized by a prolonged QT interval on the ECG and associated with an increased risk of the life-threatening arrhythmia polymorphic ventricular tachycardia, termed torsade de pointes (or torsades de pointes).

•acquired or congenital 

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Torsades de Pointes

Torsade de pointes (TdP) is polymorphic ventricular tachycardia that usually occurs in the setting of QT interval prolongation. Polymorphic VT is defined as a ventricular rhythm faster than 100 beats per min with frequent variations of the QRS axis, morphology, or both. In the specific case of TdP, the peaks of the QRS complexes appear to "twist" around the isoelectric line of the recording; hence the name torsade de pointes or "twisting of the points". TdP is usually short-lived and terminates spontaneously. However, patients may experience multiple episodes of the arrhythmia, and episodes can recur in rapid succession, potentially degenerating to ventricular fibrillation and sudden cardiac death. Thus, patients with LQTS may present with symptoms that include palpitations, near syncope, syncope or sudden cardiac death.

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acquired LQTS

Acquired LQTS usually results from drug therapy, hypokalemia, or hypomagnesemia, and the incidence of acquired LQTS is much higher than the incidence of congenital LQTS.

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congenital LQTS

Congenital LQTS is the most common of the genetic arrhythmia syndromes and is characterized by an abnormal QTprolongation on ECG and an increased risk of sudden death.

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QT interval

The QT interval is the measurement for assessing the duration of ventricular depolarization and repolarization and is measured on the ECG from the beginning of the QRS complex (either a Q or R wave) until the end of the T wave. The upper limit of normal for the QT interval varies with age and gender. The QT interval is dependent upon the heart rate; it is shorter at faster heart rates and longer when the rate is slower. A rate-corrected QT interval permits the comparison of QT values obtained at different heart rates.

QT interval corrected for heart rate (QTc) is most often calculated based on Bazett’s formula as follows:

QTc = QT interval ÷ square root of the RR interval (in sec)

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normal QTc for men

<440 msec men

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normal QT interval for women

<460 msec women

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drugs associated with LQTS and TdP - antiarrhythmics

•quinidine

•amiodarone

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drugs associated with LQTS and TdP - antibiotics

•macrolides (erythromycin, azithromycin)

•fluoroquinolones (ciprofloxacin, levofloxacin)

 

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drugs associated with LQTS and TdP - antipsychotics

•haloperidol

•chlorpromazine

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drugs associated with LQTS and TdP - emetics

•odansetron

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drugs associated with LQTS and TdP - opioid drugs

•methadone

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electrolyte associated with LQTS and TdP

•hypokalemia

•hypomagnesia

•hypocalcemia

 

increase he QT intrval through effects on the IKR potassium channel

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•monomorphic VT

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•polymorphic VT

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•atrial flutter

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•atrial fibrillation with RVR