Long QT syndrome Flashcards
What is LQTS?
Genetic or acquired condition
Associated with mutations in 13 identified genes causing a variety of channelopathies affecting myocardial repolarisation, thus prolonging the QT interval
(KCNQ1 gene - LQT1, KCNH2 - LQT2, SCN5a - LQT3)
Increased risk of SCD due to polymorphic ventricular tachycardia (torsades de pointes)
High mortality without treatment
Signs & symptoms
Palpitations
Atypical epilepsy/ seizures
Unexplained history of syncope
Triggered by exertion, excitement or stress
LQT1 – exercise, especially swimming
LQT2 – exercise, emotional situations, surprise or sudden
noise when at rest e.g. alarm clock or phone ringing
LQT3 – when at rest or during sleep
Diagnosis of LQTS
Related symptoms
Family history
Genetic testing – can indicate a predisposition, but not whether a patient will develop symptoms
ECG QT interval prolongation, altered T wave morphology
Can be normal at rest and brought out by standing or during recovery from exercise
Management
Aim: prevent symptoms
Avoid trigger events (no participation in competitive sports) and QT prolonging medications (anti-histamines, antibiotics, some cardiac medications)
Long acting Beta-blockers – nadolol, propanolol – AHA Class I (highly recommended) for LQT and suspected arrhythmogenic syncope
If unsuccessful - Left cardiac sympathetic denervation (surgical)
Genetic counselling – helps to understand implications for family members, recurrence risk, family planning options, and psychological adjustment to disease and/or carrier status
ICD implantation - AHA Class IIa (is reasonable) in patients with LQT and suspected arrhythmic syncope on beta-blocker therapy or intolerant to beta-blocker therapy
How to measure the QT interval
Measure in lead II or V5-6 (or if prominent U waves use aVL)
- Measure several successive beats and record the maximum interval
To define the end of the T wave:
maximum slope intercept method - the intercept between the isoelectric line with the tangent drawn through the maximum down slope of the T wave (top). When notched T waves are present (bottom), the QT interval is measured from the beginning of the QRS complex extending to the intersection point between the isoelectric line and the tangent drawn from the maximum down slope of the second notch, T2
Relationship between QT and heart rate
QT interval is inversely related to heart rate
QT shortens at faster heart rates
QT lengthens at slower heart rates
Prolonged QT is associated with an increased risk of ventricular arrhythmias, especially Torsades de Pointes
Congenital short QT syndrome has been found to be associated with an increased risk of paroxysmal atrial and ventricular fibrillation and sudden cardiac death
Normal QTc values: Men or Women <440 msecs
A QTc is >500 is associated with increased risk of torsades de pointes
QTc is abnormally short if < 350ms
A useful rule of thumb is that a normal QT is less than half the preceding RR interval
Normal QTc values: Men or Women <440 msecs
A QTc is >500 is associated with increased risk of torsades de pointes
QTc is abnormally short if < 350ms
A useful rule of thumb is that a normal QT is less than half the preceding RR interval
Potential causes of long QT
Causes of a prolonged QTc (>440ms)
Hypokalaemia (low potassium)
Hypomagnesaemia (low magnesium)
Hypocalcaemia (low calcium)
Hypothermia
Myocardial ischaemia
Post-cardiac arrest
Raised intracranial pressure
Drugs – Sotalol, Amiodarone
Antibiotics - Erythromycin, Clarithromycin
Anti-depressants (SSRI) – Fluoxetine, Citalopram
Congenital long QT syndrome
Causes of a short QTc (<350ms)
Hypercalcaemia
Congenital short QT syndrome
Digoxin effect
