Prolonged QT syndrome

 

QT interval:

Definition

·     Start of depolarization to end of repolarization

·     Start of QRS to end of T

·     Start of phase 0 to end of phase 3

·     K and Ca channels most important

Correction

·     QT ∝ heart rate

·     Hence correction for heart rate

·     QTc = QT/sqrt(RR) (Bazett formula)

Normal values

·     Male: <440

·     Female: <460

·     Worry ++: >500

Significance

·     ↑QTc = ↑risk of Torsades de Pointes

·     It’s the JT interval that matters (ignores QRS)

·     Not every drug that increases QTc increases risk of TdP

 

Torsades de Pointes:

Nature

·     Type of polymorphic VT

·     Due to early afterdepolarisation

·     Cyclic alteration of QRS axis

·     Triggered by a “’long-short” pattern

·     Causes syncope or sudden death

Causes

·     Congenital (Romano-Ward)

·     Electrolytes (↓K, ↓Ca, ↓Mg)

·     Low metabolic rate (hypothermia, hypothyroidism, starvation)

·     Drugs

o   High: class 3 and 1a antiarrhythmics

o   Mod: anti-psychotics, -depressant, -emetic, -biotic, methadone

o   Low: many others

Treatment

·     Shock

·     Mg

Prevention

·     Avoid triggers

·     β-blockade

·     ICD (kind of)