Expansion
Low potassium, magnesium and calcium, plus many drugs, prolong the QT interval
Expansion
The QT interval measures total ventricular depolarisation and repolarisation. It must be rate corrected (QTc), since it shortens as the rate rises; the upper limit is about 440 ms in men and 460 ms in women.
Causes of prolongation:
- Electrolytes: hypokalaemia, hypomagnesaemia, hypocalcaemia
- Drugs: many antiarrhythmics, macrolides, quinolones, antipsychotics, methadone, ondansetron
- Congenital: long QT syndromes, including Romano-Ward and Jervell and Lange-Nielsen
- Other: hypothermia, hypothyroidism, raised intracranial pressure, myocardial ischaemia, bradycardia
Prolonged repolarisation permits early afterdepolarisations, which can trigger torsades de pointes, a polymorphic ventricular tachycardia with a twisting axis. Treatment is intravenous magnesium, correction of electrolytes and withdrawal of the offending drug; increasing the rate by pacing or isoprenaline shortens the QT and suppresses it.