Synopsis
Measure QT accurately, distinguish congenital from acquired long-QT states and treat torsades without adding further QT-prolonging risk.
- Measure QT from QRS onset to T-wave end in a lead with a clear T wave and correct for heart rate; automated QTc values require manual review.
- Repeated QTc of 480 ms or more, a diagnostic score of 3.5 or more, or a pathogenic LQTS variant can establish congenital LQTS after acquired causes are addressed.
- Torsades is polymorphic VT whose QRS axis twists around the baseline, usually in the setting of QT prolongation and pause dependence.
Key red flags
Polymorphic broad complexes wax and wane in amplitude and appear to twist around the baseline, often after a short-long-short sequence.
Investigation priorities
Confirm that QT prolongation is real and reproducible.
Management branches
The patient is unresponsive, not breathing normally and has no definite pulse.
- Start CPR, call the arrest team and attach a defibrillator.
- Treat as VF/pulseless VT with immediate unsynchronised defibrillation under the RCUK ALS algorithm.
Key medicines
Magnesium sulfateRCUK: 8 mmol IV over 10 minutes for polymorphic VT with QT prolongation.
IsoprenalineRCUK adult bradyarrhythmia guidance gives a 5 micrograms/min IV infusion starting dose; in recurrent acquired pause-dependent torsades, titrate only with expert critical-care monitoring to increase heart rate.