Notes & Sources

• The headline figure is Fridericia, which is generally preferred for drug-safety assessment because Bazett over-corrects at high heart rates and under-corrects at low ones.
• Thresholds used here — prolonged: >450 ms (male) or >460 ms (female); >500 ms marks substantially increased torsades de pointes risk at any sex.
• An increase of >60 ms from a patient’s own baseline is clinically meaningful even when the absolute value stays under the threshold. Record a baseline before starting a QT-prolonging drug.
• Correct hypokalaemia, hypomagnesaemia and hypocalcaemia before attributing prolongation to a drug — electrolytes are the reversible half of the problem and are usually the pharmacist’s first move.
• All correction formulae are unreliable in atrial fibrillation and in wide-QRS rhythms.