see also:

• The basics of reading an ECG
• patterns of ECGs
• torsade de pointes VT
• short QTc
• LITFL - a case of prolonged QTc with R on T bigeminy leading to torsades

• the QT interval on the ECG is measured from the start of QRS complex to end of the T wave
• QTc refers to a QT interval “corrected” for the effect of differing heart rate to give a QT value as if the heart rate is 60bpm
• prolonged QTc represents delayed repolarisation & predisposes to early after-depolarisations, re-entrant tachycardias, especially potentially fatal torsade de pointes VT which is most likely if bradycardia is present too.

• if rate 60-100bpm, then QT should be less than half the RR interval
• Bazett's formula: QTc = QT / square root (RR interval)

• V-W class Ia drugs: quinidine, disopyramide, procainamide
• V-W class III drugs: amiodarone, sotalol
• other drugs with class Ia activity: tricyclic antidepressants, particularly in overdose, phenothiazines, non-sedating antihistamines (H1)
• quetiapine, haloperidol, erythromycin, methadone, citalopram, pentamidine, ziprasidone
• high dose iv ondansetron
• ischaemic heart disease (IHD)
• cerebrovascular disease incl. subarachnoid haemorrhage (SAH)
• rheumatic fever
• myocarditis
• mitral valve prolapse
• hypocalcaemia
• hypokalaemia - actually a flat T with a prominent U wave
• hypomagnesaemia
• hypothyroidism
• cirrhosis
• liquid protein diets
• congenital prolonged QT syndrome