QT interval Prolongation

Definition

The QT interval is the time from the start of the QRS complex to the end of the T wave, and it represents the total duration of electrical activity in the ventricles

Usually measured in aVL where the U waves are least prominent (U waves can be mistaken for T waves)

The QT interval is rate dependent, and the corrected QT interval needs to be calculated

QTc = QTm / square root of RR

Where QTm is the measured QT interval, QTc the calculated QT interval, and RR is the measured RR interval (all measurement in seconds)

Upper limit of QTc is 0.42 seconds (10.5 small squares)

QT prolongation is associated with ventricular arrhythmia, esp.Torsade de Pointes, syncope and sudden death.

QT prolongation – causes

Congenital: Romano-Ward (Autosomal dominant, normal hearing)

Lange-Nielsen (Autosomal recessive, deafness)

Hypokalaemia, Hypocalcaemia, Hypomagnesaemia

Hypothermia

Drugs antiarrhythmics (quinidine, sotalol, amiodarone etc)

antihistamines: terfenadine, astemizole

antimalarials: esp. halofantrine

antibiotics: erythromycin, co-trimoxazole

antipsychotics: esp. droperidol, thioridazine

misc: tricyclics, cisapride

QT prolongation – principles of management

Exclude congenital syndrome (FHx of sudden death, deafness)

Avoid drugs in underlying cardiac disease

Avoid high doses of drugs that prolong QT

Avoid giving two drugs concurrently that prolong QT

Avoid giving a drug that raises the concentration of a QT interval prolonging drug

If patient at risk then monitor calcium, potassium, magnesium, and thyroid function, and maintain in normal range

?Routine ECG

Investigate symptoms and signs of arrhythmia whilst on drugs

Liase with cardiologists

References

Royal College of Psychiatrists, Council Report CR57

‘The association between antipsychotic drugs and sudden death’ 1997

Reilly et al, ‘Recent studies of ECG changes, antipsychotic use and sudden death in psychiatric patients’ Psychiatric Bulletin (2002), 26, 110-112

Current Problems in Pharmacovigilance, Volume 27, February 2001

‘QT prolongation in antipsychotics’

O’Brien and Oyebode, ‘Psychotropic medication and the heart’ Advances in Psychiatric Treatment (2003), 9, 414-423

Category: Cardiology Notes