QT interval varies with heart rate.
Bazett’s formula is used to correct the QT for HR:
QTc = QT measured / (√R–R interval)
Normal QTc:
Infants less than 6 months is < 0.49 seconds
Older than 6 months < 0.44 seconds
QTc is prolonged in:
- Hypocalcaemia
- Myocarditis
- Long QT syndromes e.g. Romano‐Ward
- Head injury
- Drugs: see table
QTc is short in:
- Hypercalcaemia
- Digitalis effect
