QT interval


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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


drugs prolonging QT interval


QTc is short in:

  • Hypercalcaemia
  • Digitalis effect

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