QTc Calculator

How to use it

1. 1

   Measure QT manually. From start of QRS to end of T-wave. Use lead II or V5. Average 3–5 beats.

2. 2

   Select formula. Bazett is most common but overcorrects at high HRs. Fridericia is preferred by most guidelines. Framingham is a linear correction.

3. 3

   Interpret. QTc >500 ms: high torsades risk. Review all QT-prolonging drugs immediately.

When to use it

Starting QT-prolonging drugs

Antibiotics (azithromycin, moxifloxacin), antipsychotics, amiodarone, ondansetron, methadone, hydroxychloroquine.

Hypokalaemia or hypomagnesaemia

Electrolyte abnormalities potentiate QT prolongation. Correct K+ >4.0 and Mg²+ >1.0 mmol/L.

Syncope evaluation

Long QT syndrome (congenital or acquired) can cause torsades and VF presenting as syncope.