QTc calculator (Bazett)

Correct the QT interval for heart rate.

What this calculator is for

The QT interval shortens as the heart rate rises, so a raw QT cannot be compared between ECGs. The corrected QT (QTc) estimates the QT at a heart rate of 60. A long QTc increases the risk of torsades de pointes, a polymorphic ventricular tachycardia that can cause syncope and sudden death.

The formula

Bazett: QTc = QT ÷ √RR, with RR in seconds (RR = 60 ÷ heart rate). Fridericia, QT ÷ ∛RR, is more accurate at fast or slow rates.

How to measure the QT

Use lead II or V5. Measure from the start of the QRS to the end of the T wave, where a tangent drawn down the steepest part of the T meets the baseline.
Do not include a separate U wave.
Average three to five beats, more in atrial fibrillation.

Interpreting the result

Normal: under about 450 ms in men and 460 ms in women.
Prolonged: above these values. Review drugs and electrolytes.
Above 500 ms, or a rise of more than 60 ms after starting a drug: high risk. Stop QT-prolonging drugs where possible, correct potassium and magnesium, and monitor.

Worked example

QT 400 ms at a heart rate of 100: RR = 0.6 s, so Bazett QTc = 400 ÷ 0.775 = 516 ms. Fridericia gives about 474 ms. At fast rates Bazett overestimates, so repeat the ECG when the rate settles.

Common causes of a long QT

Drugs widely used in Pakistan: azithromycin, clarithromycin, fluoroquinolones, ondansetron, domperidone, haloperidol, amitriptyline, methadone, chloroquine and quinine.
Low potassium, low magnesium and low calcium; vomiting and diarrhoea cause all three.
Congenital long QT syndromes, hypothyroidism, hypothermia and raised intracranial pressure.

Torsades is treated with IV magnesium sulfate even when the magnesium level is normal; unstable patients need DC cardioversion.

See recording and reading an ECG, fast heart rhythms, electrolytes on the ward, magnesium sulfate, ondansetron and the textbook chapter on ECG and arrhythmias.

Frequently asked questions

What is a normal QTc?

Under about 450 ms in men and 460 ms in women. A QTc above 500 ms carries a high risk of torsades de pointes.

What is the Bazett formula?

QTc = QT ÷ square root of the RR interval in seconds. It over-corrects at fast heart rates and under-corrects at slow ones.

Which common drugs prolong the QT?

Macrolides such as azithromycin, fluoroquinolones, ondansetron, domperidone, haloperidol, tricyclic antidepressants, methadone, chloroquine and quinine, among others.

What should I do if the QTc is over 500 ms?

Stop or avoid QT-prolonging drugs, check and correct potassium, magnesium and calcium, put the patient on a monitor and discuss with a senior.

A calculator supports, but does not replace, clinical judgement. Check results before acting on them.