The QTc calculator is based on heart rate (pulse rate) in beats per minute and QT interval, either in seconds or miliseconds.

■ Heart rate is defined as the number of subsequent contractions of the heart during one minute, in which blood is pumped from the ventricles to the arteries.

■ The QT interval is a cardiologic term used for the time in the heart’s electrical cycle between the start of the Q wave and the end of the T wave. This is measured in seconds or in miliseconds, the latter for more precision.

QTc formulas

The corrected QT interval is calculated based on the following:

■ Bazett’s formula: QTc = QT/√(RR in seconds)

■ Fridericia’s formula: QTc = QT/(RR0.33)

■ Framingham’s formula: QTc = QT + 0.154(1-RR)

■ Hodges’s formula: QTc = QT + 1.75(HR - 60)

According to the ACC/HRS:

■ Normal values are lower than 450 milliseconds for men and below 460 milliseconds for women;

■ Any QTc value greater than 500 milliseconds is abnormal;

■ Any QTc value lower than 340 milliseconds is abnormal and may indicate short QT syndrome.

After one comparison study, it was concluded that Fridericia and Framingham correction formulas have the best rate correction and significantly improved prediction while Bazett’s formula overestimated QTC prolongation.

Abnormally high or low QTc

Values lower than normal may be caused by short QT syndrome which is a condition that in turn leads to arrhythmia. This is not considered to have to do with any underlying structural anomaly of the heart and can appear at any age.

A shorter QT interval basically means that the heart takes less time to relax between contractions. This can lead to syncope, dizziness and even complicate to cardiac arrest and sudden death.

Prolonged QT syndrome is also indicative of an arrhythmia type and is characteristic by moments in which the heart suddenly beats faster for no particular reason.

This increase in contraction frequency that has not been supported by other physiological adaptation can lead to impaired tissue perfusion of the brain.