Measuring the QT Interval and Calculating the QTc Value
The QT interval is measured on the surface 12-lead ECG (or telemetry or smartphone-enabled mobile ECG) from the beginning of the QRS complex to the end of the T wave. The QT interval is influenced by heart rate; with a longer QT interval at slower heart rates and shorter QT interval at longer heart rates. Therefore, the QT interval must be corrected for the patient’s heart rate to generate the patient’s QTc value.
The diagram below provides a step-by-step guide to using the Windland Smith Rice Genetic Heart Rhythm Clinic’s QTc Calculator.
After measuring/verifying the patient’s QT interval, heart rate, and their QRS duration (especially if it is > 120 ms), then simply use the calculator to calculate your patient’s QTc value