How to Calculate a QT Interval

The electrocardiogram, or ECG, is one of the most common and widespread techniques for medical examination of heart function. A normal ECG is split into several waves for each heartbeat. The first positive deflection on the ECG is called the P wave. It is followed by three waves commonly referred to as the QRS complex. The final deflection is called the T wave. The QT interval is important for diagnostic purposes and is defined as the time from the start of the QRS complex to the end of the T wave.

Instructions

How to Calculate a QT Interval

• 1

  Count the number of small boxes on the standard ECG printout (with 5-mm boxes) between the beginning of the QRS complex and the end of the T wave.

• 2

  Multiply the number of boxes you counted by the speed that the ECG paper was run (in mm/sec). Usually, this is 25 mm/sec, but it is sometimes run at 50 mm/sec in cases of unusually fast heart rate.

• 3

  Multiply this number by 8 to get the QT interval in milliseconds. This is the uncorrected QT interval.