QTc Interval Calculation and Measurement

Introduction to QT Interval Measurement

The American College of Cardiology recommends measuring the QT interval from the beginning of the QRS complex to the end of the T wave 1
The end of the T wave is best identified using the "Teach-the-Tangent" or "Avoid-the-Tail" method: draw a tangent to the steepest downslope of the T wave and define the intersection of this line with the baseline 3

The American College of Cardiology suggests that linear regression functions are recommended over Bazett's formula for more accurate QT-rate correction 1
Bazett's formula overestimates QTc at heart rates >90 beats/min and underestimates QTc at heart rates <50 beats/min 3
The American College of Cardiology recommends against using Bazett's formula for patients with very fast or slow heart rates 3

For heart rates <50 beats/min, the American College of Cardiology recommends performing mild aerobic activity to achieve a heart rate closer to 60 beats/min before measuring 3
For heart rates >90 beats/min, the American College of Cardiology recommends allowing additional resting time to achieve a lower heart rate 3
The American College of Cardiology suggests that QT correction should not be attempted when RR interval variability is large or when identification of the T wave end is unreliable 1

Normal QTc values are ≤460 ms in women and ≤450 ms in men, according to the American College of Cardiology 1
The American College of Cardiology recommends against relying solely on automated measurements without visual validation 1
The American College of Cardiology suggests that failing to use the same lead for serial QT measurements in the same patient is a common pitfall to avoid 2