QT Interval - Correction For Heart Rate

Correction For Heart Rate

The QT interval is dependent on the heart rate in an obvious way (the faster the heart rate the shorter the QT interval) and may be adjusted to improve the detection of patients at increased risk of ventricular arrhythmia. Modern computer-based ECG machines can easily calculate a corrected QT (QTc), but this correction may not aid in the detection of patients at increased risk of arrhythmia. There are a number of different correction formulas.

The standard clinical correction is to use Bazett's formula, named after physiologist Henry Cuthbert Bazett, calculating the heart rate-corrected QT interval QT_B.

Bazett's formula is as follows:

where QT_B is the QT interval corrected for heart rate, and RR is the interval from the onset of one QRS complex to the onset of the next QRS complex, measured in seconds, often derived from the heart rate (HR) as 60/HR (here QT is measured in milliseconds). However, this nonlinear formula, obtained from data in only 39 young men, is not accurate, and over-corrects at high heart rates and under-corrects at low heart rates.

Fridericia has published an alternative correction formula using the cube-root of RR.

There are several other methods as well. For example a regression-based approach that had been developed by Sagie et al., as follows:

Definitions of "normal" QTc vary among being equal to or less than 0.40 s (≤400ms), 0.41s (≤410ms), 0.42s (≤420ms) or 0.44s (≤440ms). For risk of sudden cardiac death, "Borderline QTc" in males is 431-450 ms, and in females 451-470 ms. An "abnormal" QTc in males is a QTc above 450 ms, and in females, above 470 ms.

If there is not a very high or low heart rate, the upper limits of QT can roughly be estimated by taking QT=QTc at a heart rate of 60 beats per minute (bpm), and subtracting 0.02s from QT for every 10 bpm increase in heart rate. For example, taking normal QTc ≤ 0.42s, QT would be expected to be 0.42s or less at a heart rate of 60 bpm. For a heart rate of 70 bpm, QT would roughly be expected to be equal to or below 0.40s. Likewise, for 80 bpm, QT would roughly be expected to be equal to or below 0.38s.