# QT Interval: How to Measure It and What Is "Normal"

Ilan Goldenberg, M.D.; Arthur J. Moss, M.D.; Wojciech Zareba, M.D., Ph.D.

Disclosures

J Cardiovasc Electrophysiol. 2006;17(3):333-336.

The time-duration intervals are influenced by heart rate (R-R cycle length), so heart rate correction is required in the analysis of repolarization duration. Various heart rate correction formulae have been developed in order to determine whether the QT interval is prolonged in comparison to its predicted value at a reference heart rate of 60 beats/min (i.e., an RR interval of 1.0 second). These formulae have been derived mainly from resting ECGs, and therefore require a stable sinus rhythm without sudden changes in the RR interval. Exponential, logarithmic, and linear formulae have been used[1] ( Table 1 ). To assess the performance of a particular heart rate correction formula, the correlation between the corrected QT (QTc) intervals calculated using the formula and the RR intervals can be assessed. If it differs from zero, as is the case with most of the above-described formulae, the correction formula is not truly successful. We most commonly correct the interval by using Bazett's square root formula. QTc is equal to QT interval in seconds divided by the square root of the preceding RR interval in seconds. When heart rate is particularly fast or slow, the Bazett's formula may overcorrect or undercorrect, respectively, but it remains the standard for clinical use. The cube root Fridericia formula has the same limitations at slow heart rates, but is considered to reflect a more accurate correction factor in subjects with tachycardia. Linear formulae may have more uniform correction over a wide range of heart rate. The most commonly used linear formula derives from the Framingham study. The latter formulae may give QT values that are too low at slow heart rates. There is no general consensus on the best formula to be utilized in clinical practice. Of note, in resting conditions with heart rates in the 60-90 beats/min range, most formulae provide almost equivalent results for the diagnosis of QT prolongation. Even if the rate dependence of the QT interval is probably best described by an exponential relation, in the normal heart rate range the QT-RR relation is approximately linear.

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