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Upper limit of normal QT interval, corrected for heart rate according to Bazett's formula, [5] Fridericia's formula, [10] and subtracting 0.02 s from QT for every 10 bpm increase in heart rate. [13] Up to 0.42 s (≤ 420 ms) is chosen as normal QTc of QT B and QT F in this diagram.
The value for a normal QT interval is similar in males and females from birth up to adolescence. During infancy, a normal QTc is defined as 400 ± 20 milliseconds. Before puberty, the 99th percentile of QTc values is 460 milliseconds. After puberty, this value increases to 470 milliseconds in males and 480 milliseconds in females. [citation needed]
[12] [13] The QT interval normally varies with heart rate, but this variation occurs to a lesser extent in those with short QT syndrome. [1] It is therefore recommended that the QT interval is assessed at heart rates close to 60 beats per minute. [ 1 ]
This leads to overlap between the QT intervals of those with and without LQTS. 2.5% of those with genetically proven LQTS have a QT interval within the normal range. [23] Conversely, given the normal distribution of QT intervals, a proportion of healthy people will have a longer QT interval than any arbitrary cutoff. [23]
Long QT syndrome, or LQTS, is when somebody’s QT interval is longer than normal, which should typically be less than half of a cardiac cycle. In fact, for a heart rate of 60 beats per minute, the QT interval’s generally considered to be abnormally long when it’s greater than 440 milliseconds in males or 460 milliseconds in females.
The QTc is less than 450 ms in 95% of normal males, and less than 460 ms in 95% of normal females. Romano–Ward syndrome is suggested if the QTc is longer than these cutoffs. However, as 5% of normal people also fall into this category, some suggest cutoffs of 470 and 480 ms for males and females respectively, corresponding with the 99th ...
The normal range is 2.1–2.6 ... Abnormal heart rhythms can also result, and ECG findings of a short QT interval [26] suggest hypercalcaemia.
Under normal resting conditions, beat-to-beat changes in QT interval are very small with a standard deviation typically below 5 ms. Digital high resolution ECG sampled at 300 Hz or higher and dedicated computer algorithms are required for QTV assessment. [2]