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An "abnormal" QTc in males is a QTc above 450 ms; and, in females, above 470 ms. [17] 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 0.02 s from QT for every 10 bpm increase in heart rate.
Quinidine, a class Ia antiarrhythmic agent, has been shown to partially correct the QT interval and make the heart more resilient to artificially-induced abnormal heart rhythms, [3] [14] although it is still uncertain at present whether this translates to a lower risk of sudden death.
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]
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The QTc is less than 450 ms in 95% of normal males, and less than 460 ms in 95% of normal females. LQTS 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 centiles of normal values.
A prolonged QTc interval is a risk factor for ventricular tachyarrhythmias and sudden death. Long QT can arise as a genetic syndrome, or as a side effect of certain medications. An unusually short QTc can be seen in severe hypercalcemia. <440 ms U wave: The U wave is hypothesized to be caused by the repolarization of the interventricular septum.
Methadone at therapeutic concentrations is known to prolong the QTc interval, which indicates that the heart muscle repolarizes more slowly. This QTc prolongation tends to increase the risk of torsades de pointes (TdP), a heart rhythm disturbance that can lead to syncope or sudden death.
In 2001, the FDA changed the labeling requirements for droperidol injection to include a Black Box Warning, citing concerns of QT prolongation and torsades de pointes.The evidence for this is disputed, with 9 reported cases of torsades in 30 years and all of those having received doses in excess of 5 mg. [9] QT prolongation is a dose-related effect, [10] and it appears that droperidol is not a ...