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The QT interval is a measurement made on an electrocardiogram used to assess some of the electrical properties of the heart.It is calculated as the time from the start of the Q wave to the end of the T wave, and approximates to the time taken from when the cardiac ventricles start to contract to when they finish relaxing.
The value is an indication of the time it takes for a ventricle from the beginning of a contraction to the end of relaxation. 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 ...
Corrected QT interval (QTc) The QT interval is measured from the beginning of the QRS complex to the end of the T wave. Acceptable ranges vary with heart rate, so it must be corrected to the QTc by dividing by the square root of the RR interval. A prolonged QTc interval is a risk factor for ventricular tachyarrhythmias and sudden death.
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]
The cardiac features of JLNS can be diagnosed by measuring the QT interval corrected for heart rate (QTc) on a 12-lead electrocardiogram (ECG). The QTc is less than 450 ms in 95% of normal males, and less than 460 ms in 95% of normal females. In those with Jervell and Lange-Nielsen syndrome the QTc is typically greater than 500 ms. [8]
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]
The first report of short QT syndrome to be published was in 2000, describing a family with short QT intervals on the 12-lead ECG, atrial fibrillation occurring at a young age, and an unrelated patient who had a sudden cardiac death associated with a short QT interval.
T-wave alternans and prolonged QT interval in a male patient found to be in a narrow-complex tachycardia and ruled in for an acute myocardial infarction.Administered Ibutilide and converted to sinus rhythm but subsequently had an episode of Torsades de Pointes which required DC cardioversion back into sinus rhythm.