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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.
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]
Long QT syndrome is associated with a prolonged QTc, although in some genetically proven cases of LQTS this prolongation can be hidden, known as concealed LQTS. [23] 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.
The 1925 paper [1] of Chester W. Rice and Edward W. Kellogg, fueled by advances in radio and electronics, increased interest in direct radiator loudspeakers. In 1930, A. J. Thuras of Bell Labs patented (US Patent No. 1869178) his "Sound Translating Device" (essentially a vented box) which was evidence of the interest in many types of enclosure design at the time.
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.
Short QT syndrome (SQT) is a very rare genetic disease of the electrical system of the heart, and is associated with an increased risk of abnormal heart rhythms and sudden cardiac death. [1] The syndrome gets its name from a characteristic feature seen on an electrocardiogram (ECG) – a shortening of the QT interval .
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]
QT interval variability (QTV) refers to the physiological phenomenon of beat-to-beat fluctuations in QT interval of electrocardiograms. Increased QTV appears to be a marker of arrhythmic and cardiovascular death; it may also play a role for noninvasive assessment of sympathetic nervous system activity.