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In cardiology, the QT interval is a measure of the time between the start of the Q wave and the end of the T wave in the heart's electrical cycle. The QT interval represents electrical depolarization and repolarization of the ventricles. A lengthened QT interval is a marker for the potential of ventricular tachyarrhythmias like torsades de pointes and a risk factor for sudden death. ==Correction for heart rate== Like the R-R interval, the QT interval is dependent on the heart rate in an obvious way (the faster the heart rate the shorter the R-R Interval and QT interval) and may be adjusted to improve the detection of patients at increased risk of ventricular arrhythmia. Modern computer-based ECG machines can easily calculate a corrected QT (QTc), but this correction may not aid in the detection of patients at increased risk of arrhythmia. There are a number of different correction formulas. The standard clinical correction is to use ''Bazett's formula'', named after physiologist Bazett, calculating the heart rate-corrected QT interval (QTcB). Please be ware that Bazett's formula is based on observations of only 12 patients in 1920 and does meet current scientific quality standards. A more robust method is the Frammingham correction based on the Framingham Heart study, recent longterm cohort data of over 5000 subjects. Bazett's formula is: There are several other methods, such as regression-analysis: Definitions of normal QTc vary from being equal to or less than 0.40 s (≤400 ms),〔(Lesson III. Characteristics of the Normal ECG ) Frank G. Yanowitz, MD. Professor of Medicine. University of Utah School of Medicine. Retrieved on Mars 23, 2010〕 0.41s (≤410ms),〔(Loyola University Chicago Stritch School of Medicine > Medicine I ) By Matthew Fitz, M.D. Retrieved on Mars 23, 2010〕 0.42s (≤420ms)〔(ecglibrary.com > A normal adult 12-lead ECG ) Retrieved on Mars 23, 2010〕 or 0.44s (≤440ms).〔(Image for Cardiovascular Physiology Concepts > Electrocardiogram (EKG, ECG) ) By Richard E Klabunde PhD〕 For risk of sudden cardiac death, "borderline QTc" in males is 431-450 ms, and in females 451-470 ms. An "abnormal" QTc in males is a QTc above 450 ms, and in females, above 470 ms.〔(medscape.com > QTc Prolongation and Risk of Sudden Cardiac Death: Is the Debate Over? ) February 3, 2006〕 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 0.02s from QT for every 10 bpm increase in heart rate. For example, taking normal QTc ≤ 0.42 s, QT would be expected to be 0.42 s or less at a heart rate of 60 bpm. For a heart rate of 70 bpm, QT would roughly be expected to be equal to or below 0.40 s. Likewise, for 80 bpm, QT would roughly be expected to be equal to or below 0.38 s.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「QT interval」の詳細全文を読む スポンサード リンク
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