Regression equations for QT and QTc intervals of the electrocardiogram

Czech version

Authors: Š. Kujaník
Authors‘ workplace: Ústav fyziológie Lekárskej fakulty UPJŠ, Košice, Slovenská republika, prednosta doc. MUDr. V. Donič, CSc.
Published in: Vnitř Lék 2005; 51(11): 1277-1288
Category: Review

Overview

One of the parameters of the electrical stability of the heart is also the QT interval (electrical systole) duration. It is dependent on the heart rate, during tachycardia is shortening, during bradycardia prolonging. However, its borders on the ECG are not very precisely defined, especially the end of T wave. For QT interval duration the ventricular cells of type M are mainly important and they have the very long action potentials. Since 1920, when the first regression equation of QT interval was published, there were plenty of equations linear or nonlinear (parabolic, cubic, hyperbolic, exponential, logarithmic) proposed, the most frequently used is the Bazett`s one. QT duration and the mathematic formula of regression equation are dependent on several factors - gender, age, exercise, diseases, pharmacologic agents. QT duration in healthy women at the same heart rate is approximately by 10 - 20 ms longer than in healthy men, in older persons is longer than in younger ones. The slowed repolarization of ventricular cells in women is the cause of this difference. Many cardiovascular, cerebral, other diseases and some drugs are able to prolong QT interval. Change of the dependence of QT duration on the heart rate means a change of autonomic nervous system activity. QT interval is probably not a function of the heart rate or cardiac cycle duration. QT interval duration is influenced by RR interval, therefore because of better comparison at various RR the so called corrected QT interval (QTc), deprived of heart rate or RR interval dependence, is used. Every regression QTc equation is valid under certain conditions and in certain range of heart rates only. Change of the mode of QT expressing or a used regression equation are able to alter the statistical significance of obtained differences. Relation between QT and RR intervals is highly individual and specific in every person, therefore to search some universally valid equation is probably not fruitful. Orientatively, a prolonged QT interval can be a value above 500 ms without correction for the heart rate.

Key words:
electrocardiography - QT interval - heart rate - RR interval - regression equations

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