Significance of Drug-Induced QT Prolongation

Drug related Use of antiarrhythmic agents (class I or class III) 

To further complicate the issue, no international standard exists even for QT measurement itself. Several authors have already drawn the attention to the inherent difficulties involved in accurate measurement of the QT interval. These are summarized in Table 3.3. 

The QT interval is a dynamic physiological variable depending on multiple factors such as cardiac cycle length (heart rate), autonomic nervous system activity, age, gender, plasma electrolyte concentrations, genetic variations in ion channels involved in cardiac repolarization. In addition, circadian and seasonal variations of the QT interval have been described [93]. 

The determinant that mostly influences the QT interval duration is cycle length (RR interval) the longer the RR interval, the longer the QT interval and vice versa. Therefore, a number of formulas (see [94] for a list) are used to normalize the QT interval for heart rate and obtain a corrected QT interval (QTc), a key issue especially 

Oocytes of the amphibian Xenopus laevis (unreliable because poorly predictive) 

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