Precordial QT interval dispersion as a marker of torsade de pointes. Disparate effects of class Ia antiarrhythmic drugs and amiodarone

Rate-corrected QT interval: techniques and limitations

The duration of the QT interval on the surface electrocardiogram represents the time required for all ventricular depolarization and repolarization processes to occur. Among the many physiologic and pathologic factors that contribute to the QT interval, heart rate plays a major role. Several approaches have been used to correct the QT interval, all of which take into account the heart rate at which the interval is measured. The simplest and most common approach to correcting the QT interval is to divide its value by the square root of the preceding RR interval expressed in seconds, i.e., by using Bazett's formula. This calculation provides a corrected QT (QTc) interval that represents the QT interval normalized for a heart rate of 60 beats/min. However, several studies have shown that Bazett's correction formula is not optimal. Fridericia's cube-root formula has been shown to perform better in correcting the QT interval for heart rate. Other formulas require the measurement of several QT-RR pairs at various heart rates to obtain a reliable QTc interval and are therefore not easily usable. Any correction formula is likely to introduce an error in assessing the QTc interval. Although the importance of this error should not be minimized, the corrected QT interval remains useful in assessing the effects of drugs on the duration of repolarization. For this purpose, Fridericia's cube-root formula is preferable to Bazett's square-root formula.(ABSTRACT TRUNCATED AT 250 WORDS)

Relations of QTc prolongation on the electrocardiogram to torsades de pointes: definitions and mechanisms

Antiarrhythmic drug-induced ventricular tachyarrhythmias have traditionally been identified on the basis of prolongation of the QT interval and the appearance of undulating peaks of sequential QRS complexes and T waves that give the tachycardia a "twisting" aspect (so-called torsades de pointes). It is now understood, however, that a precise lengthening of the QT interval is not a necessary or sufficient criterion for the development of torsades de pointes associated with drug therapy. Indeed, not all agents that have been linked with torsades de pointes cause significant lengthening of the QT interval. Moreover, sufficiently accurate determination of QT interval duration to judge a prolongation is confounded by a number of difficulties, including considerable intraindividual variability. It is possible that the development of abnormal giant U waves may be involved in the genesis of these tachyarrhythmias. Such U waves are believed to represent the summation of early afterdepolarizations occurring throughout the myocardium. The prepause ventricular rate determines the amplitude of the U wave, and larger U waves give rise to longer and faster runs of ventricular tachycardia. These arrhythmias tend to be self-perpetuating in a bigeminal feedback pattern. At present, our knowledge base about the relation of the QT interval and torsades de pointes is grossly incomplete. More basic and clinical data are required before hard clinical judgments can be made with any precision.