Measurement and Management of QT Interval Prolongation for General Physicians

Abstract

One of the more challenging aspects of ECG interpretation is measurement and interpretation of the QT interval. This interval represents the time taken for the ventricles to completely repolarise after activation. Abnormal prolongation of the QT interval can lead to torsades de pointes, a form of potentially life-threatening polymorphic ventricular tachycardia (VT). Detection of a prolonged QT interval is essential as this can be a reversible problem, particularly in the context of the use of a variety of commonly prescribed medications in the hospital setting. Automated ECG printouts cannot be relied upon to diagnose QT interval prolongation; thus, the onus is on the clinician to identify it. This is a difficult task, as the normal QT interval is typically measured relative to the heart rate. Therefore, the QT interval often requires "correction" for the current heart rate, in order to correctly stratify the risk of torsades de pointes. A wealth of correctional formulae have been derived, but none has proven superior. We present an approach to the ECG in this context, and a step-by-step guide to manually measuring and correcting the QT interval, and an approach to management in common hospital-based clinical scenarios.

Keywords: QT interval; antiarrhythmics; antipsychotics; electrocardiogram; torsades de pointes.

Conflict of interest statement

The authors declare that they do not have a conflict of interest.

Figures

Figure 1

The tangent technique for determining the end of the QT interval. Adapted from Postema et al.

Figure 2

Flowchart for an approach to correcting QT interval. The asterisk indicates Framingham and Hodges formulae can be used in any of these situations, but are complex equations and have been omitted from this flowchart for simplicity. HR, heart rate in beats per minute.

Figure 3

The QT nomogram. Adapted from Chan et al.

Figure 4

Flowchart for investigation and management of a prolonged QTc.