Measurement and Management of QT Interval Prolongation for General Physicians
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Department of Cardiology, Prince of Wales Hospital, Sydney, NSW, Australia; 2Department of Cardiology, St George Hospital, Sydney, Australia; Faculty of Medicine, The University of New South Wales, Sydney, Australia; 4Department of Medicine, St Vincent’s Hospital, Sydney, Australia; 5 School of Medicine, The University of Notre Dame, Sydney, Australia; 6Department of Psychiatry, Bloomfield Hospital, Orange, Australia. 3
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 lifethreatening 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 Bcorrection^ 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. KEY WORDS: torsades de pointes; QT interval; electrocardiogram; antipsychotics; antiarrhythmics. J Gen Intern Med DOI: 10.1007/s11606-019-05477-7 © Society of General Internal Medicine 2019
INTRODUCTION
The QT interval represents the duration of time taken from the onset of ventricular depolarisation to the end of ventricular repolarisation as seen on the electrocardiogram (ECG, see Fig. 1). This translates to the very beginning of the QRS complex, to the end of the T wave. Prolongation of the QT interval is associated with torsades de pointes (TDP), a form of polymorphic ventricular tachycardia which, in certain cases, can lead to Received July 2, 2019 Revised September 24, 2019 Accepted October 2, 2019
fatal ventricular fibrillation. Prolongation of the QT interval has also been associated with an increased risk of atrial fibrillation.1 Additionally, the normal limit for the QT interval varies with heart rate. During tachycardia, the QT interval decreases, and in bradycardia, it lengthens. Thus, there is a requirement to Bcorrect^ the QT interval, particularly in hospital settings where the resting heart rates of patients may not be normal. The corrected QT interval is known as QTc, and it is prolongation of the QTc, rather than the QT, that determines the risk of TDP. The QTc is prolonged in patients with congenital long QT syndrome (CLQTS). CLQTS includes sixteen identified ge
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