How to measure a QT interval

A standard approach in QT measurement improves communication between clinicians.

An abnormally prolonged QT interval is associated with an increased risk of sudden cardiac death. Some professional bodies recommend national population‐based screening programs to detect QT prolongation. Familial long QT syndrome (LQTS) may remain undetected because of misdiagnosis (eg, as a seizure disorder) or through failure to measure the QT interval correctly. Psychiatrists fear the QT prolongation caused by many psychotropic medications, and it may also be seen during periods of hypothermia; electrolyte imbalance (such as hypokalaemia, hypomagnesaemia and hypocalcaemia); in the setting of raised intracranial pressure or post‐cardiac arrest; with other medications, such as type 1A, 1C and III antiarrhythmic agents; and with antihistamines and macrolide antibiotics.

Yet, despite its importance, research shows that two physicians cannot measure the same QT interval and get the same result; an observation that also includes cardiac electrophysiologists. There is no universally accepted agreement about how to measure the QT interval, which 12‐lead electrocardiogram (ECG) leads to include, how many beats to measure, or which heart rate correction formula to use. Two large international familial LQTS registries measure the end of the T wave in different ways, such that one has an average QT interval (454 milliseconds) 40 milliseconds below the other (494 milliseconds). Moreover, the QT interval has been shown to vary minute to minute, beat to beat, and day by day. Because of these problems, guidance as to how to obtain accurate and reproducible QT measurements is a matter of opinion and experience.