When the interval is measured from a single lead, the lead in which the interval is the longest (most frequently lead V 2 or V 3) and in which a prominent U wave is absent (usually aVR or aVL) is preferred. In automated ECG systems, the QT interval typically is measured from a composite of all leads, with the interval beginning with the earliest onset of the QRS in any lead and terminating with the latest end of the T wave in any lead. Because the onset of the QRS and the end of the T wave do not occur simultaneously in every lead, the QT interval duration will vary from lead to lead by as much as 50 to 65 milliseconds ( QT dispersion). Difficulties include identifying the beginning of the QRS complex and especially the end of the T wave, determining which lead(s) to use, and adjusting the measured interval for rate, QRS duration, and sex. Thus, it includes the total duration of ventricular activation and recovery and, in a general sense, reflects the duration of the ventricular action potential.Īccurately measuring the QT interval is challenging. The QT interval extends from the onset of the QRS complex to the end of the T wave. Zipes MD, in Braunwald's Heart Disease: A Textbook of Cardiovascular Medicine, 2019 The QT Interval A very short QTc interval (<390 msec) is unusual, and the rare patient with the short QT syndrome is also at risk for malignant ventricular arrhythmias. Patients with a prolonged QTc, whether congenital or acquired, may be at risk for torsades de pointes ventricular tachycardia ( Chapter 59). In these patients, the JT interval (from J point to the end of T wave) may be a more accurate index of repolarization, but normal standards have not been established. Patients with widened QRS complexes frequently have prolonged QT and QTc intervals. The QT interval is sensitive to drug effects as well as to electrolyte and metabolic derangements. The upper limit of a normal QTc is somewhat debatable, but a cutoff of 0.45 second (450 msec) in men and 0.46 second (460 msec) in women is generally used. The QTc in a given patient may vary somewhat during the course of the day and tends to be slightly longer in young and middle-aged women than in men. If the T wave “merges” into the U wave without reaching the isoelectric baseline, the U wave is included in the QT (or QTU) interval. If the isoelectric baseline is reached between the T and U waves, the U wave is not generally included in the QT interval. The presence of a U wave complicates measurement of the QT (and therefore QTc) interval it may be unclear where the T wave ends and whether the U wave should be included in a QTU interval. May be more accurate than Bazett’s formula in atrial fibrillation and better at predicting adverse outcomes, 4 but it has not achieved widespread clinical use.
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