Definition

  QT interval encompasses the ventricular depolarization and the following repolarization.

  QTc is the corrected QT interval.



How to measure the QT interval?

  QT interval starts with the onset of QRS complex. In case where there is no Q or q wave, the onset of R wave is the onset of the QT interval. End of the T wave is also end of the QT interval. End of the T wave is the point where the descending limb of the T wave intersects the isoelectric baseline.

  Standard ECG recording (at a calibration of 10mm/mV and at a paper speed of 25 mm/sec) is generally adequate for QT interval measurement.



Which lead is selected for QT interval measurement?

  For analysis, include only the leads where the end of T wave is distinct.

  Among them, the lead with the longest QT interval must be noted.

  Generally leads II, V5 or V6 give the best results.



What to do if a U wave follows the T wave?

  Some investigators suggest that U wave must not be included for the analysis. In such a case, they advocate to use leads without U waves OR that the downslope of the T wave be extended by drawing a tangent to the steepest portion of the downward limb of the T wave until it crosses the baseline.

  Some others suggest that U wave is a part of the ventricular repolarization and therefore must be included in the measurement of QT interval.



Suggested Bazett-corrected QTc values for diagnosing QT interval prolongation
  For ages between 1-15 years
Normal: < 440 msec
Borderline: 440-460 msec
Prolonged: > 460 msec.

  For adult man
Normal: < 430 msec
Borderline: 430-450 msec
Prolonged: > 450 msec.

  For adult woman
Normal: < 450 msec
Borderline: 450-470 msec
Prolonged: > 470 msec.



References

  Clinical Arrhythmology and Electrophysiology: A Companion to Braunwald's Heart Disease. 2nd edition. 2012.


  J Am Coll Cardiol 2008;51:2291-2300.