Last update: April 2020


Definition

  QT interval encompasses ventricular depolarization + repolarization.

  QTc is the corrected QT interval.



Why do we measure QT interval?

  To search for genetic mutations carrying a high risk of arrhythmia and sudden cardiac death.

  To search for electrolyte abnormalities.

  To evaluate drug-induced cardiotoxicity.



How do we 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 the 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 the 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.



Fever and QT interval measurement

  Fever shortens the QT interval in healthy patients, irrespective of the correction formulae used.

  It has been reported that fever prolongs the QT interval in patients with long QT syndrome type 2 and such patients may be at increased risk for ventricular arrhythmias during fever.

  The presence or absence of fever must be noted when evaluating the QT prolongation effects of certain drugs.



Bundle branch block (BBB) and QT interval measurement

  It has been reported that the Bazett's formula should not be used in combination with LBBB correction formulae.



Atrial fibrillation and QT interval measurement

  The irregular RR intervals in Atrial Fibrillation makes QTc estimation highly variable due to the influence of instantaneous preceding RR interval as well as the average heart rate.

  It has been reported that not Bazett's formula but Fridericia formula best correlates with that of sinus rhythm and has the least rate variation.



References

  PLoS Med. 2020 Mar; 17(3): e1003040. doi: 10.1371/journal.pmed.1003040.

  J Electrocardiol 2019;56:70-76.

  Canadian Journal of Cardiology 2019;35:855-865.

  J Am Coll Cardiol 2018;72:e91-e220 doi: 10.1016/j.jacc.2017.10.054.

  JACC Clin Electrophysiol 2017;3:756-766.

  J Electrocardiol 2017;50:570-575. J Am Coll Cardiol EP 2017;3:756–66

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

  Neth Heart J 2010;18:165-169.

  Circ Arrhythm Electrophysiol 2008;1:202-208.

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