Effect of QRS width on QT interval

Cicy Bastian, Raihanathul Misiriya Kamarudheen Jameela, Sudhayakumar N





In the surface ECG, ventricular depolarization is represented by the QRS and ventricular repolarization by the ST and T which together forms the JT interval. When the QRS duration is normal,  we take QT interval as a measure of repolarization. But when there is QRS widening, what happens to this QT interval? If there is prolongation of the QT , is it due to prolongation of depolarization alone or  prolongation of repolarization?  We tried to find out this by analyzing 50 cases of intermittent narrow and wide QRS.


50 patients with intermittent right bundle branch block (RBBB), pacing in Sick sinus syndrome (SSS), pacing in brad arrhythmias during Myocardial infarction (MI), and intermittent preexcitation were included in the study. Patients with pre existing broad QRS were excluded from the study. Simultaneous  single channel 12 lead  ECG was taken during narrow QRS and broad QRS . QT interval was calculated as the mean QT of six leads in which QT could be easily measured.  JT interval was  calculated by subtracting QRS duration from QT interval. Corrected QT interval (QTc) was calculated by Bazett’s formula.  From this corrected JT ( JTc) was also calculated. All the measurements were taken during narrow QRS rhythm and broad QRS rhythm.


As the QRS width prolonged to 110 ms or more , QT and QTc were also prolonged. There was no change in JT and JTc.


JT and JTc intervals are more reliable than QT and QTc intervals  in case of wide QRS.

Key words

Broad QRS, JT interval ,QRS width , QT interval.

Full Text:




kautzener J. QT interval measurements : Card. Electrophysiol. Rev. 2002 Sep; 6 (3): 273 – 7

Salim M A , Case C L , Gillette P C . The JT interval as a depolarization independent measure of repolarization : lessons from catheter ablation of the Wolff – Parkinson – White syndrome . Pacing Clin. Electrophysiol. 1995 ; 18 : 2158 – 2162 .

Bazette H C . An analysis of the time relations of electrocardiogram. Heart . 1920; 7:53 – 70


Zhou S. H, Wong S,Rautaharaju P M, Karnik N, Calhoun H P. Should the JT rather than the QT interval be used to detect prolongation of ventricular repolarization ? An assessment in normal conduction and in ventricular conduction defects. J Electrocardiol 1992; 25: Suppl: 131 – 6

Das G. QT interval and Repolarization time in patients with intraventricular conduction. Indian pacing and electrophysiology journal ( ISSN 0972 – 6292), 4(4): 156 – 161 (2004).

Kenny R A , Sulton R. The prolonged QT interval : a frequently unrecognized abnormality. Post graduate Med J. 1985; 61: 279 – 286 .

Richard S Crow H D , Peter J Hannan, Mstat, Aaron R Folsom , M D : increased QRS duration ( QRSd) contributes to QT interval prolongation. Circulation 2003; 108: 1985 – 1989


  • There are currently no refbacks.