Ventricular conduction defects associated with a widened QRS complex—specifically, complete and incomplete bundle branch blocks—may artificially elongate the QT interval without reflecting an actual increase in myocardial repolarization time (1).
In complete bundle branch block, the widened QRS complex may elongate the QTc interval by as much as 16% while having no effect on the JT index, defined as JT interval x (heart rate+ 100)/518), where an index >112ms is considered to be prolonged (2).
Calculation of the JT index has been suggested for patients with incomplete bundle branch block, as well (2).
Put simply, in patients with ventricular conduction defects associated with a widened QRS complex, the JT index appears to be superior to the QTc interval for assessment of repolarization time.
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1. Salik J, Muskin P. Consideration of the JT interval rather than the QT interval. Psychosomatics 2013; 54(5): 502. https://www.ncbi.nlm.nih.gov/pubmed/24034531
2. Zhou SH, Wong S, Rautaharju PM, et al. Should the JT rather than the QT be used to detect prolongation of ventricular repolarization? An assessment in normal conduction and in ventricular conduction defects. J Electrocardiology 1992; 25 (Suppl): 131-6. https://www.ncbi.nlm.nih.gov/pubmed/1297679
Contributed by Jonathan Salik, MD, Mass General Hospital, Boston, MA