Intervalle QT. 3b. QRS larges

L’intervalle QT s’allonge en cas de trouble de conduction intraventriculaire (QT = durée du QRS + intervalle JT). L’intervalle QT ne reflète plus le risque artyhmogène s’il n’est pas corrigé par la durée du QRS.

Des formules proposées récemment proposent de soustraire la durée du QRS dans le calcul du QTc (devenu JTc) ou de comparer l’intervalle J-T (intervalle Q-T – durée du QRS) avec de nouveaux standards [2]. On retiendra qu’un risque artyhmogène existe si le JTc (QTc – QRS) est > 400 ms (mais d’autres formules existent).

Comment mesurer le QT ? video P. Taboulet

Comment interpréter un intervalle QT ? 

Formules de Rautaharju: Reduction of Variance-Related Parameters in Normal Conduction and in Ventricular Conduction Defects by Adjustment Functions for JT and QT [3] 

JTRR*  = JT – 155 x (60/HR – 1) + k; k = 34 ms for men and 22 ms for women

QT(RR,QRS) = QT – 155 x (60/HR – 1) – 0.93 x (QRS – 139) + k; k -22 ms for men, -34 ms for women

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Biblio Qtc et QRS larges

Das G. QT interval and repolarization time in patients with intraventricular conduction delay. J Electrocardiol 1990;23:49–52.2.

Spodick DH. Reduction of QT-interval imprecision and variance by measuring the JT interval. Am J Cardiol 1992;70:628–629.3.

[3] Rautaharju PM, Zhang ZM, Prineas R, et al. Assessment of prolonged QT and JT intervals in ventricular conduction defects. Am J Cardiol 2004;93:1017–1021.

Chiladakis J, Kalogeropoulos A, Arvanitis P, et al. Heart rate-dependence of QTc Intervals assessed by different correction methods in patients with normal or prolonged repolarization. Pacing Clin Electrophysiol 2010;33:553–560.

Bogossian H et al. A new experimentally validated formula to calculate the QT interval in the presence of left bundle branch block holds true in the clinical setting. Annals of noninvasive electrocardiology 2016 (coming)

–> The newest formula to evaluate QT interval in the presence of LBBB suggests: modified QT during LBBB = measured QT interval minus 50% of LBBB duration.

Vandenberk B, et al. Which QT Correction Formulae to Use for QT Monitoring? J Am Heart Assoc. 2016;5(6).

« Fridericia and Framingham correction formulae showed the best rate correction. Bazett overestimated the number of patients with potential dangerous QTc prolongation »

Tabatabaei P, et al. Assessment of QT and JT Intervals in Patients With Left Bundle Branch Block. Res Cardiovasc Med. 2016;5(2):e31528.

« JTc, as an index of repolarization, is independent of ventricular depolarization. Therefore, it can be applied for predicting QTc in patients with LBBB. »

Piotrowicz K, et al. Repolarization duration in patients with conduction disturbances after myocardial infarction. Am J Cardiol. 2007 Jan 15;99(2):163-8.

« QT(RR,QRS) and JT(RR) intervals reflect better than QTc repolarization duration in patients with conduction disturbances »

Crow RS et al. Prognostic significance of corrected QT and corrected JT interval for incident coronary heart disease in a general population sample stratified by presence or absence of wide QRS complex: the ARIC Study with 13 years of follow-up. Circulation. 2003;108(16):1985-9

« The JTc is a simple measurement that is a significant independent predictor of incident CHD events in men with wide QRS complex »

Yankelson L, Hochstadt A, Sadeh B, et al. New formula for defining « normal » and « prolonged » QT in patients with bundle

branch block. J Electrocardiol. 2018 May – Jun;51(3):481-486.

« In patients with LBBB, replacing of the QRS duration after deriving the QTc interval with a fixed value of 88 ms for female and 95 ms for male provides a simple and reliable method for predicting the QTc before the development of LBBB. »