QT interval estimation in patients with right bundle branch block using validated formulas for left bundle branch block
Authors: Garcia Gomez S , Garcia Izquierdo Jaen E , Mingo Santos S , Martinez Mingo A , Toquero Ramos J , Castro Urda V , Jimenez Sanchez D , Aguilera Agudo C , Morillo Diaz JD , Veloza Urrea D , Segura Dominguez M , Portoles Hernandez A , Garcia-Rodriguez D , Remior P , Fernandez Lozano I 
Published: May 2021

Abstract

Funding Acknowledgements
Type of funding sources: None.

BACKGROUND
Adequate measurement of the QT interval is of paramount importance in order to identify patients at higher risk for ventricular arrhythmias. Previous studies have described different methods to estimate baseline QT in patients with left bundle branch block (LBBB). However, the evidence regarding the assessment of QT interval in the setting of right bundle branch block (RBBB) is scarce.

PURPOSE
We aimed to analyze the feasibility and accuracy of the different formulas described for LBBB in the estimation of the QT interval in RBBB.

METHODS
We enrolled patients who underwent left sided electrophysiologic procedures. All patients were in sinus rhythm and had narrow QRS. Pacing was performed from the left atrial appendage for baseline measurements, and from the left aspect of the interventricular septum (selective capture of the left bundle was attempted) to measure RBBB QT and QRS. Pacing cycle length was 800 ms or slightly below patients´ intrinsic rhythm at both locations. Measurements were performed manually (using digital calipers) according to current recommendations and corrected using Bazett. Validated formulas for LBBB QT considered are described in table 1.

RESULTS
50 patients (42 cryoballoon pulmonary veins isolation (PVI), 4 radiofrequency PVI, 4 concealed left accessory pathways). 70% were male. Mean age was 62 ± 11 years old. Left ventricle ejection fraction was 58 ± 10%. 66% and 60% of the patients were taking betablockers and antiarrhythmic drugs, respectively. Mean pacing cycle length was 707 ± 99 ms. Baseline measurements: QRS 95 ± 10, QT 391 ± 36, QTc 467 ± 39 ms. RBBB measurements: QRS 165 ± 21, QT 448 ± 46, QTc 531 ± 52 ms. Correlations between baseline and estimated QTc were good for all the formulas (table 1). Reliability analysis showed that both Yankelson and Wang methods had the highest intraclass correlation coefficients (ICC) when trying to estimate baseline QTc.

CONCLUSIONS
Previously described formulas for LBBB exhibit marked differences regarding reliability in the estimation of QTc interval in the setting of RBBB. According to our results, Yankelson’s method shows the most consistent agreement when estimating baseline QTc interval in patients with RBBB. Table 1.LBBB METHODFormula to estimate baseline QTcPearson’s R correlation coefficientCI (95%)Intraclass correlation coefficientCI (95%)YankelsonQTc - QRS + 95 (m) or 88 (f)0.805(0.632-0.977)0.882(0.788-0.934)Bogossian**QT - (QRS/2)0.813(0.644-0.982)0.756(-0.127-0.919)Wang**QT - (0.86*QRS - 71)0.801(0.627-0.974)0.834(0.465-0.930)Tang-Rabkin0.945*QTcRabkin - 260.722(0.521-0.923)0.711(0.019-0.885)RautaharjuQT - 155*(60/heart rate - 1) - 0.93*(QRS - 139) - 22 (m) or - 34 (f)0.780(0.599-0.961)0.105(-0.017-0.381)**Bogossian and Wang required additional HR correction (Bazett).  Abstract Figure 1. Bland-Altman