Assessment of QT interval in ventricular paced rhythm: Derivation of a novel formula.

OBJECTIVE

The objective of the study was to determine the optimal formula to estimate QT interval adjusting for QRS prolongation during right ventricular (RV) pacing.

METHODS

This observational study included individuals (n = 43) with a newly implanted permanent ventricular pacemaker, who had a narrow QRS complex before pacemaker insertion. QT interval with RV pacing was related to QT interval before pacemaker implantation. The validation cohort (n = 442) had permanent RV pacing in DDD mode.

RESULTS

A new QTc formula was derived utilizing the constants from the relationship between the spline heart rate QT correction (QTcRBK) before and after pacing; specifically, QTcRBK = QTcRBK × 0.86. The JT interval from paced complexes was highly heart rate (HR) dependent and was not accurate for QT assessment. Previous, QTc formula for paced complexes were not highly correlated with QT before pacing unless a robust HR correction is added. Formulae subtracting a fixed amount from QTc markedly overestimated QTc before pacing.

CONCLUSION

We proposed a new, simple formula for QT estimation in RV pacing. JT interval in paced complexes is highly HR dependent and is not accurate for QT assessment. The new spline approach for HR correction for the QT, once incorporated into some previously proposed formulae, blunts HR dependency and improves prediction of QT before pacing. QTcRBK0.86 and QTcRBK - (QRS0.5) demonstrated the best balance of relatively strong correlation to QTc before pacing and accurate QTc prolongation identification. Abnormal QT for QTcRBK*0.86 as defined by the 97.5th and 99th percentile are 469 and 479 ms respectively.