Graduate Student University of Toronto East York, Ontario, Canada
Background: Approximately 70% of heart failure (HF) patients have a narrow QRS duration (QRSd) at rest and are not eligible for CRT. In the general population, rate-related bundle branch block (RR-BBB) is rare ( < 1%). The prevalence and prognosis of RR-BBB in HF has not been defined. Likewise, the ability of CRT devices to detect RR-BBB in order to trigger biventricular pacing has not been studied. Therefore, we conducted this study to determine the prevalence of RR-BBB in HF patients, evaluate their cardiac outcomes, and assess whether CRT devices can detect changes in QRSd.
METHODS AND RESULTS:
Methods: We reviewed ECGs from ambulatory HF patients (LVEF < 50%, baseline QRSd ≤120ms) undergoing exercise testing. QRSd was measured at rest and peak exercise from scanned ECGs using digital calipers (3x magnification). RR-BBB was defined as an increase in QRSd >10ms at peak exercise to >120ms. Clinical outcomes were assessed for 2 years after exercise testing. In 30 consecutive CRT patients, surface QRSd and the intracardiac LV-RV interval were simultaneously measured at baseline and during atrial pacing at 100 bpm.
Results: 173 of the 811 HF patients screened met eligibility criteria. Among eligible patients 25 (14%) exhibited RR-BBB during exercise. Age, LVEF, and cardiomyopathy etiology were similar between the groups (Table). RR-BBB patients showed greater baseline QRSd, larger QRSd changes with exercise, and more significant left axis shifts. Over a mean follow-up of 2 years, 40% of RR-BBB patients experienced cardiac events (20% annual rate - including 6 HF hospitalizations, 3 LVAD/heart transplants, 1 cardiac death). Among the 30 CRT patients, 4 (13%) manifested changes in QRSd (△25±19ms) with atrial pacing at 100 bpm that were tracked by the sensed LV-RV interval (△29±20ms). Calibration of the change in LV-RV interval using the baseline QRSd allowed accurate quantification of the QRSd with atrial pacing (r =0.96, p=0.037).
Conclusion: RR-BBB during exercise occurred in 14% of ambulatory HF patients (14-fold higher than general population) and was associated with moderate cardiac morbidity. CRT devices can accurately track rate-related changes in QRSd from the LV-RV interval. Future studies should investigate the utility of demand-based CRT during exercise in HF patients with RR-BBB.
