Background: Little is known about electrical remodeling of the native conduction systems,
particularly how the PR interval changes, after cardiac resynchronization therapy (CRT). We
investigated the effects of CRT on the intrinsic PR interval (i-PRi) and QRS duration (i-QRSd).
Methods and results: In 100 consecutive CRT recipients with sinus rhythm and long-term followup
(>1 year), the i-PRi and i-QRSd were measured at baseline and at the last echocardiographic
follow-up (33.4 ± 17.9 months) with biventricular pacing temporarily withdrawn. The relative
decrease in the left ventricular end-systolic volume (LVESV) was measured to define CRTresponders
(≥15%) and super-responders (≥30%). Following CRT, the left ventricular (LV) ejection
fraction increased significantly (p < 0.001). In CRT-responders (n = 71), the LVESV and i-QRSd
decreased markedly (170 ± 39 to 159 ± 24 ms, p = 0.012). However, the i-PRi was not shortened with
CRT response and was actually likely to increase, even in the super-responder group (n = 33).
Moreover, lengthening of the i-PRi was observed consistently irrespective of the CRT response
status, beta-blocker use, or amiodarone use. CRT non-responders were associated with a remarkable
PR prolongation (p = 0.005) and QRS widening (p = 0.001), along with positive ventricular
remodeling. Conclusion: LV volume and i-QRSd decreased markedly with CRT response.
However, the i-PRi was not shortened, but rather increased regardless of the degree of CRT
response. CRT non-response was associated with a considerable increase in the i-PRi and i-QRSd,
along with positive ventricular remodeling. CRT-induced electrical reverse remodeling might occur
preferentially in the intraventricular, but not the atrioventricular, conduction system.