Transseptal Conduction as an Important Determinant for Cardiac Resynchronization Therapy, as Revealed by Extensive Electrical Mapping in the Dyssynchronous Canine Heart

Abstract

Background-Simple conceptual ideas about cardiac resynchronization therapy assume that biventricular (BiV) pacing results in collision of right and left ventricular (LV) pacing-derived wavefronts. However, this concept is contradicted by the minor reduction in QRS duration usually observed. We investigated the electric mechanisms of cardiac resynchronization therapy by performing detailed electric mapping during extensive pacing protocols in dyssynchronous canine hearts. Methods and Results-Studies were performed in anesthetized dogs with acute left bundle-branch block (LBBB, n=10) and chronic LBBB with tachypacing-induced heart failure (LBBB+HF, n=6). Activation times (AT) were measured using LV endocardial contact and noncontact mapping and epicardial contact mapping. BiV pacing reduced QRS duration by 21±10% in LBBB but only by 5±12% in LBBB+HF hearts. Transseptal impulse conduction was significantly slower in LBBB+HF than in LBBB hearts (67±9 versus 44±16 ms, respectively), and in both groups significantly slower than transmural LV conduction (≈30 ms). In both groups QRS duration and vector and the epicardial AT vector amplitude and angle were significantly different between LV and BiV pacing, whereas the endocardial AT vector was similar. During variation of atrioventricular delay while LV pacing, and ventriculo-ventricular delay while BiV pacing, the optimal hemodynamic effect was achieved when epicardial AT and QRS vectors were minimal and endocardial AT vector indicated LV preexcitation. Conclusions-Due

Detection of Incomplete Left Bundle Branch Block by Noninvasive Electrocardiographic Imaging

“…The induced dyssynchrony seen here was similar to a previous porcine and canine models [7,8], and recapitulates abnormalities found in patients with electrical cardiac dyssynchrony. That is, the QRSi produced an average increase of 81% from the mean baseline QRS width in complete LBBB, and 39% for incomplete.…”

“…Activation times (ATs) were derived from the recorded and ECGi reconstructed potentials as the time of maximum negative derivative. A number of markers have been developed in conjunction with ECGi to describe quantitatively the electrical dyssynchrony within the heart [6,7]. In this study, the following have been used: Ventricular electrical uncoupling (VEU): the difference in mean activation of the LV and RV free wall.…”

Speckle-Tracking Echocardiography Elucidates the Effect of Pacing Site on Left Ventricular Synchronization in the Normal and Infarcted Rat Myocardium

“…In terms of mechanical synchrony our results demonstrate that LV-only pacing has favorable outcome which is almost similar to the effect of BIV pacing. While many studies support the notion that LV-only pacing may have beneficial effects in HF patients [26], recent data in children with normal cardiac function and complete AV block demonstrated striking similarity to the results of our rat model in regard to LVonly pacing [27]. Moreover, LV-only pacing appears to be associated with good clinical outcome in this and other studies in the pediatric population [28,29].…”

Delayed Trans-Septal Activation Results in Comparable Hemodynamic Effect of Left Ventricular and Biventricular Endocardial Pacing

“…Our findings of slow trans-septal conduction are strikingly similar to those recently found in canines. 4 Strik et al 4 found a TSAT of 67±9 ms in a canine model of LBBB and pacing induced dyssynchrony compared with 60±21 ms in our patients with LBBB and heart failure. In the same study, there was no incremental benefit in BIV compared with LV-only pacing, and this was due to predominant LV stimulation in both modes due to long TSATs.…”

“…We measured only LV endocardial activation time (and not epicardial) and the reduction may be related to changes occurring in the epicardium as has been shown in the canine model. 4 We saw divergent findings for LV epi and BIV epi pacing with an improvement in AHR with BIV epi pacing over LV epi accompanied by a reduction in both QRSd and endocardial LVTAT. The electric findings may be accounted for by the fact that pacing from the epicardium Figure 1.…”