Abstract 3526: Biphasic Preejection Septal Deformation in Left Bundle Branch Block -“Tug Of War” Between Early-activated Septum and Late-activated Lateral Wall
Introduction: Abnormal septal motion in LBBB has been assumed to represent passive motion related to reversal of the end-diastolic left-to-right transseptal pressure gradient (TSG). Recently, we demonstrated contribution of active septal contraction to this motion during preejection. We now investigate how active septal and LV free wall contractions contribute to septal deformation in LBBB.
Methods: In 8 anesthetized dogs with ventricular manometers we measured myocardial deformation (strain) by sonomicrometry and echocardiography and electrical conduction time by intra-myocardial EMG. LBBB was induced by RF-ablation.
Results: During LBBB electrical activation of the LV free wall was delayed 51±4 ms (±SEM) relative to septum. The free wall segment was stretched 2.2±0.5% prior to onset of shortening. Preejection septal deformation was biphasic with 6.1±4.5% (p<.01) shortening followed by 3.9±2.8% (p<.01) lengthening which continued into the ejection phase (Fig⇓). The interruption of septal shortening (ISS) coincided with onset of free wall shortening (4±3 ms after). At this time the radius of curvature was increased for septum (35±5%), but reduced for the free wall (−6±2%, p<0.1), implying higher wall stress in septum.
Conclusions: In LBBB, septal preejection deformation is biphasic with active shortening followed by lengthening. The results suggest that the septal wall lengthens due to forces generated in the late-activated freewall. Presystolic stretching increases free-wall force development via the Frank-Starling mechanism, whereas increased septal radius of curvature increases septal wall stress, and both mechanisms contribute to septal lengthening.