Introduction: Reduced septal work is a main feature of left bundle branch block (LBBB) and considered as a target for cardiac resynchronization therapy (CRT). We hypothesized that septal contractile function in LBBB is modified by crosstalk with the left ventricular (LV) lateral wall.
Purpose: To test the hypothesis that reduced LV lateral wall contractility leads to recovery of septal work in LBBB.
Methods: In 10 anaesthetized dogs we induced LBBB by radiofrequency ablation and occluded the circumflex coronary (CX) artery to reduce LV lateral wall contractility. Septal and LV lateral wall segment lengths were measured by sonomicrometry and regional work calculated as the area of the pressure-segment length loop. Work performed during counterclockwise rotation of the loop was defined as positive, whereas work performed during clockwise rotation of the loop was defined as negative (figure).
Furthermore, we used speckle-tracking echocardiography to study 24 LBBB patients referred for CRT implantation; 8 patients with LV lateral wall scar and 16 patients with non-ischaemic cardiomyopathy. There was no difference in LV ejection fraction between the two groups. Using a previously validated method for non-invasive estimation of LV pressure, regional work was calculated by pressure-strain analysis.
Results: Induction of LBBB caused characteristic regional work distribution with high values of LV lateral wall work and low values of septal work in all animals. CX occlusion, however, resulted in a major loss of LV lateral wall work, which declined from 417±84 (mean±SD) to 74±65 mmHg·mm (p<0.001). This was followed by a marked increase in septal work from 5±62 to 108±47 mmHg·mm (p<0.001) (figure).
Results from the clinical study resembled findings from the experimental study. In patients with non-ischaemic cardiomyopathy LV lateral wall work was 3144±1425 as compared to 1146±836 mmHg·% in patients with LV lateral wall scar (p<0.01). On the other hand, septal work was only 272±922 in non-ischaemic cardiomyopathy patients as compared to 1722±851 mmHg·% in LV lateral wall scar patients (p<0.01) (figure).
Conclusions: In LBBB, septal function is markedly improved or normalized in hearts with LV lateral wall dysfunction. Since recovery of septal function is one of the main mechanisms of improved LV function with CRT, hearts with lateral wall infarcts may have limited potential for response.