The aim of this study was to investigate whether anatomical characteristics of the membranous septum seen on pre-TAVR CT would predict conduction abnormalities and the need for PPM implantation following TAVR. Specifically, we examined calcium in the basal ventricular septum and the length of the membranous interventricular septum. We found that calcium in the basal septum is present in 1 out of 5 patients undergoing TAVR and is predictive of new conduction abnormalities. In such cases, a more aortic deployment of the prosthesis may be warranted since 3 out of 4 patients who developed a new LBBB or required a PPM in this group were in the highest ID quartile (ID > 7.6 mm). Conversely, the risk of new conduction abnormalities was < 10% with ID < 7.6 mm (quartiles 1–3). Therefore, anatomical characteristics seen on pre TAVR CT may result in procedural modifications that can mitigate the risk of conduction abnormalities.
We found a 15% absolute difference in rates of PPM and LBBB with negative (< 0 mm) delta MSID values, but these differences did not reach statistical significance for the association of MS length or delta MSID with conduction abnormalities or the need for PPM implantation. We believe there are several possible explanations for this finding which may be at variance with previously published literature . First, while manually obtaining double oblique views as described has the advantage of demonstrating the true long axis of the LVOT, measuring the MS like we did in our study, from the virtual aortic basal ring to the inferior edge of the membranous septum, has the disadvantage that it may underestimate the true length of the membranous septum. Second, due to a small sample size the study might have lacked statistical power to demonstrate a true association between MS length and conduction abnormalities. Finally, prior studies have shown that the risk of PPM implantation with the Edwards SAPIEN 3 valve can be reduced by utilizing a higher, or more aortic, implant height . One small study suggested implanting the S3 valve no deeper than 8 mm into the LVOT while a larger study suggested maintaining an aortic percentage > 70% . The median implant depth in our study was more aortic than in previous studies (76% aortic or 4.9 mm of stent frame in the LVOT with an interquartile range of 3.9–7.6 mm). Thus, the higher implant height in our study may have decreased the interaction between the valve and conduction system and reduced the effects of short MS length and delta MSID on the risk of PPM implantation. It should be noted that the interaction of ID and the outcome of PPM or LBBB was significant for the non-coronary cusp but not the left coronary cusp. This further suggests importance of the LVOT anatomy below the non-coronary cusp (i.e. the membranous septum) in the role of conduction abnormalities following TAVR.
Conduction disturbances following TAVR are thought to occur from direct compression of the conduction system by the TAVR prosthesis . While such compression may occur anywhere along the interface of the prosthesis and conduction system, it is most likely to occur where the bundle of His emerges into the left ventricular outflow tract which has been demonstrated to be where the interventricular membranous septum meets the muscular septum . Several factors including baseline conduction abnormalities, LVOT anatomy, and procedural outcomes such as valve size and implant depth combine to varying degrees to affect the interaction between the valve and conduction system . Thus being able to balance all of these variables becomes crucial in minimizing the risk of conduction abnormalities.
In a prior study of primarily self-expanding TAVR prostheses (Medtronic CoreValve, Medtronic, Minneapolis, MN, USA) Hamdan et al. found that shorter membranous septal length was associated with increased need for PPM implantation . In the study by Hamdan et al., there was a statistically significant between MS and implantation depth (delta MSID) between patients who did and did not receive PPM following TAVR (− 1.2 ± 4.2 mm vs 3.7 ± 4.3 mm, p < 0.001) . Implant depth is accepted to be an important risk factor for PPM following CoreValve .
Maeno et al. recently showed that shorter MS length predicted PPM implantation following TAVR with the SAPIEN 3 valve . One possible explanation for the observed differences in outcomes from our study could be differences in procedural factors including ID and the degree of prosthesis oversizing . While it is difficult to definitively prove this theory without analyzing patient-level data it does appear that the ID was, in general, more ventricular in the group of patients from Maeno (mean ID 7 ± 2.3 mm in PPM group vs 5.2 ± 1.3 mm in the no PPM group) compared to our study (5 mm (IQR: 4–9) in the PPM group versus 4 mm (IQR: 3–7) in the no PPM group, p = 0.21). While the study by Maeno et al. emphasizes the importance of MS length in regards to PPM implantation, our study finds that this factor may be modifiable with a more aortic deployment.
Our study highlights several key issues regarding conduction abnormalities following TAVR with several limitations. While our principal finding of an increased rate of LBBB in patients with calcium in the basal interventricular septum undergoing TAVR with the S3 valve is notable, we showed a numerical but not statistically significant association of the interaction between membranous interventricular septum length and conduction abnormalities. The reasons for these differences may be multifactorial and include different measurements techniques of the MS length, TAVR implantation practices, and small sample size (type II error). We chose the oblique coronal view because it shows the true long axis of the LVOT, but it also may underestimate the MS length when measured from the virtual basal ring to the inferior border of the membranous septum. It should be noted that the membranous septum is a three dimensional (3D) structure and none of the measurements truly captures the 3-D essence of the MS anatomy.