Previous studies have shown that MS length CT measurements before TAVR are useful in predicting conduction abnormalities; however, there have been no reports on the relationship after SAVR. This is the first study to show that the short MS length measured by cardiac CT is associated with the incidence of PPMI/LBBB after SAVR.
The atrioventricular (AV) node, located at the apex of the triangle of Koch, continues to the His bundle piercing the MS and penetrating via the AV bundle to the left ventricular bundle branch, where it is relatively exposed and less insulated [11]. Kawashima and Sasaki reported that a so-called “naked bundle” may occur when the AV bundle passes within the MS, in 21.0% of their series [12]. However, the typical path of the AV bundle was noted to be along the lower border of the MS in approximately 46.7% of cases and within the muscular septum but close to the MS in the remainder. Therefore, in patients with a short MS, the left bundle branch may be closer to the aortic annulus, increasing the likelihood of conduction system injury during SAVR procedure.
In SAVR, mechanical trauma associated with debriding the native valve and its annulus, deeply placed annular sutures, and placing a large bioprosthesis into a small aortic annulus may lead to conduction system injury. According to the Fukuda et al., the most common reason for conduction injury was traumatic injury from suture material as opposed to calcific infiltration or pressure from the prosthesis. In addition, the most commonly injured structures were the AV node and left bundle branch [9, 13]. In our study, we found that patients with bicuspid aortic valves undergoing SAVR did not have an increased risk of PPMI, supporting the notion that invasiveness and distribution of calcium does not play a major role in increasing the risk of conduction injury [14, 13].
The larger LV mass index was also a predictor of new PPMI/LBBB in the present study. There was no significant correlation between the LV mass index and the MS length (r = -0.105, p = 0.241). LV hypertrophy may be correlated with conduction system disease that may contribute to susceptibility of SAVR-induced conduction disease.
The incidence of PPMI/LBBB after SAVR from previous reports and the present study are shown in Table 5. The incidence of new PPMI/LBBB was relatively low in the present study (2.4%/4.0%) compared to previous reports [15, 16]; this could be related to relatively high-SAVR volume SAVR operators at our center as well as a higher threshold for PPMI. Reported risk factors for PPMI after SAVR are preoperative conduction abnormality, age, severe LVEF dysfunction, the combination of AS and aortic insufficiency, prior valve surgery, and multivalve surgery that included the tricuspid valve [17, 14, 18]. Similar to Levach and colleague’s analysis of 5807 SAVR patients, our study did not identify these as risk factors for postoperative PPMI [13]. However, we showed novel independent predictors of short MS length and large LV mass index.
Table 5
Summary of Previous Studies Investigating the Incidence of Permanent Pacemaker after Surgical Aortic Valve Replacement
| Author | Study design | Reported year | Patient number | At the time of evaluation | Incidence of PPMI |
| Thyregod et al. [3] | RCT | 2015 | 135 | 30 days/12 months | 1.6%/2.4% |
| Leon et al. [1] | RCT | 2016 | 1021 | 30 days/12 months | 6.9%/8.9% |
| Reardon et al. [2] | RCT | 2017 | 796 | 30 days | 6.6% |
| Mack et al. [4] | RCT | 2019 | 454 | 30 days/12 months | 4.1%/5.5% |
| Popma et al. [5] | RCT | 2019 | 678 | 30 days/12 months | 6.1%/6.7% |
| Present study | Retrospective cohort | 2020 | 126 | 4.0 (4.0–5.0) days | 2.4% |
| Author | Study design | Report year | Patient number | At the time of evaluation | Incidence of LBBB |
| El-Khally et al. [20] | Retrospective cohort | 2004 | 262 | 54 (36–83) months | 6.4% |
| Khounlaboud et al. [21] | Retrospective cohort | 2017 | 547 | After surgery | 4.6% |
| Mack et al. [4] | RCT | 2019 | 454 | 30 days/12 months | 8.0%/8.0% |
| Tang et al. [26] | RCT | 2019 | 678 | 12 months | 11.7% |
| Present study | Retrospective cohort | 2020 | 126 | 4.0 (4.0–5.0) days | 4.0% |
| LBBB, left bundle branch block; PPMI, permanent pacemaker implantation; RCT, randomized controlled trial |
When comparing our cohort of 53 short MS (< 1.5 mm) and 73 long MS (≥ 1.5 mm) subjects, the rates of new PPMI/LBBB (13.2% vs. 1.4%, p = 0.010) and PPMI alone (5.7% vs. 0%, p = 0.072) were notably higher in the short MS group. This important observation identifies a cohort of at-risk patients in whom technical aspects may be particularly relevant to reduce the risk of conduction abnormality; such patients may demand additional attention in the avoidance of direct injury by annular sutures or indirect injury due to tissue tension caused by pledgeted suture.
The incidence of new-onset LBBB after valvular surgery is under reported compared with PPMI. LBBB has been suggested as a marker of poor long-term prognosis, especially in patients with cardiac disease [19] though this remains controversial [20–22]. The rate of new-onset LBBB after TAVR at discharge ranged from 13.3–37% [23]. A recent meta-analysis demonstrated an increased risk of all-cause death at 1-year in patients with new-onset LBBB (RR 1.32, 95% CI 1.17–1.49; p < 0.001; I2 = 49%) from 12 studies (7792 patients). The presence of new-onset LBBB was also associated with a higher risk of 1-year cardiac death (RR 1.46, 95% CI 1.20–1.78; I2 = 10%) from eight studies (5906 patients). Overall, 1-year all-cause death ranged from 6.3–27.8% and from 4.9–28.4% in the absence or presence of new-onset LBBB, respectively [24].
The RR for 1-year heart failure hospitalization from 6 studies (3435 patients) was 1.35 (95% CI 1.05–1.72; P = 0.02; I2 = 8%) in new-onset LBBB patients. The risk of new PPMI was also high at1-year in new-onset LBBB patients (RR 1.89, 95% CI 1.58–2.27; p < 0.001; I2 = 71%) [24].
The incidence of new PPMI also contributes to hospital, length of hospital stay, costs and specific complications including lead related, pocket related, generator related, and pacemaker-induced cardiomyopathy [25]. This study suggests that routine pre-op CT may be useful to decrease post-op PPMI by identifying high-risk cases that require special attention and careful techniques during surgery.
The limitations of this study include the relatively small sample size, which made it underpowered for the prediction of new PPMI or for the elucidation of specific technical aspects, such as different suture techniques.