Septal Bipolar Ablation to Prevent Left Ventricular Outflow Tract Obstruction after Transcatheter Mitral Valve Implantation

Left ventricular outflow tract (LVOT) obstruction is a life-threatening complication of transcatheter mitral valve implantation (TMVI).[1] Alcohol septal ablation may prevent or treat LVOT obstruction.[2] However, the technique relies on septal perforating artery/arteries subtending the culprit septal myocardium and adequate myocardial thickness to avoid iatrogenic ventricular septal defect. We hypothesized that bipolar radiofrequency ablation (SCORPION: Septal bipolar ablation of a nonCORonary segment of ventricular myocardium to Prevent outflow obstructION) could selectively ablate the region of the interventricular septum threatening LVOT obstruction after TMVI and overcome these limitations.

From 2018 to 2020, five patients deemed at risk of LVOT obstruction based on computed tomography measurements of neo-LVOT (<200mm²) and skirt neo-LVOT (<180mm²) (Figure 1A and B) underwent SCORPION in separate session before planned TMVI. Three patients already had baseline LVOT gradient. The median Society of Thoracic Surgeons Predicted Risk of 30-day Mortality was 12.0 (interquartile range [IQR]:6.7–13.1)%. All were women and had multiple comorbidities. Three patients had unsuccessful attempts at alcohol septal ablation due to unfavorable coronary anatomy. The conduct of this study and the waiver of consent were approved by the Institutional Review Board at Emory University. The data that support the findings of this study are available from the corresponding author upon reasonable request.

Figure 1: Septal bipolar ablation of a nonCORonary segment of ventricular myocardium to Prevent outflow obstruction.

(A and B) The virtual valve is implanted in the mitral position. The neo-LVOT is determined by measuring the smallest cross-sectional area between the most ventricular edge of THV and the basal septum (red dotted line), while the skirt neo-LVOT is determined by measuring the area between the most ventricular portion of the THV skirt and the interventricular septum (blue dotted line). A neo-LVOT < 200 mm² and/or skirt neo-LVOT < 180 mm² has been identified as a risk factor for developing LVOT obstruction following TMVI. (C) An active catheter (red) is placed in the left ventricle and a return catheter (green) is placed in the right ventricle. Energy flow between the two catheters allows for targeted, transmural, ablation in the prediction region of “skirt” obstruction. (D) TEE and (E) Electroanatomical mapping guided SCORPION ablation and assessment. Note that multiple ablations are performed (red dots) in the region near the bundle of His (yellow dots). Using TEE, the ablation catheter is positioned approximately 2 cm from the aortic valve—in the predicted region of interaction between the THV skirt and the interventricular septum. (F) Change in neo-LVOT and “skirt” neo-LVOT in each patients.

LVOT = left ventricular outflow tract, TAVR = transcatheter aortic valve replacement, TEE = transesophageal echocardiography, THV = transcatheter heart valve, TMVI = transcatheter mitral valve implantation

Under general anesthesia, electroanatomic mapping first identified ablation targets, with a reference quadripolar catheter advanced in the right ventricular apex. Using retrograde aortic approach, 3.5mm tip irrigated radiofrequency ablation catheter (FlexAbility, Abbott, IL) or 8mm non-irrigated ablation catheter (IntellaNav MiFi XP, Boston Scientific, MN) was inserted into LVOT and served as active ablation electrode for unipolar and bipolar ablation. An 8mm non-irrigated catheter (Blazer II, Boston Scientific, MN) was placed against the opposing right ventricular septum (Figure 1C and 1D) as the return electrode connected to the dispersive skin electrode in unipolar ablation, or used in tandem with the LV catheter for bipolar ablation. This configuration ensured high current density on the septum. Combining echocardiography and the three-dimensional electroanatomic map (Figure 1E), ablation was then performed on the predetermined target site using both a bipolar “kissing catheter” technique as well as unipolar ablation from the retrograde ablation catheter.[3] All ablations were performed at power of 35–40 Watts for irrigated catheters or temperature-limited to 60 degrees for non-irrigated ablation. Ablation lesion count ranged from 9–30, and the mean total ablation time of 22 minutes (ranged from 15–30 minutes). Endpoints for ablation included echolucency, hypocontractility of targeted segments, drop in impedance, and loss of pacing capture at the ablated site.

All patients underwent a technically successful SCORPION procedure. All patients developed complete heart block and required implantation of a permanent pacemaker. No patients developed ventricular septal defect, stroke, cardiac perforation, or major vascular complication. One patient with severe mitral stenosis and hypertrophic cardiomyopathy with existing LVOT obstruction underwent TMVI less than 24 hours following SCORPION because of decompensated heart failure. Despite SCORPION and subsequent anterior mitral leaflet modification (LAMPOON), TMVI increased the LVOT gradient, and the patient died on day seven from shock and multiorgan failure.

Figure 1F shows the predicted neo-LVOT and “skirt” neo-LVOT before and after SCORPION. Four patients discharged from the hospital had computed tomography performed at a minimum of four weeks following SCORPION. In these patients, the predicted neo-LVOT increased from median 0 (IQR 0–0)mm² before SCORPION to median 30 (IQR 0–60)mm² after SCORION (median increase of 30 [IQR 0–60]mm²) and the “skirt” neo-LVOT increased from median 188 (IQR 170–219)mm² to 224 (IQR 203–257)mm² (median increase of 29 [IQR 10–53]mm²). Of the five patients who underwent SCORPION in preparation for TMVI, one was lost to follow-up, one (described above) died from worsening LVOT obstruction following urgent same-admission TMVI, and three underwent successful TMVI with LAMPOON at 51–334 days post SCORPION. Out of these three patients, one had a baseline LVOT gradient that further increased after TMVI.

Despite its technical feasibility, our experience with SCORPION revealed important shortcomings; 1) There was a 100% pacemaker implantation rate, and 2) SCORPION failed to change predicted neo-LVOT area enough to avoid adjunctive LAMPOON at the time of TMVI. This corroborates the finding of a recently reported case series of pre-emptive radiofrequency ablation before TMVI.[4] In their report of four patients who underwent pre-emptive septal ablation, all patients required pacemaker implantation, and the increase in neo-LVOT and “skirt” neo-LVOT was modest, ranging from 8–110mm² and 41–86mm², respectively. In addition, three out of four of their patients remained at risk of LVOT obstruction despite LVOT radiofrequency ablation, similar to our observation.

Pacemaker implantation following radiofrequency ablation is likely related to the required transmural ablation of the high basal interventricular septum near the conduction system and apparently remains an inevitable consequence. It is unlikely that technical refinement in mapping or ablation would reduce this risk or significantly increase debulking. There is need for other approaches that might overcome these limitations.

Abbreviations

IQR

interqurtile range

LAMPOON

intentional laceration of the anterior mitral leaflet to prevent left ventricular outflow tract obstruction

LVOT

Left ventricular outflow tract

SCORPION

Septal bipolar ablation of a nonCORonary segment of ventricular myocardium to Prevent outflow obstructION after transcatheter mitral valve implantation

TMVI

transcatheter mitral valve implantation