Abstract Body (Do not enter title and authors here):
Background:
Ventricular arrhythmia is an important factor in determining the risk of sudden cardiac death (SCD) in patients with hypertrophic obstructive cardiomyopathy (HoCM). Despite septal myectomy being recommended for symptomatic relief, its impact on ventricular arrhythmias remains minimally explored.

Objectives:
This study investigates baseline echocardiographic and cardiac magnetic resonance imaging (CMR) parameters in patients with HoCM and evaluates septal myectomy's effect on subsequent ventricular arrhythmia burden.

Methods:
Adult patients with HoCM followed at a single center (AGH) were included in this study from 2016 to 2023. Patients were divided into two groups: those who underwent septal myectomy (M+ group) and those who did not (M- group). Baseline and follow-up imaging data were collected and analyzed along with ICD interrogation and arrthymia montoring devices. Using statistical comparisons and multinomial logistic regression modeling, we investigated the relationship between myectomy and subsequent ventricular arrhythmia outcomes.

Results:
In total, 158 patients, 46 underwent septal myectomy. The M+ group had significantly higher baseline left ventricular outflow tract (LVOT) gradients and late gadolinium enhancement (LGE) compared to the M-group. Post-myectomy, the LVOT gradient decreased significantly (from 42mmHg to 10 mmHg at rest), along with interventricular septal thickness and LV mass index (p<0.05 for all). The incidence of post-surgical ventricular arrhythmia was nearly 2-fold less in the M+ group (31%) as compared to the M-group (55%); p<0.05, despite an initial higher burden in M+ group (p<0.05). Significant associations were shown through multivariate analysis between a decrease interventricular septal thickness, a reduction in LVOT gradient and a drop in ventricular arrhythmia incidence. The model's area under the curve (AUC) was 0.91, indicating a very strong predictive capability.

Conclusion:
Septal myectomy may reduce the short and mid-term incidence of ventricular arrhythmia and thus impact outcomes related to sudden cardiac death in HocM. This is likely a consequence of removal of arrhythmogenic scar tissue. Larger studies are required to further evaluate the burden and correlation of ventricular arrhythmia with scar tissue volume (LGE/T1 mapping/ECV) measured on cardiac MRI. Whether this observation will be apparent with novel pharmacologic agent remains of great interest.