Abstract Body (Do not enter title and authors here): Background:
Previous data indicate that D-dimer levels predict long-term risk of mortality in patients with ischemic heart disease, while its prognostic value in hypertrophic cardiomyopathy remains uncertain.

Objective:
We aimed to investigate the prognostic significance of D-dimer in patients with hypertrophic cardiomyopathy undergoing septal myectomy.

Methods:
We retrospectively analyzed D-dimer levels in patients who underwent septal myectomy at our center between 2009 and 2018. The primary endpoint was all-cause mortality, with cardiovascular mortality as a secondary endpoint. Cox regression and competing risk models were employed to evaluate risk factors for all-cause mortality and cardiovascular mortality, respectively.

Results:
Baseline D-dimer levels from 728 patients were categorized into quartiles (<0.21, 0.21-0.3, ≥0.3 mg/L). Higher levels were associated with older age, female sex, lower body surface area, more severe mitral regurgitation, and elevated NT-pro B-natriuretic peptide levels (each p<0.05). Over a median follow-up of 4.2 years, 31 (4.3%) patients reached the primary endpoint, with 23 of these deaths attributed to cardiovascular causes. A D-dimer level of 0.29 mg/L was identified as the optimal cutoff for predicting 5-year mortality. After adjusting for covariates, D-dimer >0.29 mg/L was significantly associated with increased risk of all-cause mortality (HR 3.12; 95% CI 1.42-6.86, p=0.005) and cardiovascular mortality (HR 3.29; 95% CI 1.12-9.62, p=0.030). Additionally, body mass index and left atrial diameter were independent predictors of both all-cause mortality (HR 1.12; 95% CI 1.01-1.23, p=0.026 and HR 1.08; 95% CI 1.02-1.15, p=0.006, respectively) and cardiovascular mortality (HR 1.12; 95% CI 1.00-1.24, p=0.043 and HR 1.11; 95% CI 1.04-1.20, p=0.004, respectively). The inclusion of D-dimer >0.29 mg/L improved the net reclassification index for all-cause mortality (0.50, p=0.016) and cardiovascular mortality (0.64, p=0.006).

Conclusion:
D-dimer levels are a robust predictor of mid-to-long-term all-cause and cardiovascular mortality in patients undergoing septal myectomy.