Abstract Body (Do not enter title and authors here): Background:Mid-ventricular obstruction (MVO) is a rare subtype of hypertrophic cardiomyopathy (HCM) , but it is associated with severe symptoms and worse clinical outcomes. Whereas the mechanisms of MVO has not fully elucidated. Therefore, we sought to investigate the mechanisms of MVO via an echocardiographic analysis.

Method: We retrospectively enrolled 239 patients diagnosed with HCM in this study. All patients were divided into non-obstructive HCM group (N=51), isolated left ventricular outflow (LVOT) obstructive HCM (oHCM) group (N=130), and MVO group (N=58) based on the initial echocardiographic evaluation.

Result: Compared to patients with oHCM, patients with MVO were much younger (43.48±11.45 vs. 49.25±12.10, p <0.05), less likely to be female (54 (41.5%) vs 12 (20.7%), p <0.05), had a lower left atrial diameter (41.64±5.82 vs. 43.99±6.30, p <0.05), a higher mid-septal thickness (21.40±4.24 vs. 18.88±3.30, p <0.05), a lower mean global longitudinal strain (-10.59±4.31 vs. -14.41±3.83, p <0.05), a longer anterior papillary muscles (APM) length ( 48.91±7.37 vs. 41.13±6.93, p< 0.05), a shorter distance between APM and septum (APM-IVS) ( 11.19±2.53 vs. 17.97±3.45, p<0.05), a higher prevalence of mid-septal bundle (56 (96.6%) vs. 74 (56.9%), p <0.05). Multinomial logistic analysis indicated that compared to patients with non-obstruction HCM patients, BMI (Odd ratio [OR] 1.321, 95% confidential interval [CI] 1.116-1.564, p< 0.001) , APM-IVS (OR 0.404, 95%CI 0.316-0.517, p< 0.001), APM length (OR 1.147, 95% CI 1.051-1.251, p= 0.002), and mid-septal thickness (OR 1.202, 95% CI 1.01-1.43, p= 0.038) were independent risk factors of MVO. Furthermore, compared to patients with oHCM, APM-IVS (OR 0.430, 95%CI 0.341-0.542, p<0.001), the presence of mid-septal bundle (OR 8.376, 95%CI 1.513-46.365, p=0.015), and APM length (OR 1.081, 95%CI 1.011-1.157, p = 0.023) were significantly associated with MVO.

Conclusion: APM-IVS, APM length and mid-septal thickness played vital roles in the development of mid-obstruction in hypertrophic cardiomyopathy, which may offer a clue for surgical strategy.