Abstract Body (Do not enter title and authors here): Background
Idiopathic non-ischemic cardiomyopathy (iNICM) includes patients without identifiable causes of left ventricular systolic dysfunction. While dilated cardiomyopathy (DCM) has been extensively studied, the entity of non-dilated iNICM remains poorly understood.

Methods
We retrospectively identified 197 consecutive patients with AHA/ACC Stage B heart failure (HF) (without clinical HF) who underwent cardiovascular magnetic resonance (CMR) for suspected cardiomyopathy from 2023-2024. Of these, 79 met criteria for non-dilated iNICM, defined by sex-specific normal LV cavity size thresholds (LVEDVi ≤78 mL/m² for females, ≤95 mL/m² for males). CMR consisted of bSSFP cine short-axis stack for evaluation of LV volumes and ejection fraction (LVEF), native T1 mapping, extracellular volume, and late gadolinium enhancement (LGE) images. LGE was quantified using a semi-automated technique (cvi42, Circle Cardiovascular Imaging), with threshold ref=5SD. We compared cardiac structure and function characteristics of patients with DCM and non-dilated iNICM and assessed outcomes over a median follow-up of 10.9 ± 4.9 months. We conducted multivariable Cox proportional hazards models, adjusting for age, sex, LVEF, and the presence of LGE. We conducted mediation and interaction analyses to assess the contribution of LVEF to the association between LV size and incident HF outcomes.

Results
Characteristics of patients with non-dilated and dilated iNICM are presented in the TABLE. Patients with non-dilated iNICM were older (p=0.04), more often male (p=0.02), and had significantly lower LV mass, LVEDVi, and LGE presence (p=0.02) and quantification (p<0.001). Despite preserved ventricular size and higher LVEF (p<0.001), the non-dilated group carried a substantial clinical risk, with 11% developing clinical HF during follow-up, and no difference in the ventricular arrhythmia rates. In adjusted analysis, each 10 mL/m² increase in LVEDVi was associated with a >10% increase in HF risk (HR 1.014; 95% CI 1.006–1.023; p=0.001). The addition of LVEF to models attenuated this association (HR: 1.002, p = 0.68). Mediation analysis demonstrated that LVEF mediated the association between LVEDVi and incident HF, but not between LVEDVi itself and HF (FIGURE). There was no significant interaction between LVEDVi and LV EF (p=0.07).

Conclusion
Despite more favorable characteristics of LV morphology and function compared with DCM, non-dilated iNICM carries a significant risk of incident clinical HF.