PURPOSE: Left atrial reservoir strain (LASr) is an established marker of atrial dysfunction and elevated filling pressures in heart failure; however, its role in mediating downstream right ventricular (RV) mechanical impairment in advanced heart failure with reduced ejection fraction (HFrEF) remains incompletely defined. This study aimed to evaluate the association between LASr and RV myocardial mechanics using multichamber deformation imaging and to elucidate the mechanistic contribution of atrial dysfunction to RV impairment within the atrioventricular continuum. METHODS: This cross-sectional observational study included 50 stabilized patients with Stage C and Stage D HFrEF. All participants underwent comprehensive transthoracic echocardiography incorporating conventional parameters, multichambered speckle-tracking strain analysis, and three-dimensional RV assessment. Associations between LASr and RV functional indices were assessed using Pearson correlation and multivariable regression analyses was applied to identify the key determinants of RV impairment. RESULTS: LASr was markedly reduced across the cohort and demonstrated a significant inverse association with RV free-wall strain and RV global longitudinal strain. Left atrial strain parameters and left atrial volume index did not differ significantly between Stage C and Stage D HFrEF, suggesting early plateauing of atrial mechanical dysfunction in advanced disease. In contrast, left ventricular global longitudinal strain worsened significantly with advancing heart failure stage. RV dysfunction was uniformly present across both stages and was more sensitively detected by strain-based indices than by conventional RV parameters. On multivariable regression analysis, LASr emerged as an independent determinant of RV mechanical dysfunction, along with chronic kidney disease. CONCLUSION: In advanced HFrEF, impaired left atrial reservoir function is closely linked to RV mechanical dysfunction, supporting atrioventricular interdependence. Multichambered deformation imaging provides important mechanistic insights beyond conventional LV-centric assessment and highlights the value of integrated atrial and RV strain evaluation in advanced heart failure.