Aim To analyze echocardiographic parameters that reflect left ventricular (LV) myocardial contractility, using a novel method for evaluation of myocardial performance in patients with chronic heart failure (CHF) and atrial fibrillation (AF) during heart contractility modulation (HCM).Material and methods Standard echocardiographic parameters and indexes of myocardial strain and work were analyzed for 66 patients (52 men and 14 women; median age, 60 [54; 66] years). 36 patients had paroxysmal AF and 30 patients had permanent AF. All patients had CHF with a duration of 17 [4; 60] months; duration of AF was 12 [6; 36] months. At baseline, the left ventricular ejection fraction (LV EF) was 33 [27; 37] %.Results After one year of HCM, LV EF significantly increased from 33 [27; 37] to 38 [33; 44] % (р=0.001). Also, there were improvements in the myocardial global longitudinal strain (from -6.00 [ - 8; - 4] to -8 [ - 10; - 6] %; р=0.001) and parameters of myocardial work, including the global work efficiency (from 74 [65; 79] to 80 [73; 87] mm Hg%; р=0.001), global constructive work (from 699 [516; 940] to 882 [714; 1242] mm Hg%; р=0.001), and global myocardial work index (from 460 [339; 723] to 668 [497; 943] mm Hg%; р=0.001). A segmentary analysis of LV work parameters showed positive changes in the myocardial constructive work in the area of the interventricular septal apical segment (at baseline, 844 [614; 1224]; after HCM, 1027 [800; 1520] mm Hg%; р=0.05) and the medium segment of the LV anteroseptal wall (at baseline, 593 [312; 1000]; after HCM, 877 [494; 1145] mm Hg%; р=0.05).Conclusion This method for analysis of the myocardial work provides a more detailed examination of LV structural and functional remodeling and mechanisms for its effects on the LV contractile function in patients with CHF. This method is promising and merits further study in various clinical situations.