To date, most published echocardiographic methods have assessed left ventricular (LV) dyssynchrony (DYS) alone as a predictor for response to cardiac resynchronization therapy (CRT). We hypothesized that the response is instead dictated by multiple correctable factors.

A total of 161 patients (66 ± 10 years, EF 24 ± 6%, QRS > 120 ms) were investigated pre- and post-CRT (median of 6 months). Reduction in NYHA Class ≥1 or LV reverse remodelling (end-systolic volume reduction ≥ 10%) defined response. Four different pathological mechanisms were identified. Group1: LVDYS characterized by a pre-ejection septal flash (SF) (87 patients, 54%). Elimination of SF (77 of 87 patients) resulted in reverse remodelling in 100%. Group 2: short-AV delay (21 patients, 13%) resolution (19 of 21 patients) resulted in reverse remodelling in 16 of 19. Group 3: long-AV delay (16 patients, 10%) resolution (14 of 16 patients) resulted in NYHA Class reduction ≥1 in 11 with reverse remodelling in five patients. Group 4: exaggerated LV–RV interaction (15 patients, 9%) reduced post-CRT. All responded clinically with fall in pulmonary artery pressure (P = 0.003) but did not volume respond. Group 5: patients with none of the above correctable mechanisms (22 patients, 14%). None responded to CRT.

CRT response is dictated by correction of multiple independent mechanisms of which LVDYS is only one. Long-axis DYS measurements alone failed to detect 40% of responders.