Haemodynamic and functional effects of cardiac resynchronization therapy (CRT) have been studied mostly at rest. CRT effects on left ventricular (LV) dyssynchrony and function during stress have not been evaluated in detail.

We studied the electromechanical effects of CRT at rest and during Dobutamine stress echocardiography (DSE), during active and withheld CRT.

Twenty‐one responders to CRT (62±12 yr) were assessed by walking test, quality of life, and BNP with active CRT (“off”) and 2 weeks after pacing withdrawal (“off”). DSE (10 μ/kg/min) was performed both at “on” and “off” to evaluate dyssynchrony parameters, systolic and diastolic function.

At rest, CRT withdrawal was associated with an increased interventricular mechanical delay (IVMD, from 21±18 ms to 49±24 ms, p<0.001) and impaired intraventricular synchrony. There was a significant decrease in LV systolic function and LV filling time. Dobutamine infusion had no impact on inter‐ and intraventricular synchrony. During stress, there was an improvement in LV performance both at “on” and “off” However, LV dp/dt, aortic VTI, cardiac output, mean systolic peak velocities and LV filling time during dobutamine stress were significantly greater with CRT “on”.

In long‐term responders, CRT improves LV performance both at rest and during dobutamine stress. This is attributable to an improvement in LV synchrony, which is maintained during stress.