Techniques are provided for measuring ventricular evoked response within the heart of a patient during atrial fibrillation. An intrinsic ventricular depolarization (i.e. a QRS complex) is sensed within the right ventricle and then a pacing pulse is applied to the left ventricle prior to intrinsic depolarization thereof. In this manner, the left ventricle contracts solely in response to the pacing pulse and possible fusion between the pacing pulse and a conducted P-wave/A-pulse from the atria is avoided. The evoked response generated in the left ventricle is then measured by the pacemaker. The technique permits reliable detection of left ventricular evoked response, even during atrial fibrillation when atrial contractions are frequent and erratic, but the technique may also be employed during normal sinus rhythm. Features of the evoked response are analyzed to detect heart failure, evaluate its severity and track its progression. Appropriate therapy is then automatically delivered to the patient.