A system for controlling delivery of cardiac resynchronization therapy to a patient, the system comprising: a plurality of electrodes (34,36, 38, 40, 42, 44, 46, 48); an implantable medical device configured to deliver ventricular pacing to the patient; sensing circuitry configured to sense electrical activity of the heart via the plurality of electrodes; and processing circuitry configured to: control the implantable medical device to deliver ventricular pacing according to a sequence of different values of a CRT parameter, wherein the system is characterized in that the processing circuitry is further configured to: acquire, via the sensing circuitry and during the delivery of ventricular pacing according to the sequence, a first electrogram and a second electrogram from respective ones of a first vector formed from the plurality of electrodes and a second vector formed from the plurality of electrodes; determine, for each of the different values of the CRT parameter, a first activation interval between an occurrence of a first fiducial of a cardiac cycle and a second fiducial of a cardiac cycle detected in the first electrogram and a second activation interval between the occurrence of the first fiducial of the cardiac cycle and the second fiducial of a cardiac cycle detected in the second electrogram; determine, for each of the different values of the CRT parameter, a value of a metric of comparison of the first activation interval to the second activation interval, wherein the metric of comparison comprises at least one of a ratio of the first and second activation intervals or a time difference between the first and second activation intervals; determine an updated value of the CRT parameter using the metric of comparison; and control the implantable medical device to deliver the ventricular pacing with the updated value of the CRT parameter. a plurality of electrodes (34,36, 38, 40, 42, 44, 46, 48); an implantable medical device configured to deliver ventricular pacing to the patient; sensing circuitry configured to sense electrical activity of the heart via the plurality of electrodes; and processing circuitry configured to: control the implantable medical device to deliver ventricular pacing according to a sequence of different values of a CRT parameter, wherein the system is characterized in that the processing circuitry is further configured to: acquire, via the sensing circuitry and during the delivery of ventricular pacing according to the sequence, a first electrogram and a second electrogram from respective ones of a first vector formed from the plurality of electrodes and a second vector formed from the plurality of electrodes; determine, for each of the different values of the CRT parameter, a first activation interval between an occurrence of a first fiducial of a cardiac cycle and a second fiducial of a cardiac cycle detected in the first electrogram and a second activation interval between the occurrence of the first fiducial of the cardiac cycle and the second fiducial of a cardiac cycle detected in the second electrogram; determine, for each of the different values of the CRT parameter, a value of a metric of comparison of the first activation interval to the second activation interval, wherein the metric of comparison comprises at least one of a ratio of the first and second activation intervals or a time difference between the first and second activation intervals; determine an updated value of the CRT parameter using the metric of comparison; and control the implantable medical device to deliver the ventricular pacing with the updated value of the CRT parameter. acquire, via the sensing circuitry and during the delivery of ventricular pacing according to the sequence, a first electrogram and a second electrogram from respective ones of a first vector formed from the plurality of electrodes and a second vector formed from the plurality of electrodes; determine, for each of the different values of the CRT parameter, a first activation interval between an occurrence of a first fiducial of a cardiac cycle and a second fiducial of a cardiac cycle detected in the first electrogram and a second activation interval between the occurrence of the first fiducial of the cardiac cycle and the second fiducial of a cardiac cycle detected in the second electrogram; determine, for each of the different values of the CRT parameter, a value of a metric of comparison of the first activation interval to the second activation interval, wherein the metric of comparison comprises at least one of a ratio of the first and second activation intervals or a time difference between the first and second activation intervals; determine an updated value of the CRT parameter using the metric of comparison; and control the implantable medical device to deliver the ventricular pacing with the updated value of the CRT parameter.