AV and VV delay optimization

AV delay is tuned so atrial-sensed or paced activity is followed by ventricular pacing at a timing that maximizes diastolic filling without causing fusion with the patient's own conduction; VV delay (the timing offset between right and left ventricular lead pacing) is tuned to maximize mechanical synchrony between the two ventricles. The overarching practical goal across both is maximizing the percentage of time the patient is actually being biventricularly paced, since CRT benefit is closely tied to a high percentage (generally >98%) of effective biventricular pacing.