1. A method of operating a cardiac pacing device that delivers pacing pulses to a heart of a patient, the method comprising: delivering first pacing pulses to the patient's heart; sensing first cardiac signals associated with the first pacing pulses; detecting a peak of each of the first cardiac signals; measuring a first feature of each of the detected peaks; determining a first variability of the measured first features; providing a variability threshold based on the first variability; delivering, after providing the variability threshold, a second pacing pulse to the patient's heart; sensing a second cardiac signal associated with the second pacing pulse; detecting a peak of the second cardiac signal; measuring a second feature of the detected peak of the second cardiac signal; determining a second variability of the measured second feature; comparing the second variability to the variability threshold value; and classifying a cardiac response to the second pacing pulse as fusion if the second variability exceeds the variability threshold value.