Methods and devices for identification of intrinsic ventricular activity occurring within a ventricular signal. Ventricular signal morphology is analyzed to determine if the signal contains intrinsic ventricular activity while delivering pacing pulses separated by nearly constant time intervals. Furthermore, an extension of a pacing interval is specified based on whether or not the signal contains autonomous intrinsic ventricular activity. In this manner, the pacing interval is only extended when it is likely for autonomous intrinsic ventricular activity to occur.