A medical device system that includes a brain stimulating element, cardiac monitoring element and a processor. The processor is configured to receive a brain stimulation signal from the brain stimulating element and a cardiac signal from the cardiac monitoring element. The processor is further configured to determine at least one reference point for a stimulation event time period by evaluation of the brain stimulation signal. The processor further identifies a first portion of the cardiac signal based on the at least one reference point of the stimulation event time period.