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.