A cardiac pacing system for delivering a cardiac pacing therapy, comprising: an implantable cardioverter defibrillator , ICD , the ICD comprising a housing ; a lead having a lead body and electrically coupled to the housing of the ICD ; a plurality of electrodes positioned along the lead body ; a control circuit positioned within the housing of the ICD and configured to determine a cardiac signal sensed between two electrodes of the plurality of electrodes positioned along the lead body or between the housing of the ICD and an electrode of the plurality of electrodes positioned along the lead body , determine signal characteristics of the cardiac signal within a signal evaluation window positioned along a first portion of a cardiac cycle, determine whether a P-wave occurs within the signal evaluation window associated with the first portion of the cardiac cycle in response to the determined signal characteristics, adjust the signal evaluation window to be positioned along a second portion of the cardiac cycle in response to the P-wave not occurring within the signal evaluation window , determine signal characteristics of the cardiac signal within the signal evaluation window positioned along the second portion of the cardiac cycle, determine whether a P-wave occurs in response to the signal characteristics determined within the signal evaluation window positioned along the second portion of the cardiac cycle, and deliver a trigger signal in response to a P-wave being determined to occur; a leadless pacing device comprising a housing ; a plurality of electrodes positioned along the housing of the leadless pacing device ; and a control circuit positioned within the housing of the leadless pacing device and configured to receive the trigger signal from the ICD and deliver a ventricular pacing therapy via the plurality of electrodes positioned along the housing of the leadless pacing device . an implantable cardioverter defibrillator , ICD , the ICD comprising a housing ; a lead having a lead body and electrically coupled to the housing of the ICD ; a plurality of electrodes positioned along the lead body ; a control circuit positioned within the housing of the ICD and configured to determine a cardiac signal sensed between two electrodes of the plurality of electrodes positioned along the lead body or between the housing of the ICD and an electrode of the plurality of electrodes positioned along the lead body , determine signal characteristics of the cardiac signal within a signal evaluation window positioned along a first portion of a cardiac cycle, determine whether a P-wave occurs within the signal evaluation window associated with the first portion of the cardiac cycle in response to the determined signal characteristics, adjust the signal evaluation window to be positioned along a second portion of the cardiac cycle in response to the P-wave not occurring within the signal evaluation window , determine signal characteristics of the cardiac signal within the signal evaluation window positioned along the second portion of the cardiac cycle, determine whether a P-wave occurs in response to the signal characteristics determined within the signal evaluation window positioned along the second portion of the cardiac cycle, and deliver a trigger signal in response to a P-wave being determined to occur; a leadless pacing device comprising a housing ; a plurality of electrodes positioned along the housing of the leadless pacing device ; and a control circuit positioned within the housing of the leadless pacing device and configured to receive the trigger signal from the ICD and deliver a ventricular pacing therapy via the plurality of electrodes positioned along the housing of the leadless pacing device . <EOS>