A system and method for monitoring at least one chamber of a heart (e.g., a left ventricular chamber) during delivery of extrasystolic stimulation to determine if the desired extra-systole (i.e., ventricular mechanical capture following refractory period expiration) occurs. The system includes an implantable or external cardiac stimulation device in association with a set of leads such as epicardial, endocardial, and/or coronary sinus leads equipped with motion sensor(s). The device receives and processes acceleration sensor signals to determine a signal characteristic indicative of chamber capture resulting from one or more pacing stimulus delivered closely following expiration of the refractory period. A threshold optimization method optionally evaluates capture and at least one of: runs an iterative routine to establish or re-establish chamber capture for the PESP therapy, sets a logical flag relating to chamber capture status and stores parameter(s) relating to successful chamber capture for one or more subsequent cardiac cycles.