This study is a double-blind, randomized, two-arm (with crossover to other arm) trial that will investigate the benefit of rate responsive pacing with either MV or XL in pacemaker patients that have chronotropic incompetence (CI). Blinding will occur for both the patient and the research personnel who are collecting primary endpoint data. The study will enroll patients that have an implanted dual/single chamber pacemaker, which has both the minute ventilation (MV) and accelerometer (XL) rate-adaptive sensors.