Some patients' heart rates do not increase as needed during activities and exercise, which can make them feel tired and fatigued easily. The patients in this study have a pacemaker with a FDA approved rate response sensor, which senses activity level by sensing motion (through a component known as an accelerometer) and/or breathing changes (known as minute ventilation). These changes in motion or breathing make the pacemaker increase the patient's heart rate. This study is being conducted to see which rate response sensor is better (motion driven or breathing driven). The study is also investigating whether optimizing the sensor based on the individual patient will give better results in terms of increasing the patient's exercise capacity. The hypothesis is that rate responsive pacing driven by the minute ventilation sensor results in improved functional capacity compared to accelerometer in chronotropically incompetent patients.