Participants will be asked to lie, semi-erect, on an exercise bed for up to 90 minutes. The exercise will be in the form of a pedal bike. The pacemaker will be programmed into four different modes of heart pacing. Whilst in each mode participants will be asked to exercise to three different stages of exercise. Each stage will last 4 minutes. The levels of difficulty will be worked out from a treadmill exercise test completed by the participants prior to the pacemaker implantation. Participants will be allowed to rest between each stage of exercise. If participants complete the whole study they will have cycled for a total of 48 minutes. During each stage 4ml of blood will be taken from the needle that will have been placed in a vein during the earlier part of the study.