Bradycardia (brad-e-KAHR-dee-uh) is a slow heart rate. The hearts of adults at rest usually beat between 60 and 100 times a minute. If you have bradycardia, your heart beats fewer than 60 times a minute. Bradycardia can be a serious problem if the heart rate is very slow and the heart can't pump enough oxygen-rich blood to the body. If this happens, you may feel dizzy, very tired or weak, and short of breath. Sometimes bradycardia doesn't cause symptoms or complications. A slow heart rate isn't always a concern. For example, a resting heart rate between 40 and 60 beats a minute is quite common during sleep and in some people, particularly healthy young adults and trained athletes. If bradycardia is severe, an implanted pacemaker may be needed to help the heart maintain an appropriate rate.