what causes heart to beat fast and then slow
1 Tachy-brady syndromeâ It is possible for the same person to have a slow heartbeat when the heart is in normal rhythm and periods of fast, irregular heartbeats (supraventricular tachycardias) at other times. 2  Sometimes, medications needed to control the fast heart rate can worsen the already slow heart rate.hat counts as an abnormally slow heartbeat for one person may not be the same for another. For example, a young, strong, healthy athlete may have a resting heart rate of 30-40 beats per minute, but may easily increase his heart rate to 180 beats per minute with exercise. This is normal.