Paul McCartney wrote this as Hey Jules, a song meant to comfort John Lennon's 5-year-old son Julian as his parents were getting a divorce. The change to Jude was inspired by the character Jud in the musical Oklahoma! (McCartney loves show tunes)
The medical term for a slow heart rate is bradycardia; generally defined as a heart rate below 60 bpm (beats per minute). In contrast, tachycardia means fast heart rate above 100 bpm.
Bradycardia is a slow heart rate of fewer than 60 beats per minute. A slow heart rate is normal for some people, such as athletes, and needs no treatment. Bradycardia may also be caused by health conditions that do need treatment.