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.
An arrhythmia (also called dysrhythmia) refers to what is usually an irregular heartbeat. The heart rate can be fast, slow, or irregular, taking into consideration age and activity. For example, a newborn's heart beats much faster than a 5-year-old child's heart. A fast heart rate is called tachycardia. A slow heart rate is called bradycardia. Some irregular heartbeats are normal: Sinus arrhythmia is the normal variation in heart rate that accompanies breathing in and out.