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 means a slowed heart rate. In premature babies, bradycardia is a heart rate of less than 80 beats per minute. In premature babies, apnea and bradycardia often occur together, along with low blood oxygen levels. First, apnea occurs and the baby will stop breathing.