Introducing the Sinus Rhythms
Sinus rhythms originate in the SA Node. The Heart Rates (HR) are particularly important: Please study/learn/know.
Normal sinus rhythm (NSR) HR: 60-100 b/min
Sinus bradycardia HR: <60 b/min
Sinus tachycardia HR: >100 b/min to ~150 b/min (it is sometimes a little faster that 150 b/min)
Sinus arrhythmia: ~60 to ~100 b/min (yes, it sometimes is slower than 60 bpm and sometimes a little faster than 100 b/min)

For YOUTUBE Videos of Sinus rhythms (when I taught EKG during COVID-19 see below:

Normal Sinus Rhythm: CLICK Below:
https://youtu.be/LAENRy8xlIA

Sinus Rhythms Part 1 Youtube video:
https://www.youtube.com/watch?v=KvEzl2ljyOw

Sinus Rhythms Part 2 Youtube video:
https://www.youtube.com/watch?v=anhsDI3Uoz0
EKG Page