• SA node rates: 60–100 beats per minute (bpm)
• AV node rates: 40–60 bpm
• Purkinje fibers: 20–40 bpm 

These rates are not fixed, and each site can sustain rates lower than their usual lowest limit. However, the inherent rates are important to consider when reading an ECG strip. For example, if a patient has a heart rate of 72 bpm, the electrical impulse that generated the rate probably came from the SA node. 

The fastest inherent rate overrides any other stimuli and becomes the pacemaker of the heart. In a normal heart, the SA node is the governor of the fastest rate. Thus, a person’s resting heart rate usually falls between 60 bpm and 100 bpm. For this reason, a normal heart rate on ECG is “sinus” in origin.