If the body’s requirement for blood is increased (such as at times of exercise), the autonomic nervous system boosts the rate at which the sinus node initiates heartbeats.
As the sinus node is still pacing beats of normal conduction the rhythm is termed sinus tachycardia. The lower threshold of the rate varies but is normally considered as 120 bpm.
Causes of inappropriate sinus tachycardia, which may occur at rest or during sleep, include anxiety, excess caffeine, some drugs, and heart failure.
Conduction: sinus Tachycardia