1. An essentially harmless increase in heart rate due to increase firing of the normal pacemaker of the heart.
2. Usually happens with fever, excitement, and exercise. There is no need for treatment, except in rare cases when it is caused by an underlying problem such as low blood count or overactive thyroid gland.
3. Rarely, there is no apparent cause for the increase in the sinus rate (inappropriate sinus tachycardia), in these circumstances, medications are given to slow the heart rate down and rarely an ablation procedure may be needed.

ECG of sinus tachycardia