Sinus arrhythmia is the normal increase in heart rate that occurs during inspiration. This is a natural response and is more accentuated in children. The “sinus” refers to the natural pacemaker of the heart which is called the sinoatrial (or sinus) node. It is located in the wall of the right atrium. Normal cardiac impulses start there and are transmitted through specialized cells to the atria and down to the ventricles. Sinus tachycardia refers to a fast heartbeat because of rapid firing of the sinoatrial (sinus) node. This occurs in response to exercise, exertion, excitement, pain, fever, excessive thyroid hormone, low blood oxygen (hypoxia), stimulant drugs (such as caffeine), etc.