Sinus rhythm is the regular heartbeat controlled by the sinus node, or sinoatrial, the natural pacemaker of the heart. The sinus node is located in the right upper chamber of the heart, claims MedicineNet.
The heartbeat begins in this node and spreads down to the ventricles through the atria. Sinus arrhythmia is the increase in the rate of cardiac impulses when a person inhales. It is a typical response that occurs more in children than in adults. Sinus tachycardia is when the heart beats fast due to rapid firing of the sinus node. Fever, pain, exercise, hypoxia and stimulant drugs cause sinus tachycardia, according to MedicineNet.