Sinus rhythm refers to the normal rhythm of the heart, according to MedicineNet.com. The sinoatrial node, which is the natural pacemaker of the heart, sets the rhythm in the upper right heart chamber. The heartbeat then travels to other areas of the heart.
The sinus rhythm of the heart can change and become too slow or too fast, states MedicineNet.com. Sinus tachycardia occurs when the sinoatrial node begins firing too quickly in response to stimuli such as overexertion, pain, fever, caffeine or exercise. Typically, sinus tachycardia resolves itself when the person returns to their normal activity. Sinus arrhythmia is a normal function of the sinoatrial node that results in an increased heart rate while inhaling.