Normally, heart rate varies with breathing, increasing during inspiration and decreasing during expiration. This is known as respiratory sinus arrhythmia and is controlled by the parasympathetic nervous system. In this test, the patient will be instructed to take deep breaths (6 breaths/minute) and his/her heart rate will be monitored. The minimum and maximum heart rate during the breathing cycles will be determined. A difference between the minimum and maximum heart rate of at least 15 is considered normal.