Postural orthostatic tachycardia syndrome (POTS) occurs in approximately 500,000 Americans, but predominates in women with a 5:1 ratio. Patients with POTS experience debilitating tachycardia upon postural changes such as standing that impairs their quality of life. Tachycardia is clinically defined as a heart rate greater than 100 beats/min; and in POTS patients, the prolonged heart rate increase is greater than 30 beats/min or increases to 120 beats/min within the first ten minutes of a diagnostic tilt table test without postural hypotension. There are currently no effective treatment methods for POTS. However, several studies suggest Ivabradine could be a main treatment option for POTS because Ivabradine specifically inhibits the f-channels (If) within the sinoatrial (SA) node, which slows the heart rate. Currently in the US, Ivabradine is mainly prescribed to treat chronic heart failure. It is well tolerated in patients, but it is not commonly prescribed for POTS. It has been also used for treatment of inappropriate sinus tachycardia with good benefit. The hypothesis for this experiment is that Ivabradine will reduce tachycardia and improve functional status in patients with POTS.