The Effect of Ivabradine on Patients With Postural Tachycardia Syndrome

Inclusion Criteria: * Patients diagnosed with POTS by the following criteria: 1. The presence of characteristic orthostatic symptoms of the POTS for at least six months. 2. An increase in the heart rate of at least 30 beats per minute (without concomitant significant decrease in blood pressure of more than 20/10 mmHg) within 10 minutes after assuming a standing position (or during tilt test) on at least 3 separate occasions. 3. No other concomitant diseases that could explain the symptoms of POTS. Exclusion Criteria: <!-- --> 1. History of systemic illness capable of affecting of affecting autonomic function (eg, diabetes mellitus, SLE). 2. History of cardiovascular disease. 3. History of smoking, drug or alcohol abuse. 4. Pregnancy, and also uncontrolled thyroid or adrenal disorders. 5. Using any drug metabolized by cytochrome P450 3A4 enzyme during the last 72 hours.