Crossover Study of Propranolol vs Ivabradine in POTS

Inclusion Criteria: * Diagnosis of Postural Tachycardia Syndrome * Age between 18-60 years * Men and women are eligible * Able and willing to provide informed consent Exclusion Criteria: * Seated resting heart rate \< 70 bpm in the absence of rate-lowering medications * Supine blood pressure \< 90/60 mmHg * Overt cause for postural tachycardia, i.e., acute dehydration, prolonged bed rest * Presence of underlying structural heart disease including: * Valvular disease (i.e. moderate or greater valvular stenosis or regurgitation) * History of heart failure * Greater than mild left ventricular systolic impairment * Hypertrophic cardiomyopathy * Known coronary artery disease or prior myocardial infarction * History of tachyarrhythmias including: * Supraventricular tachycardia, atrial tachycardia, atrial fibrillation / flutter * Ventricular tachyarrhythmias * Pre-existing long QT interval (e.g. known congenital long QT syndrome, or QT interval \>500 ms) * History of sick sinus syndrome or high degree AV block * Presence of a pacemaker, implantable cardioverter defibrillator or cardiac resynchronization therapy device * Diabetes mellitus or history of hypoglycemic episodes * History of bronchospasm or uncontrolled asthma * Pregnant (by history or serum test) or breast-feeding * Inability to safely withdraw from existing beta-blockers or ivabradine prior to the study day * Concomitant use of class I and III antiarrhythmic agents, or non-dihydropyridine calcium channel blockers * Concomitant use of strong cytochrome P450 3A4 (CYP 3A4) inhibitors * Severe hepatic impairment * Any contraindication to propranolol or ivabradine not specifically listed above as per the product monographs. * Unable to give informed consent