causes: Hyperdynamic circulation ( Valvular Incompetence , Thyrotoxicosis , Hypercapnia , Pyrexia , Anemia , Pregnancy ) Sympathetic overdrive ( Panic disorders , Hypoglycemia , Hypoxia , Levocetirizine antihistamines , Anemia , Heart Failure , Mitral valve prolapse . [ 2 ] ) Arrhythmias ( Premature atrial contraction , Junctional escape