Comparisons of tachycardia syndromes including postural tachycardia syndrome (POTS), inappropriate sinus tachycardia (IST), and paroxysmal sinus tachycardia (PST). During supine position, the heart rate is normal (<100) in POTS, consistently elevated in IST, and transiently elevated in PST. The excessive tachycardia during the tilt is seen in all shown syndromes, being continuous in POTS and IST, and intermittent in PST. Blood pressure is stable in all examples. CBFv is reduced in POTS, normal in IST, and intermittently elevated in PST.