Inappropriate sinus tachycardia (IST). Note elevated resting heart rate (>100 BMP) which is further increased during the tilt. This response satisfies criteria for POTS (last baseline HR = 101.3, last HR during the tilt = 132.6, and increment > 30 BPM and BPM > 120) except that the continuous resting supine tachycardia is inconsistent with POTS. Second clue that this is not POTS is an episode of HR increment exceeding 120 BPM (green oval) at supine position after completing the tilt. Mean supine HR = 106.6 ± 2.8, range 97.1–114.3 BPM, mean orthostatic HR 128 ± 2.4, and range 121.8–134.0 BPM.