Ken Turnbull
Senior Member (Voting Rights)
The original criterion in the 1993 Schondorf-Low paper was 2SD above the mean for a sex-matched control population which excluded children and adolescents. I'm not sure who originally proposed the 30bpm value but in the 2011 consensus statement the definition is characterized by a sustained heart rate increment of 30 beats/minute within 10 min of standing or head-up tilt. . . For individuals aged 12–19 years the required increment is at least 40 beats/minute.
I don't have the energy to check right now but the history is fairly well-documented in the Gall et al. "Postural Tachycardia Syndrome" book.
I had a look in the textbook but it doesn’t seem to cover the decision on the 30 BPM, unfortunately.