The sympathetic baroreflex remains intact and HR is excessively increased in the absence of parasympathetic counterregulation. The cardiovagal baroreflex couples BP to HR to buffer BP changes. Large low frequency BP oscillations, representing a resonance within the sympathetic baroreflex loop, occur if there is central hypovolemia, an intact sympathetic baroreflex, and reduced parasympathetic buffering of BP by HR; conditions found in upright POTS. This leads to the following hypothetical paradigm: