Patients with POTS experience tachycardia, palpitations, and shortness of breath on standing, which are attributed to baroreflex-mediated overactivation of the sympathetic nervous system. Autonomic anticipatory responses, which occur before the evoking stimulus is presented and are characteristic of classical conditioning, however, have not been investigated in these patients. 

In patients with POTS, the onset of tachycardia preceded the movement of the tilt table, triggering a greater anticipatory heart rate increase while still supine (8±1 vs. 4±1 bpm, p=0.015). The magnitude of the anticipatory tachycardia was related to the severity of somatic vigilance scores (R=0.367, p=0.018) and the extent of hyperventilation during tilt (R=-0.352, p<0.022). Only POTS patients had a significant increase in circulating epinephrine when tilted (30±6 to 54±7 pg/ml, p<0.008). Compared to controls, tilt also induced a greater increase in norepinephrine levels (542±53 vs. POTS 644±56 pg/ml, p=0.018). 

The onset of tachycardia in POTS occurs prior to orthostatic stress and is related to mental attention to surveying bodily sensations. This anticipatory autonomic response suggests classic conditioning and a possible role for the amygdala-adrenal medulla neuronal circuitry in triggering a sympathoadrenal response with hyperventilation and tachycardia, which can be brought on by the thought of standing.