Background: Chronic orthostatic intolerance (COI) occurs in postural tachycardia syndrome (POTS) and in some individuals with repeated neurocardiogenic syncope/presyncope (NCS), without POTS. This study addressed whether patients with COI and POTS or NCS have neurocirculatory abnormalities during supine rest.
Methods and results: Adult patients referred for COI who had POTS (n=90, mean+/-SEM age 40+/-1 years, 86% women) or NCS (n=36, 41+/-2 years old, 78% women) underwent measurements of plasma levels of catecholamines and forearm hemodynamics. Comparison data were obtained from 32 age- and gender-matched normal volunteers (39+/-2 years old, 81% women). The POTS group had a relatively fast mean heart rate (79+/-2 bpm) during supine rest compared with the NCS group (69+/-1.6 bpm, P=0.03) and normal volunteers (66+/-3 bpm, P=0.0004). The POTS group also had higher mean arterial norepinephrine (1.61+/-0.11 nmol/L, n=37) and epinephrine (0.39+/-0.03 nmol/L, n=37) concentrations than the NCS group (1.03+/-0.12 nmol/L, n=20, P=0.0012; 0.21+/-0.03 nmol/L, n=20, P=0.0005) and normal volunteers (1.13+/-0.11 nmol/L, n=20, P=0.006; 0.17+/-0.03 nmol/L, n=15, P=0.0001). The NCS group had higher mean forearm vascular resistance (52+/-6 U) than the POTS group (36+/-2 U, P=0.003).
Conclusions: Overall, POTS features increased heart rate and sympathetic nervous and adrenomedullary hormonal system outflows during supine rest. Increased sympathetic outflow may contribute to the relative tachycardia in POTS. NCS features forearm vasoconstriction during supine rest but not sympathoneural or adrenomedullary activation.