Increased plasma angiotensin II in postural tachycardia syndrome (POTS) is related to reduced blood flow and blood volume

POTS (postural tachycardia syndrome) is associated with low blood volume and reduced renin and aldosterone; however, the role of Ang (angiotensin) 11 has not been investigated. Previous studies have suggested that a subset of POTS patients with increased vasoconstriction related to decreased bioavailable NO (nitric oxide) have decreased blood volume. Ang 11 reduces bioavailable NO and is integral to the renin-Ang system. Thus, in the present study, we investigated the relationship between blood volume, Ang 11, renin, aldosterone and peripheral blood flow in POTS patients. POTS was diagnosed by 700 upright tilt, and supine calf blood flow, measured by venous occlusion plethysmography, was used to subgroup POTS patients. A total of 23 POTS patients were partitioned; ten with low blood flow, eight with normal flow and five with high flow. There were ten healthy volunteers. Blood volume was measured by dye dilution. All biochemical measurements were performed whilst supine. Blood volume was decreased in low-flow POTS (2.14 +/- 0.12 litres/m(2)) compared with controls (2.76 +/- 0.20 litres/m(2)), but not in the other subgroups. PRA (plasma renin activity) was decreased in low-flow POTS compared with controls (0.49 +/- 0.12 compared with 0.90 +/- 0.18 ng of Ang 1 (.) ml(-1) (.) h(-1) respectively), whereas plasma Ang 11 was increased (89 +/- 20 compared with 32 +/- 4 ng/1), but not in the other subgroups. PRA correlated with aldosterone (r=+0.71) in all subjects. PRA correlated negatively with blood volume (r = - 0.72) in normal- and high-flow POTS, but positively (r = + 0.65) in low-flow POTS. PRA correlated positively with Ang 11 (r=+0.76) in normal- and high-flow POTS, but negatively (r=-0.83) in low-flow POTS. Blood volume was negatively correlated with Ang 11 (r = - 0.66) in normal- and high-flow POTS and in five low-flow POTS patients. The remaining five low-flow POTS patients had reduced blood volume and increased Ang 11 which was not correlated with blood volume. The data suggest that plasma Ang 11 is increased in low-flow POTS patients with hypovolaemia, which may contribute to local blood flow dysregulation and reduced NO bioavailability.