Postural tachycardia syndrome (POTS) is associated with low blood volume,
reduced renin and aldosterone. Angiotensin-II has not been investigated.
Past work suggested that a subset of POTS, with increased vasoconstriction
related to decreased bioavailable nitric oxide, had decreased blood volume.
Angiotensin-II reduces bioavailable NO and is integral to the
renin-angiotensin system. Thus, we investigated the relationship among blood
volume, angiotensin-II, renin, aldosterone and peripheral blood flow in
diagnosed by 70o upright tilt. Supine calf blood flow measured by
venous occlusion plethysmography was used to subgroup POTS patients. 23 POTS
patients were partitioned among 10 with low blood flow, 8 with normal flow,
and 5 with high flow. There were 10 healthy volunteers. Blood volume was
measured by dye dilution. Biochemical measurements were performed supine.
Blood volume was decreased in low flow POTS (2.14±.12
L/M2) compared to control (2.76±.20 L/M2) but not
in other subgroups. PRA was decreased in low flow POTS (0.49±.12 vs
0.90±0.18 ng/ml/hr), while plasma angiotensin-II was increased (89±20 vs
32±4 ng/L), but not in other subgroups. PRA correlated with aldosterone
(r=+0.71) over 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 angiotensin (r=+0.76) in normal and
high flow POTS but negatively (r=-.83) in low flow POTS. Blood volume was
negatively correlated to angiotensin II (r=-0.66) in normal and high flow
POTS, and in five low flow POTS patients. The remaining five patients had
reduced blood volume and increased angiotensin-II uncorrelated to blood
volume. Data suggest that plasma angiotensin-II is increased in low flow
POTS patients with hypovolemia which may contribute to local blood flow
dysregulation and reduced NO bioavailability.
The shows results for plasma
renin activity (PRA) in left panel, serum aldosterone in the middle panel,
and plasma angiotensin II concentration in the right panel for all subjects.
PRA is significantly decreased and angiotensin is increased in low flow POTS
patients compared to controls subjects. Angiotensin II concentrations seem
to follow a bimodal distribution.