Persistent splanchnic hyperemia during upright tilt in postural tachycardia syndrome

Abstract

Previous investigations have allowed for stratification of patients with postural tachycardia syndrome (POTS) on the basis of peripheral blood flow. One such subset, comprising "normal-flow POTS" patients, is characterized by normal peripheral resistance and blood volume in the supine position but thoracic hypovolemia and splanchnic blood pooling in the upright position. We studied 32 consecutive 14- to 22-yr-old POTS patients comprising 13 with low-flow POTS, 14 with normal-flow POTS, and 5 with high-flow POTS and 12 comparably aged healthy volunteers. We measured changes in impedance plethysmographic (IPG) indexes of blood volume and blood flow within thoracic, splanchnic, pelvic (upper leg), and lower leg regional circulations in the supine posture and during incremental tilt to 20 degrees, 35 degrees, and 70 degrees. We validated IPG measures of thoracic and splanchnic blood flow against indocyanine green dye-dilution measurements. We validated IPG leg blood flow against venous occlusion plethysmography. Control subjects developed progressive vasoconstriction with incremental tilt. Splanchnic blood flow was increased in the supine position in normal-flow POTS, despite marked peripheral vasoconstriction, and did not change during incremental tilt, producing progressive splanchnic hypervolemia. Absolute hypovolemia was present in low-flow POTS, all supine flows and volumes were reduced, there was no vasoconstriction with tilt in all segments, and segmental volumes tended to increase uniformly throughout tilt. Lower body (pelvic and leg) flows were increased in high-flow POTS at all angles, with consequent lower body hypervolemia during tilt. Our main finding is selective and maintained orthostatic splanchnic vasodilation in normal-flow POTS, despite marked peripheral vasoconstriction in these same patients. Local splanchnic vasoregulatory factors may counteract vasoconstriction and venoconstriction in these patients. Lower body vasoconstriction in high-flow POTS was abnormal, and vasoconstriction in low-flow POTS was sustained at initially elevated supine levels.

Postural Hyperventilation as a Cause of Postural Tachycardia Syndrome: Increased Systemic Vascular Resistance and Decreased Cardiac Output When Upright in All Postural Tachycardia Syndrome Variants

“…Decreased upright CO in POTS has been reported by our group and by others,27, 30, 31 and is associated with increased SVR,27, 31 consistent with sustained BP and marked reduction in upright stroke volume 32. However, a subset of POTS may have increased CO during exercise 33.…”

“…We placed paired electrodes using anatomic landmarks to estimate thoracic, splanchnic, pelvic, and calf segmental blood volumes, by high‐frequency impedance plethysmography 27. Changes in electrical impedance were used to calculate changes in segmental or regional blood volumes.…”

“…Δsegmental blood volume (mL)=ρ(L 2 /R0R1)ΔR, where ρ is electrical conductivity of blood estimated as 53.2·exp(hematocrit·0.022) 27. R0 is the initial impedance, R1 is its final value, and ΔR=R1−R0 and L is the distance between the electrode pairs.…”

“…Impedance plethysmography was measured using a tetrapolar, high‐resolution, 4‐channel digital impedance plethysmograph, as previously described 14, 27. These quantities were obtained within 4 anatomic segments, defined by electrode placement on anatomic landmarks that delimit respective regional circulations, designated the thoracic segment or CBV (supraclavicular area to xyphoid process), the splanchnic segment (xyphoid process to iliac crest), the pelvic segment, including lower pelvis to the knee (iliac crest to knee), and the leg or calf segment (upper calf just below the knee to the ankle).…”

Hemodynamics and brain blood flow during posture change in younger women and postmenopausal women compared with age-matched men

“…It has been speculated that female sex hormones might modulate reflex neural or vascular properties, resulting in the greater incidence of orthostatic hypotension in young women (5,14,15,52,69). An additional indication of impaired orthostatic responses in women includes the observation that women have a greater propensity towards postural orthostatic tachycardia syndrome (18,60).…”