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.

Fig. 1

Cardiac index (A) estimated by using indocyanine green (ICG) dye-dilution technique vs. impedance plethysmography (IPG), splanchnic blood flow (B) estimated using ICG dye-dilution technique vs. IPG, and calf blood flow (C) measured by venous occlusion plethysmography and estimated by IPG (abscissa). Data from all subjects were used. Correlation coefficients (r) are shown. In general, data showed fixed and proportionate biases. Nl, normal.

Fig. 2

Changes in mean arterial pressure (MAP) and heart rate (HR) during incremental upright tilt. MAP was increased in low-flow postural tachycardia syndrome (POTS) patients at 20° and 35° upright tilt. HR was significantly increased at 35° and 70° upright tilt. bpm, beats/min. *Significantly different from control, P < 0.05.

Fig. 3

Percent changes in thoracic, splanchnic, pelvic, and leg blood volumes during incremental upright tilt averaged over subject groups. Splanchnic changes dominate normal-flow postural tachycardia syndrome (POTS). Blood pooling is widespread in low-flow POTS patients. Blood pools in the lower body in high-flow POTS patients. *Significantly different from control, P < 0.05.

Fig. 4

Percent changes in thoracic, splanchnic, pelvic, and leg (calf) blood flow. Blood flow decreases for control in all segments. Blood flow does not change in splanchnic segment for normal-flow POTS, while leg blood flow is markedly reduced. Blood flow does not change in peripheral circulation (pelvic and leg) in high-flow POTS patients. Changes in blood flow are uniformly blunted in low-flow POTS patients. *Significantly different from control, P < 0.05.