Splanchnic-mesenteric capacitance bed in the postural tachycardia syndrome (POTS)

Background: Gastroenterologic symptoms are common in the postural tachycardia syndrome (POTS), and postprandial worsening of orthostatic symptoms often occurs. We, therefore, investigated splanchnic-mesenteric vasoregulation in POTS. Subjects and methods: Eleven patients with POTS (one man, 10 women, 29.4±7.7 (S.D.) years) and 10 controls (two men, eight women, 27.9±5.6 years) participated in this study. The protocol included 5 min of 70° head-up tilt (HUT) before and after a liquid meal, as well as 1.5 min of hyperventilation. Blood pressure (BP), heart rate (HR), endtidal CO₂, and cardiovascular indices derived from thoracic electrical bioimpedance were continuously monitored. Superior mesenteric artery (SMA) blood flow was measured by real time Doppler ultrasound and analyzed off-line. Cross-sectional area of SMA (SMA-area) and time-averaged velocity (SMA-TAV) were measured; SMA blood flow (SMA-BF) and vascular resistance (SMA-VR) were derived. Results: The following significant results were found: at supine rest, the POTS group had higher HR, BP, SMA-TAV and SMA-BF and a lower SMA-VR than the control group. HUT resulted in a reduction of pulse pressure, CO₂ level, SMA-area, SMA-TAV and SMA-VF and increment of HR and SMA-VR in both groups. The POTS group underwent greater increment of HR and greater reduction of CO₂ than controls. Hyperventilation induced increment of HR and cardiac index (CI) and reduction of SMA-VR in controls; no significant change occurred in POTS. The test meal induced increments of HR, CI, SMA-area, SMA-TAV and SMA-VF and reduction of SMA-VR in patients and controls for both supine rest and HUT. Conclusion: The main novel observations of increased resting SMA-BF, SMA-TAV supine, and reduced SMA-VR when compared with controls support the notion that there is excessive splanchnic capacity (pooling) at rest in POTS. Copyright (C) 2000 Elsevier Science B.V.