Effects of High Sodium Intake on Plasma Volume and Physical Fitness in Patients with Postural Tachycardia Syndrome and Healthy Females

Abstract

Background: Patients with postural tachycardia syndrome (POTS) oftenhave lowblood volumeand are advised to increase sodium intake to restore intravascular volume. However, the benefit of a high-sodium diet has not been systematically examined in POTS. Methods: We studied 12 female POTS patients (mean± SD; 34 ±9 years, BMI 23 ± 3 kg/m) and 8 female healthy control subjects (HC; 29 ± 4 years, BMI 24 ± 3 kg/m) randomly assigned to 6 days of low (LS; 10 mEq/day) or high sodium (HS; 300 mEq/day) diet and then crossed-over. Procedures performed on Day 6 included: posture study, plasma volume (PV) measurement by I-albumin, maximal oxygen consumption (VO2max) using a supine bicycle ergometer, and cardiac output (CO) and stroke volume (SV) assessments by the inert gases rebreathing technique in the flat and tilted head-up positions. Results: PV was significantly higher after HS than LS in POTS patients (PV: 2706 ± 110 ml vs. 2390 ± 89 ml, P b 0.001). Orthostatic tachycardia was reduced in POTS with HS (49 ± 16 bpm vs. 63 ±11 bpm, P = 0.001), but still increased compared with HC (49 ± 16 bpm vs. 23 ± 11 bpm, P = 0.001). Upright SV was significantly higher after HS than LS in POTS (35 ± 11ml vs. 25 ± 7ml, P = 0.023) but remained lower than HC (35 ± 11 ml vs. 52 ± 16ml, P = 0.033). Upright CO and VO2max did not differ significantly between POTS for either LS or HS. Conclusions: HS diet caused a reduction in orthostatic tachycardia, and increased PV and SV in POTS patients. Patients with POTS experienced improvement in their condition in association with higher sodium intake, but did not ‘normalize’ compared with HC.