Changes in cardiac output (CO in L/min) in control (top panel) and POTS patients with OI (bottom panel) during imposition of a controlled orthostatic challenge using LBNP following no treatment (No TX – white bars), IV sal ine (Saline – gray bars) and ingestion of oral rehydration solution (ORS – black bars). Treatment effects for controls were not significantly different. For POTS, CO was significantly higher following ORS, compared with no treatment (* = No Tx, p<0.05), both at baseline and −45mmHg and saline had no significant effect.