 Patients with chronic OI appear to be hypovolemic with abnormalities in hormones that regulate salt water handling. Increases in dietary salt have salutary effects on orthostatic tolerance in a physiological laboratory. The infusion of intravenous saline acutely decreased heart rate in this patient population. Preliminary data from Vanderbilt suggests abnormal salt handling in patients with chronic OI in a few patients. These data need to be confirmed and a better understanding of sodium handling in response to acute salt loads is required in these patients. 