<qa>  What is the cause of the infusion of intravenous saline?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. 