Dietary Sodium's Effect on Urinary Sodium and Dopamine Excretion in Patients With Postural Tachycardia Syndrome

• Urinary sodium [ Time Frame: after 6 days of each dietary sodium level ] [ Designated as safety issue: No ]
  Amount of sodium excreted in urine over 24hr will be measured.
  
  
• Urinary dopamine [ Time Frame: after 6 days of each dietary sodium level ] [ Designated as safety issue: No ]
  Amount of dopamine excreted in urine over 24 hours will be measured.
  
  

• Blood Volume [ Time Frame: after 6 days of each dietary sodium level ] [ Designated as safety issue: No ]
  DAXOR (131-I labelled albumin) blood volume assay
  
  
• Change in Heart Rate with Standing [ Time Frame: After 6 days of each dietary sodium level ] [ Designated as safety issue: No ]
  Heart rate is measured while supine and standing during a Posture Study and the change in heart rate with standing is calculated.
  
  
• Orthostatic Symptoms [ Time Frame: After 6 days of each dietary sodium level ] [ Designated as safety issue: No ]
  Symptoms will be assessed in the supine and upright postures to determine the Vanderbilt Orthostatic Symptoms Score.
  
  
• Change in Urinary Sodium following Change in Dietary Sodium [ Time Frame: Days 1-5 of each dietary sodium level ] [ Designated as safety issue: No ]
  Urinary sodium excretion will be measured every 24 hours as the participant adapts from the 150 mEq Na/day diet to the 10 and 300 mEq Na/day diets.
  
  
• Change in Urinary Dopamine following Change in Dietary Sodium [ Time Frame: Days 1-5 of each dietary sodium level ] [ Designated as safety issue: No ]
  Urinary dopamine excretion will be measured every 24 hours as the participant adapts from the 150 mEq Na/day diet to the 10 and 300 mEq Na/day diets.
  
  

Study Day 1

• Start 150 mEq Na+/day diet (POTS patients as inpatients; healthy control subjects with CRC provided outpatient diet); consume 1.5-2 liters of water per day
• Start a 24h urine collection (for Na+, K+, Cr, fractionated catecholamines)
• Blood work

Study Days 2-5

• Continue 24h urine collection
• Start STUDY DIET (10 mEq Na+/day or 300 mEq Na+/day in a random order) after 3 meals of 150 mEq Na+/day are complete; consume 1.5-2 liters of water per day
• On Day 5, a 24 hr Holter combined ECG monitor and BP monitor will be placed on the subjects.

Study Day 6

• Continue STUDY DIET; consume 1.5-2 liters of water per day
• Remove 24h Holter combined ECG monitor and BP monitor from subject
• Continue 24h urine collection (for Na+, K+, Cr, fractionated catecholamines)
• Admit to CRC in afternoon (healthy control subjects only, as POTS patients will have already been admitted). Each subject will spend the night in the CRC and remain supine
• NPO after midnight for study next day

Study Day 7

• Awaken early (~6am) to void (still collecting 24h urine)
• Patient returns to bed, IV catheter inserted
• Posture Study (in morning; between 7-8am ideally)
• Blood pressure and heart rate will be measured while supine and then while standing for up to 30 minutes
• We will draw blood in each body position to measure electrolytes and hormones that regulate blood pressure and blood volume
• Subjects will rate symptoms during supine period and at end of stand using Vanderbilt Orthostatic Symptoms Score (VOSS)
• Total Blood Volume (DAXOR)- using injection of iodinated I-131 tagged human serum albumin nominally 25 micro-Ci of radiation blood samples drawn through IV catheter before injection and for ~30 minutes post-injection (total - 25 ml)
• This will be done after supine assessment, but before standing the subject up
• Exercise Capacity Test (in the afternoon) Will estimate maximal oxygen consumption (VO2 max) This test will be conducted on a stationary bicycle. Effort will be gradually increase while expired air is measured during exhaustive physical work.

All procedures are repeated at least a month later with the 2nd level of dietary salt. (Randomized to high or low salt in the first phase, the second phase is the remaining level)