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

Study Day 1

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

Study Days 2-5

• Continue 24hr 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 24hr Holter combined ECG monitor and BP monitor from subject
• Continue 24hr 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
• Nothing by mouth (NPO) after midnight for study next day

Study Day 7

• Awaken early (~6am) to void (still collecting 24hr 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 increased 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)

Inclusion criteria

• Premenopausal patients with POTS and healthy volunteers, 18-50 years old, who are non-smokers and free of medications with the potential to influence blood pressure
• Patients diagnosed with postural tachycardia syndrome by the Vanderbilt Autonomic Dysfunction Center
• Patients who Increase heart rate ≥30 beats/min with position change from supine to standing (10 minutes)
• For patients, chronic symptoms consistent with POTS that are worse when upright and get better with recumbence
• Only female participants are eligible. Since 80-90% of POTS patients are female, and there can be differences in measures with the menstrual cycle, including a small number of males might introduce a significant amount of noise.
• Able and willing to provide informed consent

Exclusion criteria

• Smokers
• Overt cause for postural tachycardia, i.e., acute dehydration
• Significant cardiovascular, pulmonary, hepatic, or hematological disease by history or screening results
• Positive pregnancy test or breastfeeding
• Hypertension defined as BP>145/95 off medications when supine or needing antihypertensive medication
• Other factors which in the investigator's opinion would prevent the participant from completing the protocol, including poor compliance during previous studies or an unpredictable schedule
• Unable to give informed consent