Autoimmune Basis for Postural Tachycardia Syndrome

Phenylephrine is a selective α1-adrenergic receptor agonist. It will be given in IV bolus injections starting from 12.5 ug. Incremental doses will be given every \~3 min up to 800 ug or until systolic blood pressure increases by 25 mmHg

Isoproterenol is non-selective beta-adrenergic agonist. It will be given in IV bolus injections starting from 0.025 ug. Incremental doses will be given every \~3 min until heart rate increases by 25 bpm. This intervention is optional.

Using injection of iodinated I-131 tagged human serum albumin nominally 25 micro-Ci of radiation, blood samples are drawn before and 30 minutes after injection.

Blood pressure and heart rate will be measured while supine and then while standing for up to 30 minutes. Blood will be drawn in each position to measure hormones that regulate blood pressure and blood volume. An additional sample will be collected in the supine position for the autoantibody assessment.

Blood pressure, heart rate and ECG monitoring for 24 hours

The QSART assesses the ability of sympathetic nerve terminals in the skin to release acetylcholine and increase sweat production. The test is performed at 4 sites over the forearm, proximal lateral leg, medial distal leg and proximal foot.

The autonomic function tests will determine how well the autonomic nervous system regulates blood pressure and heart rate. These tests include breathing deeply for two minutes, breathing fast for 30 seconds, maintaining a handgrip for 3 minutes, breathing against pressure for 15 seconds, and placing the hand in ice water for 1 minute. In addition, participants will be tilted up on a tilt table for up to 10 minutes while recording their heart rate, blood pressure and cardiac output.

Cardiac output will be measured using the rebreathing technique (Innocor)

Splanchnic capacitance will be assessed using cpap and body impedance to construct pressure volume curves

microneurography will be measured in the peroneal nerve to assess sympathetic activity.