The study includes up to 5 days spent in the Vanderbilt University Medical Center, at least one day of screening tests, followed by 2 study days and/or 2 study nights.~Screening tests include a physical examination and history, routine safety laboratory assessments, and autonomic nervous system testing. Medications for high blood pressure will be held for at least 5 half-lives before studies. Subjects may be able to participate in the daytime and/or the overnight studies.~Daytime Study:~Eligible participants will be studied on two separate days in random order: one day with a high CPAP level and one day with a low CPAP level. The active CPAP level will be determined during a CPAP titration trial~On each study day, participants will be instrumented to measure blood pressure, heart rate, hemodynamic parameters, segmental impedance, and markers of cardiovascular risk. A saline lock or IV catheter will be inserted in one of the arm's veins for blood sample collection. Urine will also be collected during studies.~After baseline measurements, active or sham CPAP will then be applied for up to 2 hours. Outcome measurements will be repeated after 1 and 2 hours of CPAP.~Overnight Studies:~Eligible participants will be studied on three separate nights in random order with a active CPAP, sham CPAP, and sleeping in a head-up tilt position. The active CPAP level will be determined during a CPAP titration trial.~On each study night, the intervention (active CPAP, sham CPAP, or head-up tilt position) will be applied for up to 9 hours. Blood pressure, heart rate, hemodynamic parameters, and markers of cardiovascular risk will be assessed throughout the night. Urine will also be collected during this period. On the following morning, participants will have a tilt table test.
This study aims to learn about the effects of continuous positive airway pressure (CPAP) on people with autonomic failure and high blood pressure when lying down (supine hypertension) to determine if it can be used to treat their high blood pressure during the night. CPAP (a widely used treatment for sleep apnea) involves using a machine that blows air into a tube connected to a mask covering the nose, or nose and mouth, to apply a low air pressure in the airways.~The study includes 3-5 days spent in the Vanderbilt Clinical Research Center (CRC): at least one day of screening tests, followed by up to 3 study days. Subjects may be able to participate in daytime and/or overnight studies. The Daytime study consists of 2 study days: one with active CPAP and one with sham CPAP applied for up to 2 hours. The Overnight study consists of 3 study nights: one with active CPAP, one with sham CPAP, both applied for up to 9 hours and one night sleeping with the bed tilted head-up.