Eight male crewmembers volunteered to participate in this countermeasure evaluation. Three subjects withdrew from participation after the collection of preflight data. Two of these subjects were waived from participation: one had a prolonged QTc interval on an electrocardiogram during in-flight screening and one experienced significant neurovestibular disturbances on landing day. The third subject voluntarily withdrew from participation after the preflight midodrine tolerance test.
Approximately 90-days before flight, the participants will undergo a drug tolerance test for midodrine and will participate in a drug familiarization session. An operational tilt test will be conducted 10-days prior to launch, and the participants will complete a brief questionnaire before they leave the testing room.
In the course of this study, countermeasure subjects participated in a familiarization session and drug tolerance test, an 80 degree head-up tilt test without medications approximately 10 days before launch (L-10), and the tilt test protocol within 60 minutes of Shuttle landing (R+0). Subjects received verbal and written explanation of all procedures and signed statements of informed consent prior to participation. Preflight Activities
Approximately 10 days before launch (L-10), crewmembers participated in a tilt test in the JSC Cardiovascular Laboratory. Data were collected for 6 minutes while the subjects were supine, before they were tilted to 80 degrees head-up tilt using an automatic tilt table. The subjects remained in this position for 10 minutes, or until symptoms of orthostatic hypotension and/or presyncope occurred.
Crewmembers participated in their scheduled in-flight activities without restriction for the duration of their Shuttle mission. After the decision for deorbit burn was confirmed on the scheduled landing day, crewmembers donned the Advanced Crew Escape Suit (ACES) and ingested 10 mg of midodrine approximately one 1 hour before the scheduled landing time. The midodrine pill and a cue card with medication instructions had been stowed in the subjects’ ACES for their convenience. All crewmembers participated in the standard oral fluid-loading protocol (equivalent to isotonic saline at a rate of 15 ml•kg-1 within 2 hours of landing) and inflated their antigravity suits during reentry and landing.
Postflight testing on landing day (R+0) was conducted on the Shuttle runway at either Kennedy Space Center, FL, or Dryden Flight Research Center, CA, in NASA’s Crew Transport Vehicle (CTV), a modified airport “people mover” used to transport the crewmembers from the Space Shuttle to the data collection facility. After Shuttle wheel stop, the CTV approached the Orbiter with the rest of the NASA convoy, and the Shuttle hatch was opened within ~20 min. After a brief medical check by the NASA flight surgeons, the crewmembers exited the Orbiter, with the midodrine test subjects exiting first. The subjects doffed their ACES within 5 min and participated in the same tilt test protocol as during the preflight assessment.
Midodrine appears to prevent orthostatic intolerance in test subjects after bed rest and in astronauts after space flight when testing is conducted in a controlled laboratory setting within 2 to 4 hours after landing. It is unclear at this time whether similar effects can be expected during reentry and immediately after landing, particularly in warmer environments and/or when the crewmembers are still wearing the ACES. Accurate interpretation of the current data requires that similar data be collected in control subjects (without midodrine) on the CTV. However, concerns about drug interactions with commonly used anti-emetics and prolonged QTc intervals observed in astronauts returning from long-duration missions make the routine use of midodrine unlikely and reliance on lower-body compression garments preferable.