Postural variations of intraocular pressure--preflight experiments for the D1-mission

Specific alterations in systemic circulation due to fluid shift in microgravity may lead to a rise in intraocular pressure (IOP). This situation can be simulated by head-down tilt. Several series of tonometry were performed using a handheld applanation tonometer: (1) Short time postural changes up to -90 degrees head down and back. (2) A period of 2 h in -10 degrees head down. (3) Repetition of series 2 after dehydration of the subjects. (4) Seven-day bedrest study in -7 degrees head-down tilt. (5) A period of lower body negative pressure (LBNP). (6) Valsalva maneuver. Immediately on tilting, the IOP rises with hydrostatic pressure and returns to normal again after about 1 h. The IOP seems to change parallel to venous pressure. Further experiments are planned for Spacelab missions.