title: Intraocular and intracranial pressures during head-down tilt with lower body negative pressure.
abstract: BACKGROUND
  Seven astronauts after 6-mo missions to the International Space Station showed unexpected vision problems. Lumbar punctures performed in the four astronauts with optic disc edema showed moderate elevations of cerebral spinal fluid pressure after returning to Earth. We hypothesized that lower body negative pressure (LBNP) imposed during head-down tilt (HDT) would reduce intraocular pressure (IOP) and transcranial ultrasound pulse amplitude, a noninvasive intracranial pressure (ICP) surrogate.
  
  
  METHODS
  Participating in this study were 25 normal healthy nonsmoking volunteers (mean age: 36 yr). Subjects were positioned supine (5 min), sitting (5 min), 15° whole body HDT (5 min), and 10 min of HDT with LBNP (25 mmHg). The order of HDT and HDT+LBNP tests was balanced. Right and left IOP, transcranial ultrasound pulse amplitude, arm blood pressure, and heart rate were measured during the last minute (steady state) of each testing condition.
  
  
  RESULTS
  IOP significantly decreased from supine to sitting posture by 3.2 ± 1.4 mmHg (mean ± SD: N = 25), and increased by 0.9 ± 1.3 mmHg from supine to the HDT position. LBNP during HDT significantly lowered IOP to supine levels. In addition, LBNP significantly reduced transcranial ultrasound pulse amplitudes by 38% as compared to the HDT condition (N = 9). Sitting mean blood pressure (BP) was significantly higher (+5 mmHg) than BP values after 10 min of LBNP during HDT. However, heart rate was not significantly different across all conditions.
  
  
  CONCLUSIONS
  These data suggest that short duration exposures to LBNP attenuate HDT-induced increases in IOP and ICP. Macias BR, Liu JHK, Grande-Gutierrez N, Hargens AR. Intraocular and intracranial pressures during head-down tilt with lower body negative pressure.
authors.affiliation: Departments of Orthopaedic Surgery and Ophthalmology, UCSD Medical Center, University of California, San Diego, CA, USA.
authors.forename: Brandon R
authors.initials: BR
authors.lastname: Macias
authors.forename: John H K
authors.initials: JH
authors.lastname: Liu
authors.forename: Noelia
authors.initials: N
authors.lastname: Grande-Gutierrez
authors.forename: Alan R
authors.initials: AR
authors.lastname: Hargens
mesh_terms: D000293:Adolescent; D000328:Adult; D000337:Aerospace Medicine; D001794:Blood Pressure; D005260:Female; D018475:Head-Down Tilt / Q000502:physiology*; D006339:Heart Rate; D006801:Humans; D007427:Intracranial Pressure; D007429:Intraocular Pressure; D008150:Lower Body Negative Pressure; D008297:Male; D008875:Middle Aged; D016683:Supine Position; D017585:Ultrasonography, Doppler, Transcranial; D018474:Weightlessness Simulation; D055815:Young Adult