title: Cranial diameter pulsations measured by non-invasive ultrasound decrease with tilt.
abstract: BACKGROUND
  Intracranial pressure (ICP) may play a significant role in physiological responses to microgravity by contributing to the nausea associated with microgravity exposure. However, effects of altered gravity on ICP in astronauts have not been investigated, primarily due to the invasiveness of currently available techniques. We have developed an ultrasonic device that monitors changes in cranial diameter pulsation non-invasively so that we can evaluate ICP dynamics in astronauts during spaceflight. This study was designed to demonstrate the feasibility of our ultrasound technique under the physiological condition in which ICP dynamics are changed due to altered gravitational force.
  
  
  METHODS
  Six healthy volunteers were placed at 60 degrees head-up, 30 degrees headup, supine, and 15 degrees head-down positions for 3 min at each angle. We measured arterial blood pressure (ABP) with a finger pressure cuff, and cranial diameter pulsation with a pulsed phase lock loop device (PPLL).
  
  
  RESULTS
  Analysis of covariance demonstrated that amplitudes of cranial diameter pulsations were significantly altered with the angle of tilt (p < 0.001). The 95% confidence interval for linear regression coefficients of the cranial diameter pulsation amplitudes with tilt angle was 0.862 to 0.968. However, ABP amplitudes did not show this relationship.
  
  
  CONCLUSIONS
  Our noninvasive ultrasonic technique reveals that the amplitude of cranial diameter pulsation decreases as a function of tilt angle, suggesting that ICP pulsation follows the same relationship. It is demonstrated that the PPLL device has a sufficient sensitivity to detect changes non-invasively in ICP pulsation caused by altered gravity.
authors.affiliation: Gravitational Research Branch, Life Science Division, NASA Ames Research Center, Moffett Field, CA, USA. tueno@ucsd.edu
authors.forename: Toshiaki
authors.initials: T
authors.lastname: Ueno
authors.forename: Richard E
authors.initials: RE
authors.lastname: Ballard
authors.forename: Brandon R
authors.initials: BR
authors.lastname: Macias
authors.forename: William T
authors.initials: WT
authors.lastname: Yost
authors.forename: Alan R
authors.initials: AR
authors.lastname: Hargens
mesh_terms: D000328:Adult; D000337:Aerospace Medicine; D005260:Female; D018495:Fluid Shifts / Q000502:physiology; D006257:Head / Q000000981:diagnostic imaging; D018475:Head-Down Tilt / Q000502:physiology*; D006801:Humans; D007427:Intracranial Pressure / Q000502:physiology*; D008297:Male; D011673:Pulsatile Flow / Q000502:physiology; D013026:Space Flight; D018617:Ultrasonography, Doppler, Pulsed / Q000379:methods; D018474:Weightlessness Simulation