Lower Body Negative Pressure Decreases Intracranial Pressure and Internal Jugular Vein Cross‐Sectional Area

Abstract

IntroductionLong‐term space flight subjects astronauts to both adaptive and deleterious health effects. Seven astronauts post six‐month space flight suffered decreased near vision, globe flattening, optic disc edema, choroidal folds, nerve fiber layer thickening, and cotton wool spots within the retinal nerve fiber layer. Additionally, lumbar punctures performed on astronauts diagnosed with optic disc edema after a six‐month mission showed increase in lumbar opening pressure, perhaps suggesting increased intracranial pressure (ICP). Although the mechanisms underlying these pathologies are not well understood, head‐ward fluid shifts experienced in microgravity may initiate these pathophysiologic processes. We hypothesized that during head‐down tilt (HDT), lower body negative pressure (LBNP) of 25 and 50 mmHg lowers ICP and decreases internal jugular vein cross‐sectional area (IJV CSA) due to lower extremity fluid shifts.MethodsFifteen (7F, 8M) healthy adult volunteers (age 19–60 years) were positioned at sitting posture (5 min), supine posture (5 min), 15° HDT (5 min), and 15° HDT with 10 min of LBNP of 25 and 50 mmHg with the order of HDT and HDT+LBNP randomized. Noninvasively, ICP was measured with the Cerebral Cochlear Fluid Pressure Analyzer, a device that measures tympanic membrane displacement (Vm). IJV CSA was measured by standard ultrasound.ResultsICP and IJV CSA both decreased after exposure to LBNP (25 and 50 mmHg) at 15HDT. Compared to Vm at 15HDT (−322.13 ± 419.17 nL), Vm significantly increased to −232.38 ± 445.85 nL and −199.76 ± 429.69 nL during 15HDT with LBNP (25 mmHg) and LBNP (50 mmHg), respectively (Fig. 1). A 40.0% increase in Vm (indicating a reduction in ICP) was evident after 10 minutes of 15HDT+LBNP (50 mmHg) relative to HDT posture. Further, compared to IJV CSA at 15HDT (1.50 ± 0.33 cm2), IJV CSA was significantly reduced to 0.83 ± 0.42 cm2 during 15HDT+LBNP (50 mmHg) (Fig. 2). With respect to 15HDT posture, 15HDT+LBNP (50 mmHg) for 10 minutes decreased IJV CSA by 44.5%. For both ICP and IJV CSA, there were no significant differences among the supine, 15HDT+LBNP (25 mmHg), and 15HDT+LBNP (50 mmHg) conditions.DiscussionLBNP mitigates the effect of head‐ward fluid shifts experienced during microgravity simulated by HDT and thus, decreases both ICP and IJV CSA. This study is the first to measure ICP and IJV CSA noninvasively during LBNP and also give quantitative evidence for a foot‐ward fluid shift concomitant with decreased ICP. Further studies are necessary to elucidate how LBNP lowers ICP and IJV dimensions.Support or Funding InformationThis study was supported by NSBRI through NCC 9‐58 to BRM, NASA grant NNXAJ12G to ARH, and NIH grant 5 T35 HL 7491‐35 to WW.