Lower-body negative pressure decreases noninvasively measured intracranial pressure and internal jugular vein cross-sectional area during head-down tilt

Abstract

Long-term spaceflight induces a near visual acuity change in ~50% of astronauts. In some crew members, postflight cerebrospinal fluid (CSF) opening pressures by lumbar puncture are as high as 20.9 mmHg; these members demonstrated optic disc edema. CSF communicates through the cochlear aqueduct to affect perilymphatic pressure and tympanic membrane motion. We hypothesized that 50 mmHg of lower-body negative pressure (LBNP) during 15° head-down tilt (HDT) would mitigate elevations in internal jugular vein cross-sectional area (IJV CSA) and intracranial pressure (ICP). Fifteen healthy adult volunteers were positioned in sitting (5 min), supine (5 min), 15° HDT (5 min), and 15° HDT with LBNP (10 min) postures for data collection. Evoked tympanic membrane displacements (TMD) quantified ICP noninvasively. IJV CSA was measured using standard ultrasound techniques. ICP and IJV CSA increased significantly from the seated upright to the 15° HDT posture (P < 0.05), and LBNP mitigated these increases. LBNP at 25 mmHg reduced ICP during HDT (TMD of 322.13 ± 419.17 nl) to 232.38 ± 445.85 nl, and at 50 mmHg ICP was reduced further to TMD of 199.76 ± 429.69 nl. In addition, 50 mmHg LBNP significantly reduced IJV CSA (1.50 ± 0.33 cm²) during 15° HDT to 0.83 ± 0.42 cm² LBNP counteracts the headward fluid shift elevation of ICP and IJV CSA experienced during microgravity as simulated by15° HDT. These data provide quantitative evidence that LBNP shifts cephalic fluid to the lower body, reducing IJV CSA and ICP.NEW & NOTEWORTHY The current study provides new evidence that 25 or 50 mmHg of lower body negative pressure reduces jugular venous pooling and intracranial pressure during simulated microgravity. Therefore, spaceflight countermeasures that sequester fluid to the lower body may mitigate cephalic venous congestion and vision impairment.

Keywords: spaceflight; tympanic membrane displacement; venous congestion; vision.

Fig. 1.

Internal jugular vein cross-sectional area (IJV CSA) during sitting, supine, head-down tilt (HDT), and HDT + lower-body negative pressure (LNBP) at −25 [LBNP(25)] and −50 [LBNP(50)] mmHg conditions. IJV CSA decreased significantly with LBNP (50 mmHg) relative to HDT with no LBNP. IJV CSA at sitting posture was significantly different from all other test conditions. 15HDT, 15° head-down tilt. **IJV CSA decreased significantly with LBNP (50 mmHg) relative to HDT with no LBNP. *IJV CSA at sitting posture was significantly different from all other test conditions.

Fig. 2.

Representative IJV ultrasound images during sitting, supine, HDT, and HDT + LNBP at −25 and −50 mmHg conditions. These images demonstrate visually the progressive increase in IJV CSA (arrows) from sitting to supine to HDT posture. The application of LBNP reduced IJV CSA.

Fig. 3.

Tympanic membrane displacement (TMD) during sitting, supine, HDT, and HDT + LBNP at −25 and −50 mmHg conditions. The direction of the TMD is inversely related to the intracranial pressure (ICP) level. Therefore, for graphical clarity the noninvasive ICP TMD is expressed as –nl (−1 × nl). Noninvasive ICP significantly increased from sitting to supine to HDT posture. However, application of 25 and 50 mmHg LBNP significantly reduced noninvasive TMD measures of ICP. TMD in the sitting posture was significantly different from all other postures/tilt angles. **However, application of 25 and 50 mmHg LBNP significantly reduced noninvasive TMD measures of ICP. *TMD in the sitting posture was significantly different from all other postures/tilt angles.

Fig. 4.

TMD waveforms generated by the CCFP Analyzer were averaged across all subjects during sitting, supine, HDT, and HDT + LNBP at −25 and −50 mmHg conditions. The sitting TMD moves in the outward direction, after tone stimulation and indicating a low ICP. The supine and HDT conditions caused the TMD to move in the inward direction, and suggesting elevated ICP. Application of 25 and 50 mmHg LBNP during HDT caused the TMD to move outward, relative to HDT, suggesting that LBNP reduced ICP.

Fig. 5.

Calculated ICP values at sitting, supine, HDT, and HDT + LNBP at −25 and −50 mmHg conditions based on known values of directly measured ICP at supine posture.