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At least 685 records · Page 38
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NASA NTRSDate not supplied

Cardiovascular Responses to Daily Use of Lower Body Negative Pressure During 30 Days of Strict Head-Down Tilt Bedrest

The purpose of this study was to (1) measure the heart rate (HR) and mean arterial pressure (MAP) response and subject tolerance before and during twice daily use of 3 hours of 25 mmHg LBNP during 30 days of HDTBR and (2) determine if this level of LBNP during HDTBR would reverse the headward fluid shift to a similar degree as upright posture.

Jason R. Lytle↗
NASA NTRSDate not supplied

Transition Aero-Propulsive Analysis of a Tilt-Wing eVTOL Aircraft Using a Surface-Vorticity Solver

This work uses FlightStream, a surface-vorticity solver, to perform an aero-propulsive analysis on the NASA LA-8 aircraft through the transition flight regime. Aerodynamic force and moment coefficient predictions for forward and transition flight of the airframe without propulsors are compared with wind tunnel data. Powered transition cases are simulated using Conway actuator disks to model the propellers and results are compared with response surface equations derived from wind tunnel data. The approach described in the paper provides mid-fidelity solutions at a reduced computational cost compared to volumetric CFD, which allows for rapid transition flight analyses and optimization in the early design stages. The best prediction results were observed in forward flight and high-speed transition. Mid-transition results were not as close in magnitude but the trends exhibited reasonable agreement with experimental data.

Advanced Air Mobility↗
NASA NTRS1996

Upper Body Venous Compliance Exceeds Lower Body Venous Compliance in Humans

Human venous compliance hypothetically decreases from upper to lower body as a mechanism for maintenance of the hydrostatic indifference level 'headward' in the body, near the heart. This maintains cardiac filling pressure, and thus cardiac output and cerebral perfusion, during orthostasis. This project entailed four steps. First, acute whole-body tilting was employed to alter human calf and neck venous volumes. Subjects were tilted on a tilt table equipped with a footplate as follows: 90 deg, 53 deg, 30 deg, 12 deg, O deg, -6 deg, -12 deg, -6 deg, O deg, 12 deg, 30 deg, 53 deg, and 90 deg. Tilt angles were held for 30 sec each, with 10 sec transitions between angles. Neck volume increased and calf volume decreased during head-down tilting, and the opposite occurred during head-up tilt. Second, I sought to cross-validate Katkov and Chestukhin's (1980) measurements of human leg and neck venous pressures during whole-body tilting, so that those data could be used with volume data from the present study to calculate calf and neck venous compliance (compliance = (Delta)volume/(Delta)pressure). Direct measurements of venous pressures during postural chances and whole-body tilting confirmed that the local changes in venous pressures seen by Katkov and Chestukhin (1980) are valid. The present data also confirmed that gravitational changes in calf venous pressure substantially exceed those changes in upper body venous pressure. Third, the volume and pressure data above were used to find that human neck venous compliance exceeds calf venous compliance by a factor of 6, thereby upholding the primary hypothesis. Also, calf and neck venous compliance correlated significantly with each other (r(exp 2) = 0.56). Fourth, I wished to determine whether human calf muscle activation during head-up tilt reduces calf venous compliance. Findings from tilting and from supine assessments of relaxed calf venous compliance were similar, indicating that tilt-induced muscle activation is relatively unimportant. Low calf venous compliance probably results from stiffer venous, skeletal muscle, and connective tissues, and better-developed local and central neural controls of venous distensibility. This research establishes that upper-to-lower body reduction of venous compliance can explain headward positioning of the hydrostatic indifference level in humans.

Watenpaugh, Donald E.↗
NASA NTRS2002

Do vestibular otolith organs participate in human orthostatic blood pressure control?

We hypothesized that vestibular otolith organ stimulation contributes to human orthostatic responses. Twelve subjects underwent three 60 degrees upright tilts: (1) with the neck flexed from 0 degrees to 30 degrees relative to the body during 60 degrees tilt, such that the head moved from horizontal to 90 degrees above horizontal (0 to 1 Gz otolith stimulation); (2) with the head and body aligned, such that they tilted together to 60 degrees (0 to 0.87 Gz otolith stimulation); and (3) with the neck flexed 30 degrees relative to the body during supine conditions, and the neck then extended to -30 degrees during 60 degrees body tilting, such that the head remained at 30 degrees above horizontal throughout body tilting (constant 0.5 Gz otolith stimulation). All three tilt procedures increased thoracic impedance, sympathetic nerve activity (N = 8 of 12), arterial pressure, and heart rate relative to supine conditions (all P < 0.04). Within the first 20 s of tilt, arterial pressure increased most obviously in the 0 to 1 Gz otolith condition. Thoracic impedance tended to increase more in otolith-constant conditions, but no dependent variable differed significantly between tilt conditions, and no significant time x tilt interactions emerged. Otolith inputs may contribute to early transient adjustments to orthostasis. However, lack of significant main effects of tilt condition and time x tilt interactions suggests that potential otolith effects on the variables we studied are relatively subtle and ephemeral, or that other mechanisms compensate for a lack of change in otolith input with orthostasis.

Non-NASA Center↗