Astronauts experience a chronic headward fluid shift when exposed to a weightless environment, causing a redistribution of blood and tissue fluid. This has unknown consequences to the overall behavior of the cardiovascular system and could potentially contribute to undesired changes in cardiovascular (CV) regulation, impaired cerebral venous flow, and the occurrence of Spaceflight-Associated Neuro-ocular Syndrome (SANS). SANS refers to the constellation of significant structural and functional neuro-ophthalmological findings in astronauts during both short- and long-duration exposure to microgravity. The etiology of SANS is currently unknown, but it is clearly related to the unique effects of microgravity on the body (e.g. loss of tissue weight, loss of hydrostatic gradients with the resulting fluid redistribution). Effective countermeasures and protocols that can reduce the headward fluid shifting in microgravity, such as lower body negative pressure (LBNP) or centrifugation, must be further investigated. However, at present, it is not possible to estimate the overall physiological response of a particular “dose” of either of these countermeasures.
This investigation seeks to characterize acute dose-response curves of cardiovascular and ocular variables due to changes in the gravitational vector with and without countermeasures and model these relationships using the following specific aims.
1) Generate dose-response relationships between CV and ocular parameters as a function of changes in the gravitational vector.
2) Develop a computational model to predict CV and ocular variables due to changes in the gravitational vector.
3) Identify, explore, and simulate the effects of countermeasures (i.e. LBNP, on CV and ocular parameters) using both experimental results and modeling techniques.
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Cardiovascular and ocular responses will be characterized by inducing orthostatic stresses using three different experimental approaches: tilt table, LBNP, and head-to-foot- centrifugation. In the tilt table test, subjects will be exposed to multiple tilt angles, covering the 360° spectrum in both prone and supine configurations for five minutes. The effects of LBNP will be quantified by exposing subjects to multiple levels of negative and positive pressure for until steady state is achieved (approximately 10 minutes). In the centrifugation arm of the experiment, subjects will be exposed to multiple levels of artificial gravity in the head-to-toe direction (Gz) on a human short-radius centrifuge. Twelve healthy male subjects between 20 to 30 years of age will participate in all three experiments.
Baseline data in upright, upright seated, and supine positions will be collected prior to testing. Cardiovascular and ocular measures will be collected once the subject has reached steady state. These data will be used to generate gravitational dose-response curves as a function of changes to the gravitational vector (Specific Aim 1) and develop a computational model to predict CV and ocular variables (Specific Aim 2). To address Specific Aim 3, this research will produce numerical models for two different but complementary lumped-parameter models: a full body model and an eye model. These models will be validated using experimental data and will provide a more versatile model for studying SANS.
The dose response curves resulting from this experiment will allow researchers to further computationally explore the effects of extreme orthostatic stressors that would otherwise be prohibitively difficult, expensive, or infeasible to collect and inform the development of current and future countermeasures and in-flight prescriptions.
Aorta cross-section area
Arterial blood saturation
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Common carotid artery
Diastolic pressure time index
Heart rate power spectrum
Heart rate variability
Heart rate variability (HRV)
Internal jugular vein area
Internal jugular vein pressure
Left ventricular ejection time
Normal heart beat (NN) interval standard deviation
Pulmonary blood flow
Rate pressure product
Total peripheral resistance