Presentation on internal jugular vein flow in microgravity, 2021 NASA HRP Investigators Workshop
The Importance of Tissue Weight and Tissue Compressive Forces in Human Spaceflight
Abstract
The fluid shift and loss of
hydrostatic forces that occur upon entering microgravity are often
presented as key stimuli for some of physiological changes in space.
Less often discussed is the effect of tissue weight. Tissue weight and
tissue compressive forces are important, and often overlooked,
contributors to the physiological changes that occur in space, and need
to be considered when trying to understand the cardiovascular,
cerebrovascular, and ocular effects of microgravity. The weight of
tissues generates compressive forces that act on the walls of veins and
venules. Upon entering microgravity, tissues no longer have any weight,
and so any tissue compressive forces produced by the tissues are lost,
removing compressive forces from the vessel walls. This likely increases
vascular capacity, and allows the same amount of blood to be contained
at a lower pressure within these structures. This would reduce mean
circulatory filling pressure—an important component of central venous
pressure (CVP). This is likely a main contributor to the reduced central
and peripheral venous pressures seen when these pressures are measured
directly in space. Similarly, tissue weight can affect the compliance of
other organs. Studies during Gx (front to back) centrifugation show
that the heart deforms with increased gravity, and that the size of the
cardiac chambers decreases. The left ventricle becomes stiffer with
transverse acceleration. If all gravitational forces were removed, the
heart may become less stiff, which could also contribute to reduced
venous pressures in space. This compliance effect could apply to tissues
throughout the body, including the eye and cerebrovascular system.
But, the effects of gravity on the cardiovascular system and eye are complex, and require integrated study. In our group, we have used numerical modeling combined with physiological measurements to build a model of the eye and cerebrovascular system that includes the effects of tissue weight. The model is able to reproduce the known cardiovascular effects of modifying tissue weight (increased supine intracranial pressures with obesity, lower central venous pressures in microgravity compared to a supine baseline on Earth) and provides insight into the effect of microgravity on intracranial and intraocular pressures. The model offers a new way to develop hypotheses about how microgravity exposure may affect the eye.
ESSENTIAL PARAMETERS FOR MODELING SPACE-INDUCED VISUAL CHANGES
Abstract
Upon entering microgravity, the hydrostatic pressure of
fluids within the body is reduced to effectively zero. In addition,
tissues no longer have any weight and tissue compressive forces are
lost. These changes have effects on pressures throughout the vascular
system and produce a headward fluid shift, potentially leading to one of
the unique effects of microgravity-induced visual changes in
astronauts. Given the complexity of the effect of gravity on the
cardiovascular system and the eye, our group has developed a numerical
model that incorporates the effects of weightlessness, including the
effects of modifying tissue weight. To our knowledge, this is the first
to consider the effects of tissue weight and likely provides a new way
of understanding tissue-weight related effects. But, a sensitivity
analysis has not been done on the model. A sensitivity analysis, an
exploration of the effects of changing tissue weight in the model, and
an examination of the effects of a proposed countermeasure (lower body
negative pressure (LBNP)) were done in this project. From the
sensitivity analysis, we find that the model is stable over wide range
of inputs. One interesting result from the sensitivity analysis is that
venous plexus diameter has an important effect on intraocular pressure
(IOP) and intracranial pressure (ICP) in microgravity. This may be
significant since the size of venous drainage pathways are often
significantly different between the eye showing microgravity changes
compared to the unaffected eye. Altering tissue weight in the model
showed that heavier individuals likely have larger decreases in ICP upon
entering microgravity, which contradicts hypotheses suggesting that
elevated ICP is related to the visual changes. Visual changes are more
common in heavier individuals and if elevated ICP were an important
cause then it should increase more in heavier individuals. From the
model, the proposed countermeasure Lower Body Negative Pressure might
not work as expected. These results offer insight not only to the effect
of microgravity on visual problems, but also to other tissue-weight
related conditions on earth, such as idiopathic intracranial
hypertension and increased supine intracranial pressures with obesity.