I read an interesting article in the Nov 13, 2019 issue of JAMA. I found it interesting because of three reasons –
- First, it enlightened me on the updates on the physiology of cerebral circulation.
- Second, it provided me an insight into the effects of space travel on the vascular pressure and fluid circulation during upright and recumbent positions and the effect of an absence of gravity that results in the above outcome. If Elon Musk or Virgin Airlines are thinking of planning to send frequent flyers across into space, they may either have to create gravity chambers inside the craft or ask the frequent flyers to collect frequent flyer miles and stagnation report. Possibly, Elon Musk should also consider something similar to dosimeters or Gamma counters.
- Last, and most commonly, I see several patients with shift edema that is idiopathic in nature. Does it have any relationship with an internal zero gravity like situation? I know this may sound dumb, but let us think on similar lines if any of the disease phenomena are creating any vascular or perivascular effects resulting in such outcome.
Excerpts from the article are included below for your easy perusal. A link to the article is provided at the end of this blog.
A gravity-induced head-to-foot (Gz) hydrostatic pressure gradient exists in the fluid-filled systems of the body in the upright position on Earth. In the supine position, the gravity vector no longer pulls in the Gz axis; therefore, blood and tissue-fluid pressures and volumes redistribute across the body. By spending approximately two-thirds of the day upright and the remaining one-third of the day supine at night, humans experience fluid shifts daily. However, crew members on the International Space Station (ISS) are weightless and thus experience a sustained redistribution of fluids toward the head that is not subject to daily diurnal posture-induced change in hydrostatic pressure. Headward fluid shifts during prolonged weightlessness result in facial puffiness, decreased leg volume, increased stroke volume, and decreased plasma volume. This fluid shift may also affect cerebral venous outflow as internal jugular vein (IJV) volume has been showed to be increased from 4.0 to 5.5 months of spaceflight exposure.
Cerebral venous outflow occurs predominantly through the IJVs in the supine position, whereas in the upright position, the IJVs partially or fully collapse due to atmospheric pressure being greater than intraluminal pressure, and cerebral venous outflow is diverted to the vertebral veins and vertebral plexus. However, in most cases, the IJVs do not completely occlude in the upright posture, and fluid communication is present between the cerebral and central venous systems. Indeed, we observed a waterfall-like blood flow pattern through the left IJV in most participants during upright posture on Earth, despite the semi-collapsed state of the vein. Cerebral venous drainage plays an important role in regulating intracranial pressure and intracranial fluid dynamics, and changes in cerebral venous pressure result in corresponding changes in intracranial pressure. In the upright position, venous pressure along the Gz body axis changes with respect to the venous hydrostatic indifference point because of hydrostatic effects. Notably, by collapsing, the IJVs act as Starling resistors, a protective mechanism preventing severely negative intracranial pressure in the upright position. The IJV cross-sectional area and flow are modulated by both cardiac and respiratory cycles and can be influenced by posture, anatomical variations, incompetence of the jugular valve, and changes in central venous or intrathoracic pressure. Pressure in the IJV is increased during short periods of weightlessness in parabolic flight, and in this study, we found that the IJV pressure remained elevated during long-duration spaceflight on the ISS relative to the upright posture. This result concurs with previously measured increases in intracranial pressure and transmural central venous pressure during microgravity exposure relative to the upright position on Earth. It is common to see engorged IJVs during short- and long-duration spaceflight and frame the extent of the engorgement by comparing the IJV area to 3 postures on Earth; however, it is possible that cross-sectional area measures underestimated the extent of engorgement compared to volume measures of the IJV. Cephalad fluid shifts, secondary to the lack of gravitational vectors and the reduction of tissue weight and tissue compressive forces in microgravity, may account for the increase in IJV area and pressure.
The Human Body in Space https://www.nasa.gov/hrp/bodyinspace