For 28 hours, six subjects will remain lying down and tilted at 12 degrees so their heads are lower than their legs. At times, they live and sleep in a carbon dioxide enriched atmosphere. With the 'SpaceCOT' study, researchers from the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) and the US National Space Biomedical Research Institute (NSBRI) are investigating how the human brain and eyes are affected by the shift of body fluids towards the head as well as the increased carbon dioxide content in the air. Either could be responsible for causing the visual impairments that are experienced by about 70 percent of astronauts during and after several months of long-term missions. The DLR research facility :envihab is the 'earthly' space station where the conditions under which astronauts in the International Space Station (ISS) live and work can be simulated.
Effects of weightlessness and carbon dioxide
Although gravity on Earth cannot be 'switched off', the human body reacts to the head-down position with a shift of fluids towards the upper body and the head. This is comparable to a stay in weightlessness when the blood no longer accumulates in the lower body due to the lack of gravity. In the :envihab facility, it is possible to create an atmosphere that is similar to the carbon-dioxide-rich ambient air in the ISS. On Earth, the carbon dioxide content is 0.04 percent; on the ISS it is 20 times greater. Astronauts who are exposed to these conditions for long periods of time experience a change in vision at close range, changes in the fibre layers of the retina and swelling of the fluid-filled chamber surrounding the optic nerve. "The astronauts' visual impairment could result from increased pressure in the skull; also, carbon dioxide expands the blood vessels and could cause an increase in intracranial pressure," explains DLR physician Edwin Mulder, who is conducting the study at :envihab for NSBRI.
Lying down for science
To better understand the movement and distribution of fluids in the brain and eyes, during a total of 28 hours, six male subjects aged between 33 and 47 will be exposed to a terrestrial atmosphere for some of the time and to an atmosphere with 0.5 percent carbon dioxide for the rest of the time. For two hours, the proportion of carbon dioxide will be increased to three percent via a breathing mask. While lying down, the subjects have to observe strict rules; sitting or standing is prohibited; instead, they must be lying down – with at least one shoulder making contact with the mattress at all times. To avoid influences resulting from different food intake, all subjects received a controlled, uniform diet.
Measurements of blood flow through to eye shape
The researchers detect the neurological changes with numerous measurements. "Visual impairment is a major risk that must be reduced before we can send astronauts on long-duration missions," said Jeffrey P. Sutton, the NSBRI Director. The investigations include near-infrared spectroscopy and measurements with a C-Flow monitor to determine blood flow in the brain. With volumetric inductive phase shift spectroscopy, changes in the volume of liquid in the skull can be detected. Based on magnetic resonance images, among other things, the diameter of the optic nerve, the shape of the eye, the volume of the brain or vessel diameter will be determined. A special measuring system determines the intracranial pressure by measuring the velocity of the blood flow. Using 'smell tests', changes in the perception of odours will be checked. Cognitive tests will provide, for example, insights into sensorimotor speed, spatial orientation, the recognition of emotions and even working memory. Even 'puffy face' – the facial bloating that occurs in space – can be detected by ultrasound measurements of the forehead tissues.
If one understands the processes in the human brain better, not only astronauts benefit from the 'SpaceCOT' study. Patients suffering from elevated intracranial pressure after a brain injury, for example, could be given less aggressive and more targeted treatment.