The Effects of Fatigue and Cognitive Tasks on Neurocognitive Outcomes during Orthostatic Challenge
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abstract
Orthostatic challenge (OC), also known as orthostatic hypertension, is a blood pressure drop that results in pooling of blood in the lower extremities leading to oxygen deprivation in the brain, impairing neurovestibular functions which are critical for determining the position and motion of the head, and therefore the rest of the body; these functions are necessary for an astronaut to perform their job (Mark & Heldt, 2005). Acute orthostatic intolerance is a change in blood flow pressure that affects 64% of astronauts on short-range missions and 80% of astronauts on long-range missions. Orthostatic challenge is encountered when astronauts return from the microgravity of space to the gravity of the earths atmosphere, this transition can alter physiological responses, which can worsen with fatigue and have a detrimental effect on performance of critical piloting tasks. The existing countermeasures utilized to lessen the impact of orthostatic intolerance are primarily focused around health promotion. Before the flight begins, astronauts are trained for improved physical fitness with an emphasis on aerobic exercises to maintain optimal cardiac function (Convertino, 2002). The objective of this study was to discern the types of cognitive functions affected by fatigue that should be studied further, starting with measurements of the psychomotor tasks needed for control, maneuvering, and the demands of high-level cognitive skills required for space flight. Data from this study would allow for greater exploration of the neural correlates of cognitive stressors that are encountered during OC especially under the fatigued conditions of long-range space flight. If fatigue does adversely affect neural correlates of performance during OC, then the types of cognitive stressors placed on the subject will influence that relationship and should be explored further to develop different training protocols for better countermeasures to assist astronauts with the transition out of microgravity. Sixteen college-aged athletes (8 M, 8 F) were recruited for this study. Participation in the study was limited to the university athletic groups to represent highly physically fit individuals. A two-way repeated measures design was employed to test the interactive effects of physical fatigue (control versus fatigue) and cognitive task (mental arithmetic versus psychomotor tracking) on neurocognitive outcomes during orthostatic challenge. In the fatigue condition, participants were physically fatigued for one hour prior to undergoing the orthostatic challenge to simulate the fatigued condition experienced by astronauts. We found that the presence of fatigue adversely affected participants ability to perform the psychomotor countermeasure tasks, over mental arithmetic, during short duration OC. The fatigued state was evident in the changes seen in the monitoring tools; heart rate and heart rate variability and PFC oxygenated hemoglobin levels. The inability to utilize countermeasures leaves astronauts vulnerable and potentially unable to overcome the orthostatic challenge during the transition from microgravity to gravity with an inability to combat fatigue with the countermeasures utilized. Findings obtained here with help identify the nature and extent to which fatigue impacts an astronauts ability to employ countermeasures used to combat orthostatic challenge and how effective those countermeasures will be in the fatigued state. These findings can provide the rationale for developing different countermeasures that are less susceptible to fatigue as well as new training and physical interventions for astronauts to employ while they are in space flight to better combat the cumulative effects of extended exposure to microgravity.
