27 August 2020
Does the impact of high-altitude impair cognitive performance?
Alexander Friend shares the role CANTAB played in the research he conducted on cognitive impairment at high-altitude while at The University of Birmingham.
Can you tell us more about your research?
Our research was conducted at the University of Birmingham as part of my undergraduate programme in Sport, Exercise, and Health Sciences.
The University of Birmingham has established the Centre of Human Brain Health; and the Birmingham Environmental Physiology Research and Education Centre. Their aim is to conduct world-leading research investigating the mechanisms that regulate human cerebral blood flow in health and disease, specifically during ageing and extreme environmental stress, and how these mechanisms impact function.
Rationale behind the study
Individuals who venture to high-altitude expose themselves to an environment which can cause a number of physiological complications. Physical symptoms such as breathlessness, headaches and dizziness, and nausea are noticeable on ascent, but individuals are less aware of the impairment to their cognitive function. The degree to which cognitive function is impaired is related to the rate of ascent and altitude reached. Moreover, complex tasks that require a higher order of cognitive ability are particularly sensitive to being impaired at high-altitude, such as decision-making and attentional processes, which are essential for such an unfamiliar and dangerous environment.
Normal brain function relies on sufficient oxygen delivery that requires a dynamic regulation of cerebral blood flow. Cerebral blood flow is sensitive to changes in oxygen and carbon dioxide. Ascent to high-altitude reduces the amount of oxygen carried in your blood, a condition known as high-altitude hypoxia. To combat the reduction in blood oxygen content, cerebral arteries dilate to increase the amount of blood flow that perfuses the brain. Simultaneously, breathing rate rises in an attempt to increase oxygen intake via the lungs, but this is at the expense of expiring the body’s store of carbon dioxide, giving rise to hypocapnia.
Hypocapnia is known to drive the cerebral arteries to constrict and reduce blood flow to preserve a level of carbon dioxide, particularly at the brain, which is vital for maintaining bodily homeostasis. Consequently, the regulation of cerebral blood flow during high-altitude hypoxia is influenced by two conflicting stimuli; hypoxia driving artery dilation, and hypocapnia driving artery constriction.
We aimed to examine the isolated effects of hypoxia and hypocapnia on simple and complex cognitive tasks, as well as exploring how changes in cerebral blood flow and cerebral perfusion under these conditions might relate to changes in cognitive performance.
What does the study involve?
Participants were exposed to the isolated and combined effects of hypoxia and hypocapnia as controlled by an automated breath-by-breath gas blender for 90 minutes, each whilst at rest. To mimic the effects of high-altitude hypoxia, the amount of oxygen available per breath was reduced and hypocapnia was allowed to develop. To assess the effects of hypoxia independent of hypocapnia, supplementary carbon dioxide was added during hypoxia. To assess the effects of hypocapnia independent of hypoxia, participants were instructed to voluntarily hyperventilate at a fixed rate in normal room air.
Cerebral blood flow and cerebral perfusion were measured non-invasively using bilateral transcranial Doppler ultrasound of the middle cerebral arteries located in the cranium, and near infrared spectroscopy of the brain tissue located at the forehead. Cognitive function was assessed via a battery of CANTAB cognitive tasks, which included two reaction time tasks and a spatial working memory task, representing simple and complex cognitive abilities.
Summary of findings and their implications
Hypocapnia impaired the performance of both reaction time tasks when combined with hypoxia and when induced independent of hypoxia, when cerebral perfusion was impeded most. Surprisingly, spatial working memory was not impaired. Furthermore, supplementation of carbon dioxide during hypoxia increased cerebral blood flow and cerebral perfusion and correlated with preserved cognitive performance. Taken together these findings reveal the significant role of hypocapnia on the development of cognitive impairment at high-altitude, and that by facilitating the appropriate dilation response to hypoxia through removal of the conflicting hypocapnic constriction cognitive performance is maintained.
Why did you choose CANTAB?
CANTAB provides an accurate, reliable, and intuitive system, which is sensitive to changes in neurocognitive function and can be easily administered via a portable touch screen device. This was particularly important for our research since participants were regularly hooked up to a lot of other equipment! Additionally, CANTAB offers a plethora of neurocognitive tasks across a range of domains, enabling us to investigate and compare the effects of high-altitude impairments between simple and complex cognitive tasks.
What are the next steps for you and your research?
I am currently undertaking my PhD in cerebrovascular physiology at Bangor University. As part of its Extremes Research Group in the School of Sport, Health, and Exercise Sciences, our research focuses on the cerebral blood vessel responses to the extreme conditions that accompany high-altitude hypoxia, such as cold exposure and dehydration, using a combination of advanced imaging techniques.
Alexander Friend, The University of Birmingham.