Most people have felt light-headed or dizzy when standing up too quickly after sitting or lying down for a while.<\/p>\n
According to Brock University Health Sciences PhD candidate Nathan Iannarelli, this feeling is particularly pronounced in individuals who have experienced long-term bed rest \u2014 or have just returned from space.<\/p>\n
Iannarelli says research shows physical deconditioning can impair blood pressure regulation.<\/p>\n
Blood vessels constrict when the body suddenly changes position, increasing blood pressure so that enough blood reaches the brain to prevent the light-headed sensation.<\/p>\n
Because the sympathetic nervous system helps keep blood pressure within a normal range, people without a properly functioning sympathetic nervous system may have reduced blood flow to the brain and be more susceptible to light-headedness.<\/p>\n
Iannarelli set out to study how long-term deconditioning impacts the sympathetic nervous system\u2019s ability to control blood pressure using bed rest data collected by researchers from the Universities of Waterloo and Western Ontario and the Universit\u00e9 de Caen Normandie in France.<\/p>\n
\u201cWe hypothesized that part of the reason why deconditioned individuals can’t regulate blood pressure properly is that when the sympathetic nervous system sends the same signal as it did before deconditioning, the signal doesn\u2019t cause the same amount of blood vessel constriction, so there isn\u2019t a sufficient increase in blood pressure,\u201d\u00a0he says.<\/p>\n
Iannarelli and his team analyzed data gathered from two studies that were performed in France and Germany in which participants lay on a bed for 60 days positioned so that their head was tilted six degrees below their feet.<\/p>\n
In this configuration, called head-down bed rest, blood moves easily to the brain.<\/p>\n
A technique known as microneurography was used to directly measure the activity of the sympathetic nervous system. Heart rate and blood pressure were also measured.<\/p>\n
The results of Iannarelli\u2019s analysis indicated head-down bed rest affects the ability of the blood vessels to respond to the sympathetic nervous system.<\/p>\n
He says these could be due to alterations in the release of neurotransmitters, which carry chemical messages from one cell to the next, fluctuations in blood vessels\u2019 sensitivity to these chemical messages, changes to the structure of the blood vessel wall or a combination of these factors. These changes can impair the ability to appropriately regulate blood pressure, causing the characteristic light-headedness and fainting, he adds.<\/p>\n
In addition to highlighting the implications of long-term hospital stays, the research also sheds light on what happens to astronauts when they return to Earth.<\/p>\n
Iannarelli says it\u2019s easier for the heart to pump blood to the brain in space because it doesn\u2019t have to work against gravity like it does on Earth.<\/p>\n
\u201cThe bed rest models we used in this study provide a ground-based analog of human spaceflight, since head-down bed rest simulates blood flow in space,\u201d he says.<\/p>\n
Iannarelli\u2019s supervisor, Assistant Professor of Kinesiology Stephen Klassen, says the research has clinical implications for individuals who are assigned to long-term bed rest due to injury or illness as well as interventions to help astronauts re-adjust blood pressure.<\/p>\n
\u201cOur findings suggest that strategies targeting the sympathetic nervous system such as the the development of medications that target the sympathetic nervous system or exercise prescription may help blood pressure regulation which could minimize the risk of fainting among individuals that have experienced prolonged physical inactivity,\u201d he says.<\/p>\n
The team\u2019s findings can be found in their study, “Long-duration head-down bed rest alters human sympathetic neurocirculatory regulation<\/a>,” published earlier this year in the Journal of Applied Physiology. <\/em><\/p>\n Funding for their work includes support from the Government of Canada\u2019s Natural Sciences and Engineering Research Council, the Ontario Graduate Scholarship Program and a Queen Elizabeth II Graduate Scholarship in Science and Technology.<\/p>\n