HOUGHTON, MI-A researcher in Michigan Technological University's Center for Biomedical Engineering hopes to find an exercise regimen that will prevent fainting in persons who are otherwise healthy.
Dr. William H. Cooke says a significant number of highly trained athletes and astronauts returning from space experience lightheadedness, tunnel vision, nausea, and fainting when standing upright.
"Approximately 30-40 percent of astronauts just back from space missions can't stand for more than 10 minutes without showing signs of fainting," he says. "Many marathon runners and triathalon participants experience the same difficulty. In fact there's a saying, 'highly trained athletes can run, but they can't stand.' Of course, many non-athletes also have fainting spells."
Cooke says that a significant number of fainting episodes are caused by a malfunction in the arterial pressure sensors responsible for maintaining adequate blood flow to the brain.
"Based on previous research our group has done with normal healthy persons and with astronauts returning from space, I believe this malfunction resides, at least partly, in an inadequate neural response to decreases in blood volume--such as occur when one stands and blood flows from the upper to the lower body. "
The most intriguing aspect of the problem to Cooke is whether astronauts and athletes faint due to fundamental adjustments in neural regulatory mechanisms, and if so, what mechanisms do cardiovascular deconditioning in space flights and conditioning in traditional terrestrial exercise regimens share in common? And how are these mechanisms altered by different types of exercise training programs? For example, does resistance training increase or decrease nerve activity and arterial pressure sensing mechanisms?
"When astronauts go into space, fluid shifts from the lower to upper body," explains Cooke. (Just the opposite occurs when people stand on earth.) "The resulting increase in cardiac filling 'tricks' the volume sensors of the heart into thinking that total blood volume has been increased. The body then secretes diuretic hormones to restore 'normal' blood volume, but this adjustment typically causes a 10-15 percent decrease in blood volume in astronauts. Reduced blood volume in space is probably the catalyst for adaptations that cause some astronauts to have difficulty standing when they return to earth.
Cooke says scientists believe that arterial pressure sensors known as "baroreceptors," located at the aortic arch and carotid sinus, reset in space in response to decreases in blood volume. Because baroreceptors normally sense even subtle decreases in blood pressure during standing on earth, any "malfunction" or "decreased gain" of these pressure sensors after space flight results in an inappropriate response, such as a lack of sufficient increase in heart rate, which is necessary to restore normal blood flow to the brain.
"When a person stands upright on earth, the sympathetic branch of the autonomic (involuntary) nervous system sends bursts of nerve messages to the veins delivering blood from the leg muscles," says Cooke. "This results in a constriction of the leg veins, which facilitates the pumping of blood toward the head. " He says there is evidence that astronauts who faint after space flights do so because of a decreased sympathetic response to standing.
Aerobic exercise on earth leads to a chronic elevation in blood volume. So theory has it that if astronauts exercise in space, they will limit their reduction in blood volume and reduce the potential for fainting when they return to earth.
"The primary exercise program of choice for astronauts is long-duration, moderate intensity aerobic exercise," says Cooke. "However, some astronauts who follow this type of program in space still faint when they return to earth. Another countermeasure, drinking large amounts of fluid prior to reentry into the earth's atmosphere, tends to restore blood volume, but still doesn't prevent fainting.
"These clues suggest that decreases in blood volume are only partly responsible for cardiovascular deconditioning. Current exercise countermeasures do not seem to be effective because they impact only one (blood volume) of the three mechanisms that potentially cause fainting. Aerobic training actually decreases arterial pressure sensitivity and nerve traffic on earth--a healthy adaptation for athletes, but bad for astronauts. The trick is to find out if people's cardiovascular systems react differently to other forms of training and if a particular training program will help prevent fainting."
Cooke says it may be better for astronauts to limit aerobic training, but increase strength training before going into space, and then maintain strength training exercises in space. He hopes that the data he will collect during the next four years will suggest an appropriate exercise training program and countermeasure for astronauts, and also for normal healthy persons who routinely suffer fainting spells on earth. In the latter instance, an appropriate exercise program could help prevent falls that cause bone fractures in elderly persons.
Cooke's research is being supported by a $255,000 Scientist Development Grant from the American Heart Association. He will be one of the few scientists in the nation to use a method called microneurography to measure the nerve activity in subjects' legs before and after different exercise training regimens to determine whether cardiovascular regulatory strategies depend on specific exercise programs.
Contact: Jim Lutzke
906-487-2343
jclutzke@mtu.edu
For more information, contact Cooke at 906-487-1740 or email: whcooke@mtu.edu.