About 20 percent of astronauts suffer from post-flight orthostatic hypotension, loss of consciousness resulting from insufficient blood flow to the brain, or presyncope, the symptoms sometimes experienced prior to fainting, such as dizziness, lightheadedness or vertigo, blurred or tunnel vision, nausea, headache or sweating. To better understand how weightlessness affects the body's autonomic function, a group of investigators led by Dr. Janice Meck will evaluate astronauts before launch, on landing day and three days after landing. The study will compare pre- and post-flight changes in responses of veins to drugs which cause the veins to dilate or constrict. The group is also looking at changes in nitric oxide production. To date, the effects of space flight nitric oxide physiology and its modulation of blood pressure has not received extensive study.
Janice Meck, Ph.D.
NASA Johnson Space Center
There is still a significant number of astronauts at Johnson Space Center who suffer from postflight orthostatic hypotension and presyncope. The mandatory use of fluid loading with salt tablets and water, anti-gravity suits and the liquid cooling garment has not eliminated the problem.
A growing body of evidence suggests that there are major physiological systems that become dysfunctional as a result of space flight. The degree of dysfunction varies from minimal to severe. Several studies have provided evidence that autonomic function is impaired during and after space flight. Additional factors such as local factors could also be involved.
An area that has not been studied in humans is the effect of space flight on nitric oxide physiology and its modulation of blood pressure. The study proposed in this application will continue the pursuit of mechanisms of autonomic dysfunction in presyncopal astronauts. In addition, it will begin to elucidate changes in nitric oxide production and the resulting effects on the cardiovascular system.
This study will not have inflight measurements. All procedures will be performed before launch, on landing day and three days after landing. The study has two specific aims: 1). to compare preflight to postflight changes in responses of veins to adrenergic agonists between presyncopal and nonpresyncopal astronauts, and; 2). to compare preflight to postflight changes in nitric oxide levels, inducible nitric oxide synthase messenger RNA and protein, cell adhesion molecules associated with endothelial activation, responses to acetylcholine with and without nitric oxide synthase inhibition, and reactive hyperemia responses in the brachial artery, the arm, and the popliteal artery in the leg between presyncopal and nonpresyncopal astronauts. Presyncopal and nonpresyncopal astronauts will be defined by their ability to complete a ten-minute upright tilt test on landing day.