A g-suit, also called an antigravity suit, is used to decrease the incidence of orthostatic intolerance in astronauts upon re-entry to Earth's atmosphere and upon landing. Orthostatic intolerance includes symptoms such as lightheadedness, fatigue, nausea, and fainting when one stands or sits upright. The primary cause of this condition is cardiovascular deconditioning that occurs during spaceflight.
The g-suit uses air-inflated sacs around the legs and abdomen to increase blood flow from the lower legs to the upper extremities. The objective of this Canadian-directed project was to investigate the efficiency of various types of g-suit inflation techniques.
On Earth, blood pools in the lower legs due to the effects of gravity. Our bodies use a variety of neurological and hormonal reflexes to minimize this pooling, and to ensure adequate blood pressure and blood flow to the brain. However, long-term exposure to microgravity diminishes the body's reflexes that counteract the blood-pooling effects of gravity.
When astronauts reach orbit, they immediately experience a head-ward blood volume shift that is caused by lack of gravity in the environment. This increase in blood volume in the upper body activates regulatory responses, which cause the body to reduce its overall blood volume. Upon re-entry into a 1g (Earth) environment, blood pools back into the legs. However, due to the space-induced low blood volume and the body's diminished reflexes, less blood is circulated to the upper body. This results in symptoms of orthostatic intolerance. The 1.2g forces experienced by astronauts for 17 minutes during shuttle re-entry further aggravate the symptoms.
A standard g-suit applies a constant and uniform pressure to the lower legs in order to minimize the amount of blood that pools in this area. Studies of the venous system have suggested that a graded pressurization or a "milking action" may be a better way of increasing blood flow to the upper body.
This project, led by Defence Research and Development Canada (DRDC-Toronto), was conducted in collaboration with NASA's Johnson Space Center Cardiovascular Laboratory.
The study tested one crewmember pre-flight and post-flight, and eight subjects who were recently bed-rested for 24 hours. These subjects rested in a 6° head-down position to mimic the fluid shift that occurs in microgravity. The study used a tilt table where the subjects rested lying down for five minutes and then were tilted to a 90° head-up position. Researchers measured the effects of this tilt on the subject's blood pressure, heart rate, stroke volume, and artery velocity both without the g-suit and with different g-suit pressurizations. The four g-suit inflation modes tested were:
It was found that all models of the g-suit provide some protection from orthostatic intolerance, but the pulsatile model may provide better protection than the standard uniform inflation model. Further post-flight experiments on long-duration crew members are needed to provide more concrete results.