Weightlessness


doc_portrait_172_01Duane Graveline MD MPH
Former NASA Astronaut
Research Scientist in Aerospace Medicine
During his ten year USAF career Doctor Graveline selected prolonged weightlessness as his major area of research.
Using bed rest and water immersion analogues he was able to accurately predict the progressive deconditioning of astronauts caused by extended zero gravity exposure. This type of research continues today in the effort to design effectve counter-measures for zero gravity deconditioning.

Man readily adjusts to weightlessness and it is this inherent adaptability which causes the problem. Having adjusted to this new environent he becomes less able to tolerate normal Earth gravity. Zero gravity deconditioning becomes functionally significant only upon return to Earth. Muscles not used become weak. Comparable changes occur in the circulatory system. The astronaut who returns is not the same as the one who left Earth weeks or months earlier.

Musculo-skeletal Changes
Weight bearing bones rapidly lose calcium in conditions of reduced gravity loading. Numerous studies on animals and humans during immobilization, bed rest, free floating in water or during extended space flight have demonstrated this tendency for disuse de-mineralization.

The natural consequence of this, if continued for a sufficient time, is a tendency for osteoporosis, weakening of the bones commonly seen in the elderly and associated with increased susceptibility to fractures. Another natural consequence of calcium mobilization is a tendency for kidney stones to form, an event not as yet seen in space flight but certainly common in bed rest and immobilization.

Cardio-vascular Changes
Heart muscle resembles skeletal muscle in that under conditions of disuse a weakening can result. Just as an athlete will strengthen his heart muscle through exercise and make it more efficient as a pump, any reduction in demand will lessen this efficiency. Associated with this, during bed rest, immersion in water or the weightlessness of space flight is a progressive loss of circulatory reflexes so important in maintaining blood pressure.

When a person is resting in bed, hydrostatic loading due to the weight of blood in the long columns of the blood vessels is reduced. The consequence of this is a diminished requirement for the many adjustments our circulatory system has to make from moment to moment to adjust to positional change. As with muscle tone, if you do not use it, you lose it. Additionally, tissue fluid present in the dependent parts of the body because of gravilty loading become mobilized into the circulation and excreted by the kidney under conditions of bed rest, water immersion or weightlessness.

Recumbency diuresis has long been studied and is inevitable any time hydrostatic loading is reduced, as in zero gravity. It represents nothing more than a re-adjustment of the circulation to meet the new, lesser demands.but the consequence of this is a decreased circulating blood volume. Upon arising and standing erect after an extended stay in bed, a tendency for faint results from this complex of cardiovascular changes. A returning astronaut is subject to these same effects unless effective counter-measures are used.

Counter-measures to weightlessness effects
Much protection can be given to the musculoskeletal system and indirectly the cardiovascular system through the routine use of exercise on the bicycle ergometer or the treadmill. These devices have been available for routine use on MIR and the shuttle and certainly will be aboard the International Space Station. Cosmonauts routinely have two forty-five minute periods programmed each day for self maintenance using such equipment.

These techniques have permitted them stays in zero gravity in excess of a year with safe return although in a profoundly weakened state requiring special support for several weeks. In no case has the cosmonaut returned in a normal state regardless of how enthusiastically he may have used available countermeasures. This indicates that even ninety minutes of aggressive exercise daily, perhaps the upper limit for operational acceptability, is not completely protective.

Other measures having theoretical usefullness but perhaps more demanding technically include the use of extremity tourniquets and the lower body negative pressure device. Bed rest studies long ago demonstrated that an oscillating bed almost completely protected a chronic bed rest patient from circulatory deterioration. Similarly, periodic inflation of extremity tourniquets provides similar benefit and has the potential for use in zero gravity.

In modified form this system was flown on one of our Gemini missions, incorporated into the spacesuit. The lower body negative pressure (LBNP)device isolates the lower half of the body with a waist seal and applies approximately thirty millimeters negative pressure in a pulsating fashion simulating the venous engorgement of erect standing on Earth. This device both challenges and maintains cardiovascular reflexes. Currently it's use is primarily to assess circulatory status in weightlessness just as a tilt table test does on Earth. Incorporating the LBNP into a sleeping chamber for use throughout the sleep period might prove an acceptable way to protect the circulatory system.Other measures include the combined use of salt retaining hormones and fluid in the few hours prior to de-orbit to help return the blood volume and body fluids to normal.

The Tragedy of Soyuz 11 - Was Weightlessness implicated in their Deaths?
After spending 24 days in orbit, Cosmonauts Dobrovolsky, Volkov and Patsayev were killed during de-orbit when an exhaust valve in their Soyuz 11 space-craft accidently opened while still at high altitude causing rapid decompression.

Two of the cosmonauts attempted to rise and close the valve located above the center seat but were unable to do so in the fifteen to twenty second period of useful consciousness available before their cabin air was exhausted. Ordinarily only a few seconds would have been sufficient to accomplish this task but these men had been 24 days in zero gravity.

Although I have no proof there is no doubt in my mind their degree of muscular deconditioning was sufficient to almost immobilize them. Rising from their seats and reaching that valve was beyond their capabilty in their weakened state. Deconditionig had robbed them of their ability to survive de-orbit emergencies. Prolonged weightlessness exposure had doomed them!

Duane Graveline MD MPH
Former USAF Flight Surgeon
Former NASA Astronaut
Retired Family Doctor

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