Studies on many effects of weightlessness on nervous functions require monitoring of the autonomic nervous system, including such autonomic effects as gastrointestinal activity, secretion, lacrimation, salivation, sweating, and the central control of respiration. Urinary estimations of catecholamines and 5-hydroxyindoleacetic acid would provide important data on autonomic system activity if collected in flight and compared with preflight and postflight controls.

Major areas have been outlined in which prolonged weightlessness may be expected to interfere with performance, judgment, and, ultimately, chances of survival. These include cardiovascular, metabolic, central nervous, psychophysiological, and biorhythmic effects. They have been dealt with separately and in sequence, but have not been intended to be viewed as hierarchic. The relative scarcity of data necessarily precludes such an evaluation.

Soviet experience with zero gravity and weightlessness has increased their emphasis on this space-flight factor and was an important topic at the May 1964 COSPAR meeting. Discussion of the postflight medical status of Bykovsky (5-day flight) and Tereshkova (3-day flight) revealed a concern for the significance of prolonged weightlessness and the presence of postflight physical debility and fatigue following Vostok flights 3 through 6. These changes persisted for several days. Among the physiological conditions singled out for mention were—

  1. Body fluids— Cosmonauts have shown a postflight weight loss of 1.9 to 2.4 kg apparently resulting from a redistribution of body fluid in response to elimination of the hydrostatic pressure gradients caused by Earth gravity. There is the suggestion that this redistribution is complete within the first 24 hours of flight. Titov is reported to have been dehydrated alter his flight with early hemoconcentration. These findings directly support predictions made from ground-based research.
  2. Cardiovascular— Postflight orthostatic tachycardia is reported for Titov as long as 23 hours after landing; at 48 hours there was significant residual intolerance to the upright posture. Cosmonauts have demonstrated a 20- to 35-percent increase in oxygen consumption during the standard postflight exercise test.

In both of these areas there was a return to normal within the postflight period of study. The Soviets have continued their biological experiments in space with the Vostok/Voshkod series. Fixing of histologic specimens in flight by Bykovsky demonstrated a critical role for man and made possible an expanded experimental program. Biopackages have become more complex with each succeeding flight.

With the exception of postflight orthostatic intolerance after the third and fourth Mercury flights, changes as a result of exposure to a zero-gravity environment have not been noted by U.S. investigations in space. Ground-based research proceeds here at an advanced pace and is supported in large measure by both the USAF and NASA. A study of the relationships among renal and systemic hemodynamics, neurohumoral cardiovascular regulation, and renal excretory function in differently positioned subjects is underway, as are studies of acceleration tolerance.

DEPRESSED METABOLISM

In anticipation of prolonged manned space flights, NASA has sponsored research related to metabolism depression. The daily food requirements, for example, of astronauts during a voyage of several months can constitute a major portion of the weight and storage capacity of the spacecraft. A somewhat promising and fundamental approach to this problem is the reduction of the astronauts' daily metabolic requirements. It has been suggested that astronauts on prolonged space missions be put in a state of suspended animation until their destination is reached. Though this sounds fantastic, 10 years ago no cell had been frozen to cryogenic temperatures and survived. Today it is commonplace for tissues to be frozen, stored at low temperatures, and thawed and then to maintain their viability and function.

Animal metabolism may be depressed by reducing body temperature, as in hibernation and hypothermia. Other means by which metabolism can be lowered include drugs and electronarcosis. Hibernation is a nonstressful state and results in a great decrease in metabolism. However, human beings are not hibernators, and much research is needed before the mechanism of hibernation is understood, and the possibility of inducing it in humans evaluated. Hypothermia is the direct cooling of the body to temperatures where metabolism is substantially depressed. Extracorporeal circulation systems combined with cooling are in routine use in most medical centers throughout the world. Hypothermia is not an ideal solution, however, since general body hypothermia is a stressful condition. Pharmacologic induction of hypothermia can be accomplished by such drugs as chlorpromazine and harbamil. Other drugs can be used to depress metabolism, but all have some disadvantage.

In recent years there has been a growing interest in electronarcosis, the induction of sleep by an electric current. Although potentially valuable, this method is far from routine application.