In the microgravity of space flight, five astronauts showed dramatic reductions in the number of sleep-related disturbances, time spent snoring, and the number of arousals associated with respiratory-related events.
These are the results of research reported in the first issue for August of the American Thoracic Society's peer-reviewed American Journal of Respiratory and Critical Care Medicine.
The investigators believe their study is the first direct demonstration that gravity plays a dominant role in the generation of apnea, hypopneas, and snoring in healthy persons.
G. Kim Prisk, Ph.D., of the Department of Medicine, University of California, San Diego, with 8 associates, studied the effect of space flight on sleep-disordered breathing before, during and after two National Aeronautic and Space Administration (NASA) space shuttle missions in 1998.
Microgravity in space was associated with a 55 percent reduction in the apnea-hypopnea index (AHI), which decreased from an average preflight value of 8.3 events per hour for the 5 astronauts to 3.4 events per hour during sleep in space.
This reduction in AHI was accompanied by a virtual elimination of snoring, which fell from 16.5 percent of total sleep time preflight to 0.7 percent inflight. A decrease in respiratory arousals in space amounted to a reduction of 19 percent from preflight levels.
Apneas constitute a state of not breathing for some seconds during sleep. Hypopnea is an abnormal decrease in the rate and depth of respiration or very shallow, slow breathing.
Three of the 4 men and 1 woman (average age 41) who participated had preflight AHI indices that were below 5 events per hour, classifying them as not having any sleep-related breathing disturbances. Hypopneas were the primary classification for their respiratory disturbances. A fourth astronaut had 6.1 events per hour of sleep preflight, and the fifth, 22.7 per hour.
The investigators studied 77 8-hour, full polysomnographic recordings from the 5 healthy subjects before the space flight on 4 occasions per subject during either a 16- or 9-day shuttle mission and after their return to earth.
According to Dr. Prisk, the respiratory system is greatly influenced by the force of gravity. Significant changes take place when an individual goes from standing upright to lying on his back (supine).
In the supine position, functional residual capacity, expiratory reserve and tidal volume are all reduced, affecting lung volume. This, in turn, reduces upper airway diameter. In this position, gravity works to reduce upper airway size and to increase upper airway resistance by causing the tongue, soft palate, uvula and epiglottis to move backward toward the posterior pharyngeal wall.
The investigators believe the supine position alone may be one of the primary causes of the mildest form of upper airway resistance syndrome.
The authors note their results during sleep in space are probably due to the passive elimination of gravitationally-induced changes in the upper airway anatomical structures.
[Contact: G. Kim Prisk Ph.D.]