Based on the proposed semiempiric nomograms, the study was aimed at selection and investigation of normoxic N2-O2 hypobaric atmosphere (HA) at 93-73 kPa, and evaluation of effectiveness of the gas mixture against the altitude decompression sickness (ADS) during 6-hr simulation of extravehicular activities at 37 kPa without prebreathing. Subjects were 22 healthy males from 20 to 50 yr. old. In all, 115 experiments were performed. Twenty-four hours in HA at 93 kPa and subsequent transition to 37 kPa led up to development of ADS symptoms by one subject in one of 27 experiments (3.7%), and emergence of venous gas bubbles (USI) with the intensity of 2, 70, and 19 points by the Spenser scale in 9 experiments (33.3%). Lower HA pressures (86 kPa, 80 kPa and 73 kPa) did not give rise to ADS though the occurrence of venous GB distinctly tended downward (30.6%, 14.3% and 11.8%) and so did the intensity of their entry into the pulmonary circulation (2, 40, 18 points; 1, 80, 19 points, and 2, 00, 15 points, respectively). Minimal ADS risk was observed in HA at 80 kPa and 73 kPa and did not reach 4% at the confidence level of 95%.

Purpose of the investigation was to assess contribution of repeated (with a 12-hr interval) decompression to the risk of altitude decompression sickness (ADS) by simulation of 6-hr extravehicular activities (EVA) of space crewmembers in altitude chamber. The protocol included "ascents" of 6 essentially healthy male subjects at the age of 24 to 51 to the altitude of 7,600 m (37 kPa) following 30-min prebreathing (elimination of nitrogen from the body by breathing pure oxygen through a mask at the ambient pressure of 73 kPa = 2,600 m). Each subject participated in 2 experimental exposures: first initial and then repeated decompression. None of 24 "ascents" produced clinical signs of ADS. Comparison of the data concerning frequency and time points of detection by ultrasonic Doppler equipment of gas bubbles (GB) in the venous bed during decompression with initial, maximal and mean values of US signal intensity failed to state a significant difference between them. Data of the investigation were confronted with anticipated length of GB dispersion in body tissues.

442 altitude experiments on 40 volunteers were performed. Gas bubbles in venous blood were detected using an ultrasonic Doppler system functioning in a continuous mode at a frequency of 5 MHz. The threshold of bubble formation was identified in 31 test subjects and that of emergency of altitude-decompression disorders, in 28 test subjects. Individual resistance to decompression-induced bubbles was revealed. It was shown that the thresholds may vary with time, in the head-down position and in the head-down position combined with exercises. A correlation was established between the rate of intravascular bubble formation and the frequency of altitude-decompression disorders.

Variations in coronary circulation, cardiac output and heart rate were examined by dopplerography in 14 test subjects kept in an altitude chamber. It was found that during acute hypoxia the effective coronary blood flow decreased distinctly prior to general circulatory disorders. It is recommended to monitor coronary circulation during altitude exposures.

The effect of altitude chamber hypoxia of different intensity and duration on epinephrine and norepinephrine excretion was investigated. The hypoxia tolerance as a function of catecholamine excretion was also examined. At altitudes 5000 and 6500-7000 m the test subjects were kept for a short time and at 3500-4500 m for 24 hours. The subjects with high hypoxia tolerance showed an increased and those with low tolerance a decreased epinephrine excretion. The epinephrine excretion did not increase with the intensity and duration of the hypoxia effect. Variations in the norepinephrine excretion were insignificant in all tests.