APPLICATION OF THE MECHANICAL OSCILLATOR TECHNIQUE TO THE INVESTIGATION OF EVA DECOMPRESSION

The ultrasonic bubble detection technique has been used as a versatile tool in the investigation of hypobaric decompression procedures for t he last two decades. However, this method can only detect relatively l arge bubbles > 30-50 mu m (usually in venous blood) and not the very f irst stages of bubble formation. Recently, the mechanical oscillator t echnique has been applied in air diving trials in order to detect smal l blood density changes due to hypothetical fluid shifts. However, sha rp density drops were found in the decompression process, and these ha ve been attributed to gas bubble formation. In this study, this method was applied together with Doppler controls to a series of altitude de compressions to 300 and 500 hPa, which represents the range of EVA ope rational pressures currently used or planned. Six subjects participate d in both series; the oscillator sample tube could be pressurized. By the application of pressure on a blood sample containing questionable gas bubbles, these are reduced in volume, like in recompression therap y, and thus the density of the sample is increased. This effect has be en shown in all 6 subjects undergoing decompression to 300 hPa, wherea s in the 500 hPa series only 3 out of 6 subjects corresponded with gas phase formation immediately after decompression. Typically, Doppler b ubbles appeared only after some latency time between 20 and 60 min; af ter this time, the density effect had nearly vanished. It is concluded that the mechanical oscillator technique represents a complementary r esearch instrument to the Doppler method, because it is able to delive r a quantitative resolution of the early stage of gas phase formation.