Air Break During Preoxygenation and Risk of Altitude Decompression Sickness
AIR FORCE RESEARCH LAB BROOKS CITY-BASE TX
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To reduce the risk of decompression sickness DCS, current USAF U-2 operations require a 1-h preoxygenation PreOx. An interruption of oxygen breathing with air breathing currently requires significant extension of the PreOx time. The purpose of this study was to evaluate the relationship between air breaks during PreOx and subsequent DCS and venous gas emboli VGE incidence, and to determine safe air break limits for operational activities. Methods Volunteers performed 30 min of PreOx, followed by either a 10-min, 20-min, or 60-min air break, then completed another 30 min of PreOx, and began a 4-h altitude chamber exposure to 9144 m 30,000 ft. Subjects were monitored for VGE using echocardiography. Altitude exposure was terminated if DCS symptoms developed. Control data uninterrupted 60-min PreOx to compare against air break data were taken from the AFRL DCS database. Results At 1 h of altitude exposure, DCS rates were significantly higher in all three break in prebreathe BiP profiles compared to control 40, 45, and 47 vs. 24. At 2 h, the 20-min and 60-min BiP DCS rates remained higher than control 70 and 69 vs. 52, but no differences were found at 4 h. No differences in VGE rates were found between the BiP profiles and control. Discussion Increased DCS risk in the BiP profiles is likely due to tissue renitrogenation during air breaks not totally compensated for by the remaining PreOx following the air breaks. Air breaks of 10 min or more occurring in the middle of 1 h of PreOx may significantly cantly increase DCS risk during the first 2 h of exposure to 9144 m when compared to uninterrupted PreOx exposures.
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