Oxygen consumption at altitude as a risk factor for altitude decompression sickness.

INTRODUCTION: The existence of a general influence of exercise on the incidence of decompression sickness (DCS) has been known for more than a half-century. However, quantification of the effect has not been done for several reasons, including isolation of exercise as the only variable. The DCS database at Brooks City-Base, TX, contains detailed physiologic information on over 3000 altitude exposures. The purpose of this study was to measure Vo2 during the activities performed during those exposures to retrospectively determine if Vo2, a quantifiable index of exercise intensity, was related to the level of reported DCS. METHODS: Ground-level activity was designed to duplicate the standardized activity during the altitude exposures. Breath-by-breath Vo2 was determined for each activity using a COSMED metabolic measurement system. Comparison of the Vo2 during four levels of activity performed under otherwise comparable conditions allowed a determination of correlation between Vo2 and DCS risk observed during the altitude exposures. RESULTS AND DISCUSSION: Four previous altitude exposure profiles at 8992 m to 9144 m (29,500 to 30,000 ft; 231 to 226 mmHg) for 4 h following a 1-h prebreathe resulted in 38-86% DCS. This study provided the Vo2 of activities during those studies. The correlation between DCS incidence and the highest 1-min Vo2 of activity was 0.89. CONCLUSION: The highest 1-min Vo2 showed a high correlation with level of DCS risk. Future exposures involving lower levels of activity could provide data that would allow improvement in modeling of DCS risk.

Muscle activity in pilots with and without pressure breathing during acceleration.

BACKGROUND: Fighter pilots are frequently exposed to high acceleration (+Gz) forces during sorties. To counter these forces the pilots wear anti-G ensembles, use positive pressure breathing for G protection (PBG), and perform anti-G straining maneuvers (AGSMs). The purpose of this study was to analyze the muscle activity during sustained high G when no positive pressure breathing was used (control) compared with that during the use of PBG. METHOD: Seven Swedish Air Force fighter pilots volunteered to be exposed to gradual and rapid onset runs to +9 Gz with and without PBG in a human centrifuge. Surface electromyography was recorded from the intercostals, rectus abdominis, vastus lateralis, biceps femoris, and gastrocnemius lateralis. Measured variables included mean muscle activity, relative time with high muscle activity levels, and individual activation preferences. RESULTS: G duration tolerance was significantly longer (p = 0.028) when PBG was used (57 s) compared with control (32 s) during rapid onset runs. The vastus lateralis and the gastrocnemius lateralis generated activity > 50% of a reference contraction for a longer relative time during control (5.8% and 33.6%, respectively) than during PBG (0.3% and 12.7%, respectively). Cumulative muscle activity during acceleration was compared between trials and indicated that some pilots preferred contracting their leg muscles and others their abdominal muscles. CONCLUSION: G duration tolerance time increased when PBG was used during rapid onset sustained exposures. Less relative time with high muscle activity was seen during the use of PBG in two groups of leg muscles. The pilots seemed to have individual muscle activation sequence preferences while performing the AGSMs.