Acceleration tolerance at night with acute fatigue and stimulants.

INTRODUCTION: The impact of pharmacological agents on aviators concerns all flight surgeons. This study tested the related hypotheses that acute fatigue reduces +Gz tolerance and endurance, and that stimulants can partially reverse this impact. Additionally, the researchers attempted to develop a test battery sensitive enough to detect subtle differences in aviator cognition and performance among conditions. METHODS: To determine the effect of fatigue on +Gz tolerance and the impact of stimulant use, 10 male centrifuge subjects, mean age 32, from Brooks City-Base, TX, were tested in a repeated measures study under five nighttime conditions following an average of 22 h of sustained wakefulness during their circadian nadir. Using a within-subject design, subjects received placebo, dextroamphetamine 10 mg, modafinil 200 mg, methylphenidate 10 mg, and pemoline 37.5 mg at night, and were tested during a daytime control session. Cognitive/performance tests were administered before each centrifuge run. RESULTS: No difference in +Gz tolerance or endurance was detected among conditions. The cognitive/performance tests also did not detect any differences. Subject perception that anti-G straining maneuver (AGSM) difficulty was greater during the night placebo condition than during the daytime control, methylphenidate and modafinil night conditions reached statistical significance (P = 0.005, 0.012, 0.022, respectively). DISCUSSION: Physiological changes during the circadian nadir following acute sleep deprivation do not appear to negatively impact +Gz tolerance. A standardized protocol sufficiently sensitive to detect subtle behavioral and performance effects would be useful to test and compare the effect of other pharmacological agents on aviators.

Pressure breathing without a counter-pressure vest does not impair acceleration tolerance up to 9 G.

INTRODUCTION: This study was to determine whether safe and adequate G-protection by pressure breathing during G (PBG) could be maintained if the COMBAT EDGE counter-pressure vest were eliminated to ensure aircrew do not unnecessarily endure a possible in-flight discomfort or distraction. METHODS: Centrifuge exposures up to +9 Gz were completed by 11 subjects, including 5 F-15 aircrew, using PBG at 60 mmHg pressure with and without the counter-pressure vest. Additional G-exposures using pressures of 0, 30, and 45 mmHg were performed without the vest. RESULTS: Elimination of the COMBAT EDGE counter-pressure vest did not significantly reduce G-tolerance. During gradual onset G exposure, the mean G level reached with PBG was 8.4 G without the vest and 8.2 G with the vest. In comparison, 6.7 G was reached without PBG. Mean times at G with rapid onset G exposure were 59 and 60 s, respectively, compared with 49 s without PBG. PBG, with or without the vest, was preferred by all test subjects. PBG at 60 mmHg produced the highest G protection and was preferred by the test subjects over lesser pressures. Subjects reported no adverse effects from the use of PBG without chest counter-pressure. CONCLUSION: The use of PBG and the anti-G straining manuever (AGSM) together enhances G tolerance and comfort more than AGSM alone. Elimination of the counter-pressure vest during use of PBG does not hinder an individual's ability to reach +9 Gz or complete a short duration simulated aerial combat maneuver G exposure. Further research is needed to determine if use of PBG without chest counter-pressure increases fatigue during multiple sorties or produces other aeromedical problems in operational environments.

Heat stress effects for USAF anti-G suits with and without a counter-pressure vest.

BACKGROUND: Aircrew have reported increased heat stress when wearing the USAF Combined Advanced Technology Enhanced Design G-Ensemble or COMBAT EDGE (CE). The perceived thermal burden has been attributed to the fact that CE includes an inflatable counter-pressure vest to ease the work of positive pressure breathing during G (PBG). This study compared the heat load of CE with that of the standard USAF anti-G system (STD) without the vest, and measured heat stress effects on G-tolerance in both suits. METHODS: This study had 12 subjects (6 of them aircrew) who participated. Simulated preflight thermal stress (20 min walking at 35 degrees C with 85% relative humidity and radiant heat) was followed by return to a cooler environment (21 degrees C). G-tolerance and subjective stress levels were determined on the human centrifuge before and after the heat stress. Body weight, rectal and skin temperatures, and blood parameters were also assessed. RESULTS: Baseline relaxed tolerance for +Gz gradual onset runs (GORs) were (mean +/- SD) 7.6 +/- 1.3 G for CE and 7.1 +/- 0.8 G for STD (p < 0.05). Maximal rectal temperature following heat stress peaked at 38.1 +/- 0.4 degrees C for both CE and STD, and mean nude weight loss was 1.10 +/- 0.24 kg for both. Relaxed GOR tolerances after heat stress were 7.1 +/- 1.3 for CE and 6.3 +/- 0.9 for STD (p < 0.01). The heat stress significantly reduced G tolerance for both CE and STD (p < 0.01). CONCLUSIONS: Simulated preflight activity in hot conditions revealed no significant difference between CE and STD with regard to maximal core and skin temperature elevations or dehydration levels. CE supported a significantly higher baseline relaxed G-tolerance than STD, an advantage that persisted after heat stress and dehydration.