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°C with 85% relative humidity and radiant heat) was followed by return to a cooler environment (21 °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°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.