+Gz tolerance, with and without muscle tensing, following loss of anti-G trouser pressure.

BACKGROUND: Fast jet aircrew are heavily reliant on anti-G trousers (AGT) and failure of the garment or its pressure supply would expose them to high levels of +Gz acceleration without protection. A previous study demonstrated the severity of this event at high +Gz, but no data exists as to the maximum +Gz level which could be tolerated.

METHODS: : While relaxed and with lower body muscle tensing, 10 experienced centrifuge subjects had their AGT deflated after 5 and 30 s of +Gz exposure. Discrete +Gz exposures of increasing intensity were performed until deflation resulted in central light loss (CLL). Visual symptoms, eye-level systolic BP (SBPeye), and mean blood flow velocity in the middle cerebral artery (MCAVmean) were recorded.

RESULTS: The mean +Gz level at CLL following AGT deflation was comparable to that without AGT inflation (+4.07 Gz) and increased by muscle tensing (+0.53 G) independent of exposure duration. Initial visual symptoms occurred more rapidly in the shorter +Gz exposures, while progression to CLL was quicker with muscle tensing compared with relaxed, but never less than 6.7 s. At CLL, the nadir in SBPeye was higher (+17 mmHg) with muscle tensing compared with relaxed, while MCAVmean was decreased by about 50% in all conditions.

DISCUSSION: +Gz tolerance following AGT deflation is comparable to that without inflation and only modestly increased by muscle tensing. Although vision is maintained for several seconds following AGT deflation, progression of light loss symptoms can be rapid, possibly resulting in insufficient time to respond before G-induced loss of conciousness (G-LOC) ensues.