Will prior hyperventilation reduce cerebral blood flow during escape from a submarine?

This study was undertaken to determine if hyperventilation would reduce cerebral blood velocity (CBV) and thereby the risk of decompression illness (DCI) during escape from a submarine and increase the depth from which escape can be made. CBV was measured in eight subjects using Doppler ultrasound as they completed a mock submarine escape exercise. The exercise involved climbing a ladder followed by immersion in cold water-in a real escape the escapee would be exposed to increased pressure and at risk of DCI during the immersion phase. Immediately before the escape exercise the seated subjects either rested or hyperventilated at a controlled rate for2 min. There was a third condition in which the subjects hyperventilated for 2 min and then sat and rested. The three conditions were each undertaken twice. Hyperventilation reduced mean CBV by 45%. In the first session during the first 90s of immersion, CBV was 10% lower (P < 0.05) when the escape procedure followed hyperventilation than when following rest. In the second session CBV was similar for the two escape conditions. Following hyperventilation the restoration of CBV was more rapid during the escape condition than when the subjects rested--the reasons for this are unclear. It is concluded that, although hyperventilation effectively reduces CBV, the reduction is neither sustained during the escape procedure nor sufficiently consistent to recommend that it should be used before escape from a submarine.