The ISLE EVA prebreathe protocol was used for the first time during Space Shuttle Mission STS-134 and has since become the regular EVA preparation protocol for Space Station spacewalks. It is required to remove nitrogen from the blood streams of spacewalking astronauts.
The former/traditional protocol involves a much longer process of getting Nitrogen out of the Astronaut's blood stream. This is done to prevent them from getting decompression sickness that can occur when astronauts are subjected to the low pressure environment of the EMU Space Suit. . Usually, the EVA crew members are camped out the night before an EVA at a lower air pressure of 10.2 psi. In the morning of a Spacewalk, the Astronauts have to wear masks to breathe pure Oxygen.
The new ISLE Protocol will accomplish the same effect but in a much shorter time frame. A campout prior to a spacewalk will not be necessary. At the day of the EVA, both Astronauts will wear oxygen masks and the Airlock pressure will be reduced to 10.2 psi for suit-up operations. When both crew members are in their spacesuits, the Airlock pressure will be brought up to 14.7psi again. The astronauts will perform a nominal 50-minute prebreathe in their suits before beginning the exercise.
The ISLE adds another 50 minutes of exercise time to the prebreathe protocol. During that time, both crew members will perform a very light exercise program while in their spacesuits. They will basically flex their knees for 4 minutes and rest for 1 minute and repeat this until 50 minutes have passed. This will remove the Nitrogen from the blood in a much shorter time than the traditional protocol.
Without the campout, the EVA-crew will be able to spend a normal night before the EVA without being separated from the rest of the crew. It will also reduce the time Oxygen masks are worn and eventually less Oxygen will be consumed to preserve ISS resources.
ISLE - Pro's and Con's