There is always a prebreathe prior to an EVA from the US side of the ISS.
Prior to each EVA, an oxygen
prebreathe is conducted to prevent
the crew from getting the bends
after depressurization of the
airlock (i.e., when the absolute
suit pressure drops to 222 mm Hg
[4.3 psi]). This is the same DCS
that ascending scuba divers as
well as aviators at high altitudes
must also prevent. Breathing
100% oxygen forces nitrogen to
migrate out of tissues and is exhaled.
If done for long enough, the chances
are greatly reduced that these gases
will create harmful bubbles that
can lodge in joints or travel in the bloodstream to critical organs,
resulting in pain, severe medical issues, and even death.
Multiple prebreathe protocols have
been available and used on the Space
Shuttle and the ISS. The various
protocols can involve oxygen masks,
exercise in the suit or on a bicycle
machine, reduction in cabin pressure
to 528 mm Hg (10.2 psi) (~equivalent
to 3 km [10,000 ft] altitude) for a
period of time, and/or prebreathe in
the spacesuit. Prebreathe methods
have evolved to incorporate
reductions in crew day length and
reduced complexity. For example,
at one point, prebreathe (Figure 23)
involved having the EVA crew
sleep in the Joint Airlock overnight
at a reduced cabin pressure; however,
this had its pitfalls. In addition to
sequestering the extravehicular
crew members from their crewmates
and the toilet, certain failures such
as a fire alarm on the ISS will cause
a repress of the airlock and will
interrupt the prebreathe process.
This can be frustrating when the
alarm is false, as has happened in
the past, since any interruptions in
prebreathe protocol require strict
penalties to “buy back” the time.
Depending on the situation, the crew
might have to breathe pure oxygen
for twice the number of minutes than
was the interruption.
Source: ISS: Operating a Outpost Chapter 17
There's a paper "EVA and Human Physiology" by Svensson, Vogt, and Herber that goes into the rationale for the prebreathes a bit more. I don't find that paper on the internet though.
Pure oxygen breathing, prebreathing, immediately before the decompression reduces the risk (of "bends"). By this method the tissue nitrogen is eliminated faster because the pure oxygen in the inhaled gas replaces the nitrogen in the lung. As a result a maximum diffusion gradient for the elimination of nitrogen from organism is established...
The reason for all the different protocols is an attempt to strike a balance between an effective/safe prebreathe and the inconvenience to the EVA crewmembers. One of the Shuttle protocols was a 4 hour prebreathe in the suit. Not how a crewperson wants to spend their time in orbit.
Source: Shuttle Environmental Control and Life Support Training Manual page 2-28.
For EVA-heavy Shuttle missions they would depress to 10.2 psi a day before the first EVA and stay there until the EVAs were done. For example, on STS-088 they did the 10.2 depress 19 hours into the mission and stayed there until 7 days, 20 hours into the mission (with a brief excursion back up to 14.7 psia to ingress the Functional Cargo Block ISS module).
Source: STS-088 Flight Plan & fond memories
