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ICU ("Intensive Care Unit") is a Resistance perk featured in Call of Duty: Ghosts. Players equipped with this perk will half both their delay before their health regen, and the time it takes to regen once it starts.
Although not mentioned in the perk description, tactical grenades' effects will not last as long when ICU is equipped. This is perhaps due to the partial damage taken from tactical grenades is eradicated quicker by ICU.
In Safeguard mode, it is referred to as "Fast Health Regen".