See also: Actions in Combat
If you are reduced to one-half your maximum Hit Points or less, you can Catch a Second Wind as a Swift Action. You can only Catch a Second Wind once per day. Only Heroic characters can Catch a Second Wind; Nonheroic characters, objects, devices, and Vehicles cannot.
This Action heals one-quarter of your full Hit Point total (rounded down) or a number of points equal to your Constitution score, whichever is greater. You can Catch a Second Wind only once per day.
Certain Feats or Talents may allow you to Catch a Second Wind more often, but never more than once in a single encounter.
Extra Second Wind[]
If you have the Extra Second Wind feat, you can Catch a Second Wind an additional time per day (but never more than once in a single encounter). A Nonheroic character that takes the Extra Second Wind feat can Catch a Second Wind once per day.