Take a BreatherSource Healer's Handbook pg. 15
You can use lapses in your opponent’s guard to recuperate from the wounds you have already sustained.
Prerequisites: Con 13, Combat Vigor, character level 4th.
Benefit: Your vigor pool gains 1 vigor point. Whenever an opponent that you threaten provokes an attack of opportunity from you, as a swift action you can spend 1 or more vigor points to regain hit points as described by the Combat Vigor feat, instead of making an attack of opportunity against that opponent. Spending vigor in this manner does not count against the total number of attacks of opportunity that you can make during a round.