Heartless

 is a perk in the Fallout: New Vegas add-on Old World Blues.

Obtaining
In the initial dialogue with Doctor Klein as part of the quest Welcome to the Big Empty, you find out that your brain, heart, and spine have been removed and replaced. This perk represents the replacement of your heart. While you technically have these perks from the start of the DLC, they are unlocked in the course of the conversation.

Effects
You cannot be poisoned ever again, the effects of healing items are boosted, and robots have a -50% chance of scoring a critical hit on you.