Heartless

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

Obtaining
In the initial dialogue with Dr. Klein as part of the quest Welcome to the Big Empty, the Courier finds out that their brain, heart, and spine have been removed and replaced. This perk represents the replacement of their heart. While technically having these perks from the start of the add-on, they are unlocked in the course of the conversation.

Effects
Immunity to poisoning, adds a +25% bonus to healing items and -50% to Critical Chance for robots against you.

Behind the scenes
Heartless is an adjective associated with consistent cold, cruel behaviour from an individual. The term takes on a much more literal meaning in-game.

Bugs
Both Cardiac arrest and Heartless do not affect healing from any items as they are missing the correct modifiers.