Your heart is a quest item in the Fallout: New Vegas add-on Old World Blues.
The Courier's heart is removed by the Sink Auto-Doc, together with the spine and brain on arrival in the Big MT.
During the quest Old World Blues, the heart appears in The Sink and from this point on, can be returned at any time in the Courier's body (as well as removed again) with the same auto-doc.
- While the heart is removed, the Courier will be granted some bonuses by the perk Heartless.
- Once the heart is replaced, the Courier will be granted some bonuses by the perk Cardiac Arrest.
- The heart can be seen in the tank on the far right in The Sink.
- Using the BaseID to add it to the inventory and then removing the quest item flag allows dropping the heart. The dropped heart has a correct world model and can be grabbed and repositioned.