This attachment scans for any signs of respiration. The sensor is able to scan through various materials including solid walls.
Modern Warfare 2
The heartbeat sensor was first shown in single player but the tech is also available for use in multiplayer modes. Enemies show up as red dots, friendlies as blue dots. The one perk to counter this attachment is the ninja perk which renders you invisible to opponent's heartbeat monitors. While it shows enemies and friendlies in your vicinity it can't indicate what floor or level they are at. This can make it difficult to locate your foes, especially on multi-tiered levels.