A flatline is an electrical time sequence measurement that shows no activity and therefore when represented, shows a flat line instead of a moving one. It almost always refers to either a flatlined electrocardiogram, where the heart shows no electrical activity (asystole), or to a flat electroencephalogram, in which the brain shows no electrical activity (brain death). Both of these specific cases are involved in various definitions of death. Some consider one who has flatlined to be clinically dead, regardless of eventual resuscitation or lack thereof, whereas others insist that one is alive until the moment of brain death. This is mostly used in the medical industry when a person's pulse has stopped, indicating a flat line on the heart monitor.
When a patient displays a cardiac flatline, the treatment of choice is an injection of vasopressin (epinephrine and atropine are also possibilities)  and chest compressions. Successful resuscitation is generally unlikely and is inversely related to the length of time spent attempting resuscitation. Despite defibrillation commonly appearing on medical dramas as a remedy for asystole, it is usually not done when there is a total absence of electrical activity, as it could actually make it more difficult to restart the heart.
A flatline will also be generated if all leads are disconnected from the patient.
- Caggiano, R. "E-Medicine: Asystole" Retrieved on 2008-11-13.