What Does a Defibrillator Do?

The community response to cardiac arrest is critical to saving lives. Each year, UK ambulance services respond to approximately 60,000 cases of suspected cardiac arrest. Resuscitation is attempted by ambulance services in less than half of these cases (approximately 28,000).3 The main reasons are that either the victim has been dead for several hours or has not received bystander CPR so by the time the emergency services arrive the person has died. Even when resuscitation is attempted, less than one in ten victims survive to go home from hospital. Strengthening the community response to cardiac arrest by training and empowering more bystanders to perform CPR and by increasing the use of automated external defibrillators (AEDs) at least doubles the chances of survival and could save thousands of lives each year.  (British Heart Foundation)

Defibrillators are designed to produce an electrical shock to disrupt certain chaotic and disorganized heart arrhythmias. 
By sending a shock to disrupt the disorganized electrical activity, defibrillation momentarily stops the heart, allowing it to reset and the natural pace making function to hopefully resume a normal heart rhythm. 
What Does Happen When The Heart Stops?
The extended flatline, means that there is no electrical pattern to disrupt at all.  Medical professionals must administer drugs into the patienta��s circulation to stimulate the electrical activity of the heart to start up again.