MODEL RELEASED. Emergency defibrillation. Doctor using a defibrillator on a patient suffering from a heart attack. An ECG trace of a heart beat is across lower frame. A defibrillator provides a strong electric shock to the heart. During a heart attack, the heart muscle contracts wildly and irregularly, a condition known as fibrillation. Fibrillation is ineffective at pumping blood around the body, and is thus a medical emergency. The defibrillator clears all electrical activity in the heart, allowing it to regain its former healthy rhythm.