"CLEAR!! BZZZZZ! CLEAR!!!! BZZZZ! CLEAR!!!!!! BZZZZZ!!!!!....piiiiiiiiiiiii...."

This is a short video from an open-heart surgery:
The heart of this patient is "shaking" in a very strange way. This is called the Ventricular Fibrillation. It is a very dangerous state in which both ventricles of the heart are contracting independently. There is no blood flow throughout the vessels so the brain is exposed to hypoxemia (low level of oxygen in the erytrocytes) and hypoxia (low level of oxygen in the cells). 
After about 5 minutes of VF we can consider a patient is dead due to irreparable brain damage. Mostly.

When we learn basics of the CPR we are taught to use the defibrillator as soon as it is possible. What does the defibrillator do? It sends an electric shock which "resets" the heart (it changes the polarization of cardiomyocytes to the state of depolarization -> when there is no contraction activity). If the patient is lucky, the heart after a few seconds will resume its normal rhythm. 

In various of medical series (dr House, Gray's Anatomy) we often see an usage of defibrillator when the ECG shows a flat line - IT IS A LIE! This flat line is a state (asystole) in which the heart is not contracting at all so what does the defibrillator need to stop?! It will not start the heart again, please remember that!

Conclusion: the defibrillator does NOT START the heart -> it STOPS it in order to help in RESUMING normal rhythm.

In the right bottom of the film you can see an ECG trace and how does it change during VF, usage of defibrillator, resumption of contraction activity.

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