Defibrillation is a common treatment for life-threatening cardiac dysrhythmias, ventricular fibrillation and pulseless ventricular tachycardia. Defibrillation consists of delivering a therapeutic dose of electrical energy to the heart with a device called a defibrillator. This depolarizes a critical mass of the heart muscle, terminates the dysrhythmia and allows normal sinus rhythm to be reestablished by the body's natural pacemaker, in the sinoatrial node of the heart. Defibrillators can be external, transvenous, or implanted (implantable cardioverter-defibrillator), depending on the type of device used or needed.

 

The contraction of heart (cardiac) muscle in all animals is initiated by electrical impulses known as action potentials. The rate at which these impulses fire control the rate of cardiac contraction or the heart rate. The cells that create these rhythmical impulses are called pacemaker cells, and they directly control the heart rate.

 

 

In humans, and occasionally in other animals, a mechanical device called an artificial pacemaker (or simply "pacemaker") may be used after damage to the body's intrinsic conduction system to produce these impulses synthetically.

 

The pacemaker is located in the wall of the right atrium.  1% of the cardiomyocytes in the myocardium possess the ability to generate electrical impulses (or action potentials) spontaneously.

A specialized portion of the heart, called the sinoatrial node (SA node), is responsible for atrial propagation of this potential.

 

The sinoatrial node (SA node) is a group of cells positioned on the wall of the right atrium, near the entrance of the superior vena cava. These cells are modified cardiomyocytes. They possess rudimentary contractile filaments, but contract relatively weakly compared to the cardiac contractile cells.

 

The pacemaker cells are connected to neighboring contractile cells via gap junctions, which enable them to locally depolarize adjacent cells. Gap junctions allow the passage of positive cations from the depolarization of the pacemaker cell to adjacent contractile cells. This starts the depolarization and eventual action potential in contractile cells. Having cardiomyocytes connected via gap junctions allow all contractile cells of the heart to act in a coordinated fashion and contract as a unit. All the while being in sync with the pacemaker cells; this is the property that allows the pacemaker cells to control contraction in all other cardiomyocytes. 

 

Cells in the SA node spontaneously depolarize, ultimately resulting in contraction, approximately 100 times per minute. This native rate is constantly modified by the activity of sympathetic and parasympathetic nerve fibers via the autonomic nervous system, so that the average resting cardiac rate in adult humans is about 70 beats per minute. Because the sinoatrial node is responsible for the rest of the heart's electrical activity, it is sometimes called the primary pacemaker.

 

In the past, defibrillator used needles to transfer electricity. It was dangerous because the it must be an open-heart surgery and the chance of success was very low. However, technology has grown for the past century. Defibrillator now is portable and can even be implanted. It is less dangerous as well and we can use it even without training. It is easier to use and available mostly everywhere.