The Heart and Electricity
The heart is predominantly made up of muscle (also known as cardiac muscle). Like all muscles, cardiac muscles contract in response to electrical stimulation. The contraction of cardiac muscle is carefully controlled to ensure that the beating of one portion of the heart is coordinated with the rest of the cardiac tissue.
Blood is pumped from the two upper chambers (called the atria) into the lower chambers (the ventricles) where it is then pumped out. As a result, the electrical activity in the heart is designed so that the cardiac muscle of the ventricles contracts after the atrial muscles. This electrical activity begins in a part of the heart called the sinoatrial node, which naturally generates periodic bursts of electrical activity that travel throughout the heart.
If the ventricles develop an abnormal electrical signal (either spontaneously or as a result of damage to the heart) they can begin to contract rapidly, which is known as fibrillation. This disorganized contraction is dangerous because it does not allow the heart to pump blood throughout the body.
Defibrillation
As Dr. Theodoros Xanthos of Sci Topics explains, the goal of defibrillation is to stop the abnormal electrical activity in the heart that is leading to its disorganized and uncoordinated contractions. Fibrillation is the result of an abnormal electrical signal that is able to propagate and maintain itself over a length of time. However, if a large enough shock is given to the heart, it will cause all of the cells in the heart to reset, which will eliminate the fibrillation signal. This reset then allows the sinoatrial node to regain control of the heart's electrical activity. Defibrillators aim to deliver a strong enough electric shock to restore normal electrical activity but not one so severe that it damages the heart tissue.
Biphasic
As BiphasicDefibrillator.com explains, the earliest defibrillators were monophasic, which means that they passed an electrical current in just one direction to try to reset the heart. Biphasic defbrillators use an electrical current that flows in two directions to shock the heart. The advantage of using biphasic defibrillators is that less electrical current is needed to successfully shock the heart, which makes these devices more effective to restore the heart's regular rhythm more quickly.
