How Defibrillators Work

A defibrillator is a machine used to deliver a shock to the human heart. Many people falsely think that the defibrillator is used to start a person’s heart when it has stopped beating. In reality it is used to reestablish a correct rhythm to the heart. When a person suffers from a myocardial infarction, (heart attack), it can be triggered by many different things. One of the most common things that can happen during a heart attack is that the normal rhythm of the heart is altered.

The heart is made up of 4 chambers, left and right atrium on the top, and left and right ventricles on the bottom. The area of the heart that is responsible for maintaining the normal rhythm of the heart is located in the right atrium and is called the sinoatrial node. It’s a small area of neurons that fires an electrical pulse causing the heart muscle to contract and squeeze blood through the four chambers. Occasionally during a heart attack the sinoatrial node may either misfire or get weak. When this happens it can cause the lower chambers to “flutter” because they are not receiving the correct signal and don’t know when to contract. This is called ventricular fibrillation and is where the defibrillator would be used to reestablish the correct rhythm to the heart.

The defibrillator essentially delivers enough of a shock to get all of the chambers to settle down and allow the sinoatrial node to begin its job again. So how does the defibrillator itself work? The defibrillator uses batteries and a series of transistors and inverters to switch the voltages rapidly enough to simulate an AC sine wave. The voltage is then fed through a high voltage transformer that steps the 12 volts up to a higher voltage. This is rectified and stored in a capacitor until the shock button is pressed on the defibrillator paddles. The shock is then delivered from one paddle through the patient and back to the other paddle completing a circuit.
Defibrillators also have a sync feature which allows them to monitor the heart and deliver the shock at the right point of the QRS wave. During the QRS complex the heart goes through a polarization period known as the P-Wave. Then the sinoatrial node fires causing the QRS spike and subsequent heart muscle contraction and then depolarizes during the T-Wave. If the defibrillator delivers the shock at the wrong time it can damage the muscles and cause death. The Shock has to be delivered after the QRS spike during the depolarization of the T-Wave.