Anatomy of a Pacemaker
Contrary to what you may have seen on television, a defibrillator is not effective at treating a patient with a flatlined pulse. That state, called asystole, occurs when there is no electrical current in the heart muscle.
A defibrillator is effective at treating ventricular fibrillation or ventricular tachycardia (when the heart's electrical signals are too rapid and circulation is impaired). In these situations, a defibrillator can deliver a shock of about 300 joules to the heart in a very short amount of time (roughly 4 to 12 milliseconds), briefly stopping the irregular electrical impulses so that the heart's natural pacemaker can -- hopefully -- restart its regular rhythm.
 A clinician readies an external defibrillator.
Two metal paddles (electrodes) with insulated handles or adhesive pads are attached to the defibrillator by insulated electrical wires called leads. The electrodes are placed on the skin of the patient's chest so that the heart is positioned roughly between them.
The defibrillator's battery will then deliver the specified charge to the defibrillator's capacitor to be stored, which can result in a high-pitched noise as the charge collects. At the right moment, the charge is released, passing electricity through one electrode, in through the skin, through the heart, and then out through the skin to the other electrode.
 The Atlas II DR, like other implantable cardioverter
defibrillators, can automatically diagnose and treat
fibrillation.Pacemaker image courtesy of St. Jude Medical, Inc.
An automated external defibrillator (AED) is like a manual defibrillator, but it uses a computer to analyze the patient's heart rate, automatically diagnose the arrhythmia, and select the correct level of charge to deliver.
Implantable Cardioverter Defibrillators
An implantable cardioverter defibrillator (ICD) is used when the patient is at a high risk for sudden cardiac death. The device, similar in function to external defibrillators, is implanted under the patient's skin. A battery-powered pulse generator, like a small computer, constantly checks the heart's electrical signals. Upon sensing an abnormal rhythm, it delivers electrical energy to the cardiac tissue through a system of leads located near the heart or in one of its chambers. As with a pacemaker, an ICD requires an external monitoring device to check and adjust its functioning when the patient comes in for a check-up.
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