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ICDs are a first‐line therapy for many patients at risk for sudden cardiac death (SCD). They are usually implanted in the left infraclavicular region and are typically palpable. All patients with these devices get identification cards that note the manufacturer and device model. ICDs have four main functions:1) sensing atrial and ventricular signals, 2) classification of those signals into programmable heart rate zones, 3) administration of electrical therapy to terminate ventricular tachycardia or ventricular fibrillation, and 4) pacing for bradycardia or cardiac resynchronization therapy (equivalent to a standard pacemaker) [27]. If ventricular fibrillation or ventricular tachycardia is detected, shocks of 1 to 40 joules can be delivered [27]. Although this is less energy than external defibrillation or cardioversion, the shock can still be painful to the patient.

The EMS physician will most likely encounter one of three possible scenarios with a patient who is suffering from an ICD‐related cardiac event. The first is device failure in the event of a ventricular arrhythmia. The second is an appropriately functioning device in the setting of a ventricular arrhythmia. The third possibility is the ICD delivering shocks inappropriately in the absence of a ventricular arrhythmia. The first steps in all cases are assessment of mental status, vital signs, and cardiac monitoring. If the patient has an unstable ventricular arrhythmia, and the ICD does not fire, it should be assumed the device is nonfunctional and resuscitation protocols should be followed. If external defibrillation is needed, the defibrillator pads should not be placed over the implanted device. If the patient has a ventricular rhythm and the ICD is giving appropriate shocks, care should be focused on additional treatment of the arrhythmia, as well as rapid transport to the hospital. The patient may benefit from analgesia and possibly sedation in the event of multiple shocks. External electrical therapy should not be needed.

In the third scenario, the ICD is giving inappropriate shocks in the absence of a ventricular arrhythmia. Some ICDs are programmed to simply detect an elevated rate and not specifically detect ventricular arrhythmias. Inappropriate shocks are most often induced by T‐wave oversensing in which the high amplitude T‐wave is interpreted as another QRS complex leading to double counting. This can also occur during CPR when compressions can trigger an inappropriate shock [28]. As with pacemaker malfunctions, ideally the device can be interrogated by an electrophysiologist at the receiving hospital. In the event the patient’s condition requires emergent intervention to stop inappropriate shocks, a special magnet can be placed over the device. The magnet will suspend detection of ventricular fibrillation and ventricular tachycardia and should stop the shocks. The magnet will not stop the pacemaker function of the ICD or place the pacemaker in asynchronous (fixed) mode [27]. In the event a magnet is used, cardiac monitoring is required because the ICD will no longer be able to sense nor shock arrhythmias. Magnet therapy is only effective while the magnet is secured to the skin over the device. It may also be prudent to apply external defibrillator pads during transport. As with a pacemaker, cutting the lead wires of an ICD will most likely permanently damage the device, is difficult to perform in the field, and is not recommended short of a dire last resort.