The placement of a magnet over a device affects pacemakers and defibrillators differently. Application of a magnet to a pacemaker will cause the reed switch to close and result in asynchronous pacing. The pacing rate is company-specific with a different rate once battery depletion has occurred. Thus, placement of a magnet over the device can assist with the determination of battery status and device identification. If exposure to electromagnetic interference (EMI) is anticipated, positioning a magnet over the device can prevent inappropriate pacing inhibition. On removal of the magnet, the pacing mode will revert to the originally programmed settings, and, in general, formal device interrogation is not required. In contrast, application of a magnet to a defibrillator will disable all antitachycardia therapies but will not affect the pacing mode. Therefore, magnets can be used to prevent inappropriate therapies due to supraventricular tachycardia (SVT), lead fracture, or EMI. On removal of the magnet, defibrillator therapies will be restored, and, in general, formal device interrogation is not required.

In hospitals, many potential sources of EMI exist. Sources of electromagnetic energy that could possibly interfere with device function include magnetic resonance imaging (MRI), electrocautery, defibrillation, radiation therapy, neurostimulators, TENS units, radiofrequency ablation, electroconvulsive therapy, video capsule endoscopy, extracorporeal shock wave lithotripsy and therapeutic diathermy [2,3]. EMI exposure most commonly results in inappropriate inhibition or triggering of pacing stimuli, inappropriate ICD tachyarrhythmia

detection and therapy and reversion to an asynchronous pacing mode (noise-reversion mode).

Inappropriate inhibition of ventricular pacing can be catastrophic in the pacemaker-dependent patient; similarly atrial oversensing with inappropriate ventricular tracking could result in a myriad of symptoms including heart failure exacerbation, hypotension, or angina. Improper ICD tachyarrhythmia detection due to EMI could potentially be arrhythmia-inducing as a result of unsynchronized inappropriate shock delivery during the vulnerable period of repolarization. Noise-reversion mode is an algorithm that reverts transiently to asynchronous pacing in response to rapid frequency signals. The algorithm is designed to protect against inappropriate inhibition of pacing when high-frequency signals are sensed. Although this algorithm is present in all pacemakers regardless of manufacturer, this is not the case for ICDs. Less frequently, EMI can result in reprogramming of the device parameters or permanent circuitry or lead damage. When EMI exposure is unavoidable, certain measures can be taken to minimize the potential risk. For example, pacemaker or defibrillator patients requiring surgery with electrocautery should have a magnet placed over the device during the operation. Other forms of EMI (e.g. MRI, radiation therapy) carry substantial risk and may prompt the revision or removal of the entire cardiac device system prior to planned exposure. Care should be taken to avoid sources of EMI in device patients or, if exposure to EMI cannot be avoided, at a minimum, measures should be taken to minimize potential harm with consideration of device interrogation following exposure.

Mode switch is a programmable pacing algorithm that automatically changes the pacing mode to a nontracking mode in response to a sensed atrial arrhythmia. The purpose of this algorithm is to prevent inappropriately fast ventricular tracking at the upper rate limit in response to a rapid atrial tachyarrhythmia. Once the device has mode switched, it will remain in a nontracking mode until the atrial rate has fallen below the mode switch threshold for a specific number of intervals. This algorithm is very useful for patients with paroxysmal atrial arrhythmias (e.g. SVT, atrial fibrillation or atrial flutter). The atrial rate at which mode switch occurs is programmable in most devices and the feature can even be programmed “off.”

The placement of central venous catheters in cardiac device patients warrants special consideration. Depending on the location and age of the device and the planned location of central venous access, a number of potential complications can occur. Reported complications associated with central venous catheters in cardiac device patients include lead damage from needle puncture [4], lead dislodgement, and inappropriate ICD therapies [5]. In addition, central venous stenosis as a consequence of prior cardiac device implantation may present a challenge to central venous catheter placement ipsilateral to the device [6]. Cardiac device infections and device-related endocarditis represent a particularly serious hazard of indwelling central venous catheters necessitating removal of the entire device system [7]. Central venous access should be performed contralateral to the device whenever possible.

The likelihood that patients with cardiac devices will require an MRI is high [8] but this imaging modality is not without risks in these patients. The potential hazards of magnetic resonance imaging in cardiac device patients include movement of the device, programming changes, asynchronous pacing, activation of tachyarrhythmia therapies, inhibition of pacing output, and induced lead currents that could lead to heating and cardiac stimulation [9], resulting in altered pacing and defibrillation thresholds, device damage, asystole, arrhythmias, or even death [10]. Although an implantable cardiac device remains a strong relative contraindication to MRI, certain centers have developed protocols for performing MRIs in cardiac device patients [11] and MRI-safe pacemakers are being developed. If an MRI is the only diagnostic imaging option in a cardiac device patient, imaging at 1.5 Tesla with appropriate programming and monitoring can likely be undertaken safely with careful assessment of the risk–benefit ratio on a case-by-case basis [11,12,13,14].

In the event of a cardiac arrest or hemodynamically unstable arrhythmia in a patient with an implantable cardiac device, resuscitative efforts should proceed as per guidelines without deviation. Defibrillation or cardioversion can result in permanent damage to the cardiac device; to minimize these risks, the defibrillation pads should be placed at least 10 cm from the pulse generator [15]. Other potential risks of external defibrillation include device reprogramming [16] and myocardial damage at the interface with the lead resulting in an acute rise in threshold [17]. Following defibrillation or cardioversion, cardiac devices should be interrogated formally to insure proper function and programming. Again, the low potential risk of damage to the device should not impede usual and necessary resuscitative efforts for the patient.

Cardiac device-related infection encompasses a disease spectrum from pocket infection to device-related endocarditis. The clinical manifestations of cardiac device-related infection are protean and can range from pain at the implant site without cutaneous manifestations to minor erythema or swelling of the device pocket (Fig. 44.4A) to overt erosion of the system (Fig. 44.4B) to device-related endocarditis (Fig. 44.4C) [18,19]. In the absence of bacteremia, systemic manifestations and leukocytosis are rare. Cultures of the device leads yield the highest results and, Staphylococci are the primary pathogen identified [20]. A high index of suspicion is warranted in a patient with implanted pacemaker or ICD and signs and symptoms of infection. Cardiac device-related infection requires prompt removal of the entire device system for complete treatment unless significant comorbidities preclude extraction [7,18]. Although no specific vegetation size has been established as a contraindication to transvenous extraction, most experts agree that vegetations greater than 3 cm in size are better treated surgically [7]. Patients with device-related endocarditis require a minimum of six weeks of intravenous antibiotics and pose a particular problem with respect to the timing of re-implant in pacemaker-dependent patients.