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Clinical course The patient was evaluated and diagnosed as having pacemaker-mediated tachycardia (PMT). A magnet was placed over the pulse generator, which terminated the PMT. The pacemaker was reprogrammed, and after a short course of observation without further rapidly paced ventricular episodes, the patient was discharged home. Comment Two common causes of rapid paced ventricular rates on a resting electrocardiogram include (1) tracking of rapid atrial rates and (2) PMT. Other causes include programmed overdrive pacing to terminate tachyarrhythmias, “runaway” pacemaker due to a malfunctioning pulse generator, and tracking of oversensed signals in the atrial channel. Given the myriad of possible programmed functions and differing devices, it can be challenging for the untrained professional to distinguish normal from abnormal function. Therefore, the most important lesson about apparent pacemaker malfunction is that it is often appropriate. Occasionally, a rapid paced ventricular rate may represent an undesirable response that requires treatment of an underlying atrial arrhythmia or pacemaker reprogramming. It is valuable for the clinician to have a basic understanding of the mechanisms that result in inappropriate rapid paced ventricular rates and of indications that intervention is required. A sensed rapid atrial rate is the most common cause of a rapid paced ventricular rate and represents appropriate pacemaker function.1When the atrial rate is rapid, as occurs with sinus tachycardia or atrial arrhythmias, the sensed atrial depolarizations result in atrial synchronous ventricular pacing at rates up to the maximum atrial tracking rate (the programmed upper pacing rate limit).1,2The maximum atrial tracking rate is typically programmed not to exceed 110 to 130/min in adults, although the upper limit may be programmed to be higher in pacemakers with sensor-based rate-adaptive pacing. With atrial-based pacing, when the sensed atrial rate exceeds the maximum atrial tracking rate, the ventricular pacing rate will be slower than the atrial rate and may even be irregular, resembling a Wenckebach phenomenon. Sensor-based pacing relies on sensors (eg, respiratory rate, body motion) to provide input to the pacemaker to adjust the paced heart rate appropriately for the level of metabolic demand. Pacemaker-mediated tachycardia is a common cause of inappropriate rapid paced ventricular rates. This is almost always due to a recurring sequence in which a paced ventricular depolarization is conducted retrogradely via the atrioventricular node to the atrium; the resultant atrial depolarization is sensed and initiates a ventricular pacing stimulus leading to another paced ventricular depolarization. Figure 2demonstrates the paced ventricular complexes with retrograde P waves that fall just after the ventricular complex. Figure 2. View LargeDownload Enlargement of initial electrocardiogram. This patient had pacemaker-mediated tachycardia near the upper rate limit of 120/min. A pacing stimulus (asterisk) precedes each ventricular complex, and a retrograde P wave (↓P) is seen just after each wide ventricular complex. The P wave axis (upright in aVR and inverted in II) indicates retrograde conduction. In this patient, a magnet was placed over the pulse generator to terminate PMT. When applied to a pacemaker, a magnet eliminates all sensing function. Sensing is the function that allows the pacemaker to either trigger or suppress its firing in response to the sensed atrial or ventricular event. Therefore, application of a magnet temporarily inactivates the pacemaker response to intrinsic atrial or ventricular depolarizations. This results in asynchronous pacing at the manufacturer-specific magnet rate.1,3On removal of the magnet, normal programmed pacing is expected to resume. When a magnet is applied to a patient with PMT, sensing is prevented and the retrogradely conducted P wave will no longer trigger ventricular pacing. This interrupts and terminates the recurring sequence. It is important to draw a distinction, however, between pacemakers and implanted cardioverter-defibrillators with pacing function. Because nearly all implanted cardioverter-defibrillators also have pacing capability, it is important to recognize that PMT can also occur in patients with implanted cardioverter-defibrillators; magnet application will deactivate defibrillator function under most circumstances but does not affect bradycardia pacing function and will not terminate PMT. This patient's PMT was considered to be due to a short postventricular atrial refractory period (PVARP). The PVARP is the programmed period of time after a paced ventricular event during which atrial events are not sensed. If the retrogradely conducted P wave arrives after the PVARP expires, it will be sensed by the atrial channel and result in initiation of a ventricular paced event. By reprogramming a longer PVARP so that the retrogradely conducted P waves fall within the PVARP period, atrial events will not be sensed and will not result in initiation of a ventricular paced event. The implications of programming a longer PVARP are beyond the scope of this discussion, but there are tradeoffs with regard to the maximum achievable pacing rate. Premature ventricular depolarizations are the most common initiating event for PMT.4Oversensing in the atrial channel with earlier-than-expected ventricular pacing and retrograde conduction is another mechanism. Other causes of rapid paced ventricular rates include “runaway pacemaker” and algorithms to terminate tachyarrhythmias. Runaway pacemaker is now a virtually obsolete cause of a rapid paced ventricular rate caused by a dysfunction within the pulse generator itself, resulting in firing rates as high as 400 to 800/min. It is associated with electrical malfunction or battery power loss.4,5Pulse generator replacement is necessary in specific cases. Oversensing of electrical signals in the atrial channel can be due to myopotential oversensing (eg, due to contraction of the pectoralis or diaphragmatic muscle), oversensing of environmental signals such as electronic surveillance devices, and lead-related problems such as fracture or insulation break. In addition, rapid paced ventricular rates can be due to antitachycardia pacing programs to terminate sensed reentrant tachycardias (primarily in implantable cardioverter-defibrillators) and overdrive pacing algorithms intended to suppress atrial fibrillation. Initial approach and management For any patient with a potential abnormality of their pacing system function, the most important step is to obtain consultation from a specialist and/or contact the pacemaker manufacturer. The most common cause of a suspected pacing system abnormality is clinician misunderstanding of a properly functioning pacemaker. Support for pacemaker evaluation can come from the local expert, the regional tertiary care center, or the device manufacturer's representative. If the patient is unaware of what type of device is implanted, the major device manufacturers can be contacted; with appropriate patient data (such as name, date of birth, and/or social security number), they can usually determine whether that patient has received their device and what type it is. Hemodynamic instability in a patient with a rapid paced ventricular rate should prompt consideration of other underlying causes (eg, sepsis, myocardial infarction, pulmonary embolism) as it would in a patient with nonpaced tachycardia. In pacemaker patients with suspected PMT, application of a magnet is therapeutic. If expert consultation is unavailable and the pacemaker manufacturer is unknown, the patient should be transferred to a higher level of care. Take-home points Patients with pacemakers should be reminded to carry the identification card that provides information regarding the manufacturer and model. Concerns about apparent pacemaker malfunction often represent appropriate pacing function that is misunderstood by the clinician—a specialist or the manufacturer should be contacted as soon as possible in any case of suspected pacemaker malfunction. Two common causes of rapid paced ventricular rates at rest in patients with dual-chamber pacemakers are tracking of rapid atrial rates or PMT. Patients with tracking of fast atrial rates may require treatment of the underlying atrial abnormality or reprogramming of the pacemaker. A magnet is a fast and effective way to terminate PMT in patients with pacemakers. Return to Questions Correspondence:Jeffrey A. Tabas, MD, Department of Emergency Medicine, University of California, San Francisco, 1001 Potrero Ave, Room 1E21, San Francisco, CA 94110 (email@example.com). Financial Disclosure:None reported. References 1. Mattu ATabas JABarish RA Electrocardiography in Emergency Medicine. Dallas, TX American College of Emergency Physicians2007; 2. Kusumoto FMGoldschlager NF Cardiac Pacing for the Clinician. 2nd ed. New York, NY Springer Science+Business Media2008; 3. Sobel RMDonaldson PRDhruva N Pacemaker-mediated tachycardia: management by pacemaker interrogation/reprogramming in the ED. Am J Emerg Med 2002;20 (4) 336- 339PubMedGoogle ScholarCrossref 4. Griffin JSmithline HCook J Runaway pacemaker: a case report and review. J Emerg Med 2000;19 (2) 177- 181PubMedGoogle ScholarCrossref 5. Makaryus ANPatrick CMaccaro P A rare case of “runaway” pacemaker in a modern CPU-controlled pacemaker. Pacing Clin Electrophysiol 2005;28 (9) 993- 996PubMedGoogle ScholarCrossref
Archives of Internal Medicine – American Medical Association
Published: Mar 14, 2011
Keywords: tachycardia,artificial cardiac pacemaker
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