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Cardiology > Electrophysiology Procedures
Pacemaker-Mediated Tachycardia
Article Last Updated: Aug 9, 2006
AUTHOR AND EDITOR INFORMATION
Section 1 of 11  
Author: Brian Olshansky, MD, Professor of Medicine, Department of Internal Medicine, University of Iowa College of Medicine
Brian Olshansky is a member of the following medical societies: American Autonomic Society, American College of Cardiology, American College of Chest Physicians, American College of Physicians, American College of Sports Medicine, American Federation for Clinical Research, American Heart Association, Cardiac Electrophysiology Society, Heart Rhythm Society, and New York Academy of Sciences
Coauthor(s):
Chirag M Sandesara, MD, Fellow, Department of Internal Medicine, Division of Cardiovascular Diseases, University of Iowa Hospitals and Clinics;
Noel G Boyle, MB, BCh, MD, PhD, Co-Director of Cardiac Electrophysiology, Assistant Professor, Department of Internal Medicine, Division of Cardiology, University of California at Los Angeles School of Medicine
Editors: Justin D Pearlman, MD, PhD, ME, MA, Director of Dartmouth Advanced Imaging Center, Professor of Medicine, Professor of Radiology, Adjunct Professor, Thayer Bioengineering and Computer Science, Dartmouth-Hitchcock Medical Center; Francisco Talavera, PharmD, PhD, Senior Pharmacy Editor, eMedicine; Marschall S Runge, MD, PhD, Marion Covington Distinguished Professor of Medicine, Vice Dean for Clinical Affairs, Chairman, Department of Medicine, University of North Carolina at Chapel Hill School of Medicine; Amer Suleman, MD, Consultant in Electrophysiology and Cardiovascular Medicine, Department of Internal Medicine, Division of Cardiology, Medical City Dallas Hospital; Leonard Ganz, MD, Associate Professor of Medicine, Temple University School of Medicine; Cardiac Electrophysiologist, Children's Hospital of Pittsburgh, University of Pittsburgh Medical Cent, West Penn Hospital
Author and Editor Disclosure
Synonyms and related keywords:
pacemaker-mediated tachycardia, PMT, reentrant tachycardia, reentrant arrhythmia circuit, endless-loop tachycardia
Background
Pacemaker-mediated tachycardia (PMT) is a form of reentrant tachycardia that can occur in patients who have a dual-chamber pacemaker. In this setting, the pacemaker forms the anterograde (atrium to ventricle [A®V]) limb of the circuit and the atrioventricular (AV) node is the retrograde limb (ventricle to atrium [V®A]) of the circuit.
Pathophysiology
Assume a patient is in a paced rhythm with the pacemaker pacing the atrium and ventricle; if the AV node conducts the ventricular-paced beat or a premature ventricular contraction (PVC) back (ie, retrograde) to the atrium and depolarizes the atrium before the next atrial-paced beat, the impulse can then trigger the pacemaker to repeatedly pace the ventricle, thereby generating a reentrant arrhythmia circuit known as PMT or endless-loop tachycardia. Hence, the pacemaker forms the antegrade limb of the circuit, and the AV node is the retrograde limb.
Another common situation that causes PMT is failure to capture the atrium with a paced beat, which means the atrium then is amenable to depolarization by the impulse conducted retrograde from the ventricle. The tachycardia continues until retrograde conduction is lost or the atrium becomes refractory. Note that as many as one third of patients with antegrade complete AV block have intact retrograde (V®A) conduction, which may be intermittent. Depending on the retrograde conduction time, the PMT rate may be at or below the programmed upper rate limit of the pacemaker. This form of pacemaker behavior only can occur in a dual-chamber system.
Mortality/Morbidity
Patients may experience chest palpitations, rapid heart rates, lightheadedness, syncope, or chest discomfort.
History
Consider PMT in patients with a dual-chamber pacemaker who experience palpitations, rapid heart rates, lightheadedness, syncope, or chest discomfort.
Physical
- The examination findings may confirm tachycardia and may be otherwise unremarkable, except for presence of a pacemaker.
- Findings on ECG demonstrate intermittent or continuous ventricular-paced rhythm at or near the upper rate limit, which typically is set at 120/min.
Causes
See Pathophysiology.
Other Problems to be Considered
Any rapid atrial rhythm, such as atrial tachycardia, atrial flutter, or atrial fibrillation, may be sensed by the atrial lead, and this drives the pacemaker at or near the upper rate limit. This is confirmed when intracardiac electrograms are examined or by evaluation of the underlying rhythm. In unipolar pacemakers, myopotentials from the chest wall muscles or electromechanical signals from an electrocautery device in the operating room may be oversensed and also may drive the pacemaker at the upper rate limit. Extremely rarely, in older pacemakers, the pacemaker circuitry may malfunction, resulting in so-called runaway pacemaker.
Other Tests
- Ventricular pacing at or near the upper rate limit of the pacemaker is evident on ECG.
- A Holter monitor or event recorder also shows ventricular pacing at or near the upper rate limit.
- Telemetering the intracardiac electrograms using the pacemaker program generally proves the diagnosis.
Medical Care
Treatment of PMT typically involves altering the pacemaker programming to make the atrial lead insensitive to the retrograde P wave, rapid atrial activation, or electromagnetic interference. The former condition is most easily fixed by prolonging the postventricular atrial refractory period (PVARP). During the PVARP, the atrial lead does not sense any atrial activity; hence, ventricular pacing is not triggered. Note that this may affect the upper tracking rate of the pacemaker as this is defined by the total atrial refractory period (TARP), ie, TARP = AV delay + PVARP. For example, if the AV delay is 170 milliseconds and the PVARP is set to 430 milliseconds, the TARP then is 600 milliseconds, which corresponds to an upper rate of 100/min (rate = 60,000/cycle length [milliseconds]). This means the pacemaker could not track atrial rates above 100/min and would develop 2:1 block for this and higher rates, thus limiting the activity of many patients. Other options include
programming apacemaker AAI or DDI, if possible, so as not to track the P waves.
In acute situations, when a patient is very symptomatic, when the pacemaker model is unknown, or when a programmer is unavailable, applying a magnet over the pacemaker inhibits sensing and makes the pacemaker pace asynchronously in the atrium and ventricle, thus terminating the PMT by blocking the antegrade limb of the circuit. Carotid sinus massage or AV nodal–blocking drugs such as adenosine, verapamil, or beta-blockers can block VA (ie, retrograde conduction) directly and terminate and prevent tachycardia.
Reprogramming a dual-chamber, dual-mode, dual pacing, dual-sensing (DDD) pacemaker to AAI, VVI, or DVI (DDI) also prevents any sensing in the atrium and prevents PMT. The atrial sensitivity also may be programmed so that intrinsic P waves are detected but not retrograde P waves (which often are smaller).
Many modern pacemakers are capable of detecting PMT and initiate PMT intervention by automatically prolonging the PVARP for the beat after a ventricular-sensed event that is not preceded by atrial pacing, ie, a PVC (PVARP extension). This problem also can be minimized by the use of adaptive PVARP in rate-responsive (DDDR) pacemakers; the PVARP is long when the patient is at rest and shortens when the sensor indicates activity, allowing the pacemaker to track to higher atrial rates. Other pacemaker algorithms include dropping a ventricular-paced beat when the pacemaker is pacing at the maximum tracking rate for a specific period of time or shortening the AV interval for a single beat to induce retrograde AV block and terminate the tachycardia.
PMT generally is treated by reprogramming the pacemaker.
Further Inpatient Care
- Inpatient care involves standard pacemaker follow-up.
Deterrence/Prevention
Complications
- PMT rarely is associated with any serious complications such as presyncope or syncope.
- In many patients, the condition may be asymptomatic and is noted only with ECG or Holter monitoring.
- With the appropriate programming interventions described above, the problem usually is resolved, and, in most modern pacemakers, it can be detected and treated by the device itself.
- In patients who develop chest pain (angina pectoris) associated with the rapid pacing rate, consider a stress test to evaluate for coronary artery disease.
Prognosis
- Prognosis is not directly altered by an episode of PMT and is defined by the patient's underlying cardiac or medical condition.
- Indirectly, in a rare event such as PMT-induced syncope, a patient could sustain injury as a result of the syncope.
Medical/Legal Pitfalls
| Media file 1:
Telemetered ECG tracing with surface lead II (top) and intracardiac electrograms (atrial electrogram [center] and ventricular electrogram [lower]) and marker channel (bottom) showing pacemaker-mediated tachycardia (PMT). The intracardiac markers indicate that the retrograde P waves, labeled AS for atrial-sensed event, occur 280 milliseconds after the ventricular-paced beats, labeled VP. |
 |  View Full Size Image | |
Media type: ECG
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| Media file 2:
Telemetered ECG tracing (see Image 1) showing atrioventricular (AV)–paced rhythm at 60/min after termination of the pacemaker-mediated tachycardia (PMT). The tracing, from top to bottom, shows lead II, atrial electrogram, ventricular electrogram, and marker channels. The intracardiac markers indicate the rhythm is atrial paced (AP) and ventricular paced (VP). |
 | View Full Size Image | |
Media type: ECG
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- Furman S, Hayes DL, Holmes DR. A Practice of Cardiac Pacing. New York, NY:. Futura;1993:77-81.
- Griffin J, Smithline H, Cook J. Runaway pacemaker: a case report and review. J Emerg Med. Aug 2000;19(2):177-81. [Medline].
- Moses HW, Miller BD, Moulton K. A Practical Guide to Cardiac Pacing. Philadelphia, Pa:. Lippincott Williams & Wilkins;2000:96-98.
Pacemaker-Mediated Tachycardia excerpt Article Last Updated: Aug 9, 2006
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