Implantable Pacemakers 

Permanent pacemakers are implantable devices that sense intrinsic cardiac electric potentials and, if these are too infrequent or absent, transmit electrical impulses to the heart to stimulate myocardial contraction.

Magnetic resonance imaging (MRI) is generally contraindicated in patients with pacemakers. However, in February 2011, the US Food and Drug Administration (FDA) approved the Revo MRI SureScan Pacing System, which is the first cardiac pacemaker designed to be used safely during MRI examinations.

Category

Cardiac pacemakers

Device details

Biotronik Pacemaker devices

• Cylos DR-T/DR/VR
• Evia
• Philos DR/SR
• Philos II DR-T/DR/SR

Biotronik CRT (cardiac resynchronization therapy) devices

• Lumax 540 HF-T
• Lumax 340 HF-T
• Stratos LV

Boston Scientific Pacemaker devices

• Altrua™ Pacemaker Family

Boston Scientific CRT devices

• COGNIS® CRT-D
• CONTAK RENEWAL® 3RF CRT-D
• CONTAK RENEWAL® TR CRT-P
• LIVIAN™ CRT-D

Medtronic Pacemaker devices

• Adapta™ (see the image below)Medtronic Adapta. Image courtesy of Medtronic.
• EnRhythm™
• Sensia™ (see the image below)Medtronic Sensia. Image courtesy of Medtronic.
• Versa™ (see the image below)Medtronic Versa. Image courtesy of Medtronic.
• Revo MRI™ SureScan® Pacing System (see the image below); this is the first MRI-safe pacemakerMedtronic Revo MRI SureScan MRI-safe pacemaker. Image courtesy of Medtronic.

Medtronic CRT devices

• Concerto® II CRT-D (see the image below)Medtronic Concerto II CRT-D. Image courtesy of Medtronic.
• Consulta II CRT-D
• Maximo™ II CRT-D (see the image below)Medtronic Maximo II CRT-D. Image courtesy of Medtronic.

St. Jude Medical Pacemaker devices

• Accent®
• Affinity®
• AutoCapture™
• Entity™
• Identity® ADx
• Identity®
• Integrity® ADx
• Integrity®
• Microny™
• Regency®
• Verity® ADx
• Victory®
• Zephyr™

St. Jude Medical CRT devices

• Anthem® CRT-P
• Atlas® + HF CRT-D
• Atlas® II HF CRT-D
• Epic® HF CRT-D
• Epic™ II HF CRT-D
• Frontier® II CRT-P
• Promote Accel® CRT-D
• Promote® Plus CRT-D
• Promote® RF CRT-D
• Unify™ RF CRT-D

Sorin Group Pacemaker devices

• Esprit™ DR
• Reply™ DR
• Rhapsody™ 2530 and 2510
• Symphony® DR 2550

Sorin Group CRT devices

• Ovatio™ CRT-D
• Paradym™ CRT-D

Pacing systems consist of a pulse generator and pacing leads. The pulse generator contains a battery, as well as sensing, timing, and output circuits. The battery (most commonly lithium-iodide) typically has a lifespan of 5-10 years.

Pulse generators can be set to fixed-rate (asynchronous) or demand (synchronous) modes. In the asynchronous mode, impulses are produced at a set rate independent of intrinsic cardiac activity. In the synchronous mode, the sensing circuit searches for an intrinsic depolarization potential. If this is absent, a pacing response is generated. This mode closely mimics intrinsic myocardial electrical activity.

The Revo MRI SureScan Pacing System, the pacemaker approved for use with magnetic resonance imaging (MRI), is based on the Medtronic EnRhythm pacemaker and CapSureFix Novus pacing lead, with the following modifications to improve MRI compatibility^[1] :

• Leads modified to reduce the lead tip heating induced by radiofrequency (RF) energy
• Internal circuits changed to reduce the potential for cardiac stimulation
• Amount of ferromagnetic materials limited
• Internal circuit protection improved to prevent disruption of internal power supply
• Pacemaker reed switch replaced with a Hall sensor, with predictable behavior in a static magnetic field

Permanent pacemakers are implantable devices that sense intrinsic cardiac electric potentials and, if these are too infrequent or absent, transmit electrical impulses to the heart to stimulate myocardial contraction. A specialized type of pacemaker therapy, cardiac resynchronization therapy (CRT) with biventricular pacing, with or without an implantable cardioverter-defibrillator (ICD), has been used as adjunctive therapy for patients with heart failure.^[2]

Absolute indications for pacemaker placement include the following:

• Sinus node dysfunction (sick sinus syndrome)^[3]
• Third-degree atrioventricular block (complete heart block)
• Symptomatic sinus bradycardia
• Atrial fibrillation with sinus node dysfunction
• Chronotropic incompetence (inability to increase the heart rate to match a level of exercise)
• Long QT syndrome

Relative indications for pacemaker placement include the following:

• Dilated cardiomyopathy
• Severe refractory neurocardiogenic syncope

Patients with heart failure and reduced left ventricular ejection fraction (LVEF) often have electrical conduction abnormalities, with bundle branch block (BBB) being common. BBB leads to delayed activation of myocardium (ie, mechanical dyssynchrony), which results in inefficient contraction of the left ventricle (LV), increased LV end-diastolic and pulmonary wedge pressures, increased mitral regurgitation, decreased cardiac output, and progression of heart failure symptoms.

Biventricular pacing can restore mechanical synchrony (ie, CRT), which may lead to LV reverse remodeling, decreased cardiac pressures and mitral regurgitation, and improved LVEF and exercise tolerance. Guidelines developed by the American College of Cardiology (ACC) and the American Heart Association (AHA) on the diagnosis and management of heart failure in adults recommend the indications below for CRT^{[4, 5]} :

• CRT, with or without an ICD, is indicated for patients with an LVEF of 35% or lower, sinus rhythm, and New York Heart Association (NYHA) functional class III or IV heart failure symptoms who are on optimal medical therapy and have evidence of cardiac dyssynchrony (as demonstrated by a QRS duration of more than 120 ms)
• CRT, with or without an ICD, is reasonable for the treatment for NYHA functional class III or IV heart failure symptoms in patients on optimal medical therapy with chronic atrial fibrillation, an LVEF of 35% or lower, and a QRS duration of 120 ms or longer
• CRT, with or without an ICD, is reasonable for patients who have frequent dependence on right ventricular pacing, an LVEF of 35% or lower, and NYHA functional class III or IV symptoms on optimal medical therapy

Magnetic resonance imaging (MRI) is generally contraindicated in patients with pacemakers, and the AHA guidelines recommend consideration of MRI only in exceptional circumstances, thereby excluding the vast majority of pacemaker patients who might benefit from MRI examination (see Complications).

In February 2011, the US Food and Drug Administration (FDA) approved the Revo MRI SureScan Pacing System, the first cardiac pacemaker designed to be used safely during MRI examinations.

The Comparison of Medical Therapy, Pacing and Defibrillation in Heart Failure (COMPANION) trial demonstrated a reduction in hospital admissions among patients who underwent cardiac resynchronization therapy (CRT).^[6] In this study, 1520 patients with advanced heart failure were randomized to optimal medical therapy, optimal medical therapy plus CRT (CRT-P), or an implantable cardioverter-defibrillator (ICD) plus CRT (CRT-D).

In comparison with optimal medical therapy alone, CRT-P and CRT-D were associated with 21% and 25% reductions, respectively, in all-cause hospital admissions; 34% and 37% reductions, respectively, in cardiac-related hospital admissions; and 44% and 41% reductions, respectively, in heart failure–related hospital admissions per patient-year of follow-up.^[6] The improvements in heart failure hospitalization rates in both CRT groups were evident within days after CRT implantation and were sustained throughout the study.

CRT-P and CRT-D were also found to reduce the risk of death from any cause by 24% and 36%, respectively, as compared with medical therapy alone.^[6]

In the Multicenter Automatic Defibrillator Implantation Trial with Cardiac Resynchronization Therapy (MADIT-CRT), CRT was associated with a significant reduction in LV volume and improvement in LV ejection fraction (LVEF), although not with a significant difference in the overall risk of death.^[7]

This trial studied the potential benefit of CRT with biventricular pacing in patients with an LVEF of 30% or less, a QRS duration of 130 ms or longer, and New York Heart Association (NYHA) class I or II symptoms. Over the course of 4.5 years, 1820 patients were randomly assigned to receive either CRT plus an ICD or ICD alone.^[7]

Several major randomized trials have compared the merits of dual-chamber (ie, atrial and ventricular) pacing, single-chamber atrial pacing, and single-chamber ventricular pacing.^{[8, 9]} These include the Canadian Trial of Physiologic Pacing (CTOPP),^[10] the Mode Selection Trial (MOST), the United Kingdom Pacing and Cardiovascular Events (UK-PACE) trial, the Pacemaker Selection in the Elderly (PASE) study,^[11] and a large Danish study. Results have been inconsistent; direct comparisons are limited by differences in study design.

No complications related to magnetic resonance imaging (MRI) occurred during or after MRI in the EnRhythm MRI SureScan Pacing System Study,^[1] a prospective, randomized, controlled, multicenter clinical trial designed to investigate the safety and effectiveness of the Revo system in patients who met class I or II dual-chamber pacemaker implant indications according to the 2008 American College of Cardiology (ACC)/American Heart Association (AHA)/Heart Rhythm Society (HRS) Guidelines.^[5]

In this study, patients were randomly assigned to undergo MRI 9-12 weeks after pacemaker implantation (MRI group, n = 258) or not to undergo MRI (control group, n = 206).^[1] MRI scans were performed on 1.5T systems from 3 different manufacturers. Fourteen non – clinically indicated head and lumbar spine sequences were performed on each patient in the MRI group. No MRI-related sustained ventricular arrhythmias, pacemaker inhibition or output failures, electrical resets, or threshold or sensitivity changes occurred during or after MRI.

Pacemaker endocardial leads are inserted transvenously and advanced to the right ventricle, the right atrium, or both, where they are implanted into the myocardial tissue or, in the case of biventricular pacing, to the coronary sinus, from which point the lead is advanced to a terminal vein adjacent to the posterior surface of the left ventricle. The pulse generator is placed subcutaneously or submuscularly in the chest wall.

During pacemaker placement, the amplitude and width of the electrical impulse are set high enough to reliably achieve myocardial capture but low enough to maximize battery life. Subsequent pacemaker programming can be performed noninvasively by an electrophysiology technician or cardiologist.

The Heart Rhythm Society (HRS) and the British Pacing and Electrophysiology Group (BPEG) have developed a code to describe various pacing modes (see Table 1 below).

Table 1. Pacemaker Code Used to Describe Various Pacing Modes (Open Table in a new window)

A typical pacing code consists of 3-5 letters, which have the following meanings:

• First letter - Indicates the chamber(s) paced (A, V, or D)
• Second letter - Indicates the chamber in which electrical activity is sensed (A, V, or D; O is used when the pacemaker discharge is not dependent on sensing electrical activity)
• Third letter - Refers to the response to a sensed electric signal (T, I, D, O)
• Fourth letter - Represents programmability and rate modulation (P, M, R, C, O)
• Fifth letter - Represents the antitachyarrhythmia function (P, S, D)

The most common setting, DDD mode, denotes that both chambers are capable of being sensed and paced. This setting requires 2 functioning leads, one in the atrium and the other in the ventricle.

Implementation of MRI-safe pacemaker

A dedicated programming care pathway was developed for the magnetic resonance imaging (MRI)-safe pacemaker (ie, the Revo MRI SureScan Pacing System) to facilitate the choice between asynchronous and nonstimulation modes, to increase the pacing output to 5.0 V/1.0 ms during MRI, to prevent programming of the MRI mode if the device has failed any of the 7 system integrity checks (see below), and to facilitate restoration of prescan program states and values.

Pacing system integrity checks are as follows:

• Pacemaker and both leads implanted for more than 6 weeks
• Pectoral implantation
• No other active pacemakers, implantable cardioverter-defibillators (ICDs), or leads
• No abandoned leads, lead extenders, or adapters
• Leads electrically intact, with stable and normal function
• Lead impedance between 200 and 1500 W
• Capture threshold less than 2.0 V at 0.4 ms

Schedule visits after device or lead implantation as follows:

• 1-2 weeks for wound check
• 1 month for pacemaker interrogation
• 3 months for pacemaker interrogation
• Every 6 months thereafter for pacemaker interrogation

Because of the myriad of existing pacemaker types, patients should carry a card with them providing information about their particular model. This information is crucial when there is a need to communicate with the cardiologist about a pacemaker problem. However, most pacemaker generators have an x-ray code that can be seen on a standard chest radiograph. The markings, along with the shape of the generator, may assist with deciphering the manufacturer of the generator and pacemaker battery.

As with other pacemakers, an identification card is provided to patients implanted with the magnetic resonance imaging (MRI)-safe Revo pacemaker. Like other pacemaker systems, the Revo system includes radiopaque markers that can be seen on a standard chest radiograph. These identification markers are placed on pacemakers and leads.

Advanced life support protocols, including defibrillation, may safely be performed on patients with a pacemaker in place. Sternal paddles are placed at a safe distance (10 cm) from the pulse generator. Temporary pacing may become necessary in cases of myocardial infarction because the current pacemaker discharge settings may be insufficient to stimulate ventricular contraction.

Treatment of pacemaker complications depends on the etiology. Common complications include the following:

• Pneumothoraces may necessitate medical observation, needle aspiration, or even chest tube placement
• Erosion of the pacer through the skin, though rare, necessitates replacement of the device and administration of systemic antibiotics
• Hematomas may be treated with direct pressure and observation; surgical drainage is rarely required
• Device-associated venous thrombosis is rare but generally presents as unilateral arm edema; treatment includes extremity elevation and anticoagulation
• Lead dislodgment generally occurs within 2 days of device implantation pacer and may be seen on chest radiography; free-floating ventricular leads may trigger malignant arrhythmias

Additional complications arise from malfunction due to mechanical factors. Although true pulse generator failure is very rare, pacing system malfunction occurs occasionally. This can be due to malfunctioning of the leads, the electrode-tissue interface, or the pulse generator. Most of such malfunctions can be corrected by simple reprogramming of the device. The majority of malfunctions, in fact, are due to normal programmed pacemaker function. Airport metal detector gates and hand-held metal detectors are safe for patients with pacemakers.^[12]

Major pacemaker malfunctions include the following:

• Failure to output
• Failure to capture
• Failure to sense
• Pacemaker-mediated tachycardia
• Runaway pacemaker
• Pacemaker syndrome (loss of atrioventricular synchrony, retrograde ventriculoatrial conduction, absence of rate response to physiologic need)
• Twiddler syndrome (patient manipulation of the pulse generator within the pocket, resulting in lead displacement or fracture)

Pacemaker or implantable cardioverter-defibrillator (ICD) leads placed in the venous system often have surrounding thrombosis, with 20% of patients experiencing complete occlusion of the subclavian vein at 2 years.

If a patient with a pacemaker or ICD requires a central venous catheter and the metal guide wire contacts the lead system during central line placement, there may be enough noisy artifact to trigger an inappropriate shock. Consideration should be given to either avoiding a metal guide wire or deactivating the ICD during central line placement. Although the contralateral subclavian or internal jugular vein can be cannulated with care, the femoral vein access is a much safer option.

Pacing with MRI

Magnetic resonance imaging (MRI) is generally contraindicated in patients with pacemakers,^[13] and American Heart Association (AHA) guidelines recommend consideration of MRI only in exceptional circumstances,^[14] excluding the vast majority of pacemaker patients who might benefit from MRI examination. However, in February 2011, the US Food and Drug Administration (FDA) approved the Revo MRI SureScan Pacing System, the first cardiac pacemaker designed to be used safely during MRI examinations.

For patients without an MRI-safe pacemaker, the electromagnetic fields and radiofrequency (RF) energy generated by MRI may pose risks (eg, interference with pacemaker operation, damage to system components, inappropriate therapy, lead or pacemaker dislodgement, or change in capture threshold). A scientific statement from the AHA on the safety of MRI in patients with cardiovascular devices includes the following limited recommendations on the performance of MRI in patients without an MRI-safe pacemaker or ICD^[14] :

• Performing the examination at centers with expertise in MRI and electrophysiology
• Having a physician with pacemaker/ICD expertise decide whether it is necessary to reprogram the device before the MRI examination
• Having a person with expertise in MRI physics and safety involved in planning the scan, with consideration for the use of scanning parameters that minimize risk (eg, the lowest RF power levels and the weakest/slowest necessary gradient magnetic fields)
• Testing pacemaker functions before and after the examination
• Observing the patient closely throughout the examination, including monitoring of heart rhythm and vital signs