7/6/23, 11:01 AM Pacing system malfunction: Evaluation and management - UpToDate Official reprint from UpToDate www.uptodate.com 2023 UpToDate, Inc. and/or its affiliates. All Rights Reserved. Pacing system malfunction: Evaluation and management : Mark S Link, MD : N A Mark Estes, III, MD : Todd F Dardas, MD, MS All topics are updated as new evidence becomes available and our peer review process is complete. Literature review current through: Jun 2023. This topic last updated: Dec 19, 2019. INTRODUCTION Periodic evaluations of an implanted pacemaker are required to maintain optimal programming and to identify any system problem that should be corrected. Common pacing system problems of single and dual chamber pacemakers and the methods of evaluation and therapy will be reviewed here. The malfunctions discussed will be limited to those that are manifest on an electrocardiogram (ECG) rhythm strip. Complications not related to pacing system malfunction Other complications not related to pacing are presented separately. These include infections, venous thrombosis and emboli, pacemaker syndrome, tricuspid regurgitation, and specific problems associated with dual- chamber pacemakers. (See "Infections involving cardiac implantable electronic devices: Epidemiology, microbiology, clinical manifestations, and diagnosis" and "Cardiac implantable electronic devices: Long-term complications", section on 'Tricuspid regurgitation' and "Dual chamber pacing system malfunctions of timing, sensing, and capture: Evaluation and management".) PACING SYSTEM COMPONENTS The traditional pacing system is comprised of the pulse generator ( picture 1), also called the pacemaker, and the transvenous or epicardial lead or leads that connect the pulse generator to the heart. https://www.uptodate.com/contents/pacing-system-malfunction-evaluation-and-management/print 1/25 7/6/23, 11:01 AM Pacing system malfunction: Evaluation and management - UpToDate The phrase "pacing system malfunction" includes problems that might arise from any of the components of the system. Inappropriately programmed pacemaker parameters, although they do not represent abnormal pacing system function, may be suboptimal for the patient. The normal characteristics and unique timing systems and algorithms of a given pacemaker are also an issue, as they may be interpreted as a malfunction by a clinician who is not familiar with the specific pulse generator. Recording system artifacts must always be considered in the differential diagnosis of a pacing system malfunction. INCIDENCE The incidence of pacing system malfunction is difficult to determine due to inconsistent definitions and the lack of any comprehensive reporting mechanism or registry [1,2]. Overall, device hardware is highly reliable [3]. In terms of comparative reliability, there is a higher incidence of complications of leads compared with pulse generators. In terms of lead malfunction, more complex implantable cardioverter-defibrillator leads have a higher incidence of failure than simpler pacemaker leads. As a result of a series of lead malfunctions, a policy was published by Heart Rhythm Society with lead performance guidelines [4]. PACING STIMULI PRESENT WITH LOSS OF CAPTURE One of the principal requirements of a pacing system is that it prevents a heart rate that is slower than a predefined rate. This is accomplished by the release of an output pulse or stimulus from the pacemaker that is delivered to the myocardium via the pacing lead, inducing a depolarization of either the atrium or ventricle, depending upon the location of the lead. The failure of that stimulus to produce electrical activation of the heart and a subsequent cardiac contraction is called noncapture or loss of capture. It is differentiated from capture by the absence of an evoked potential following the pacing stimulus. The diagnosis of loss of capture is based upon the presence of a stimulus, without a subsequent P wave or QRS complex, which occurs at a time when the myocardium is physiologically capable of being depolarized. One explanation for apparent loss of capture is failure to sense a native QRS complex followed by the release of the pacing stimulus at a time when the myocardium is physiologically refractory and incapable of being stimulated. While this may reflect a sensing malfunction, it is a normal phenomenon with regard to capture and should not be considered true capture malfunction. This is designated "functional failure to capture." https://www.uptodate.com/contents/pacing-system-malfunction-evaluation-and-management/print 2/25 7/6/23, 11:01 AM Pacing system malfunction: Evaluation and management - UpToDate To establish true noncapture, it must be determined that a pacing stimulus was present on the ECG. Historically, on analog recording systems, unipolar stimuli were large and easily visible, and bipolar stimuli were diminutive and at times difficult to recognize. However, with current digital recording systems, this distinction is no longer reliable. If the output energy has been reduced, the stimulus may be virtually invisible on some ECG leads or may be eliminated entirely by high-frequency filters in the recording system. If no pacing stimuli are present in any of the available recording leads, then the problem is not loss of capture but a circuit interruption, which prevents delivery of the stimulus to the myocardium or oversensing. Causes of loss of capture The differential diagnosis of loss of capture is relatively limited, and the likelihood of a given problem is highly correlated with the time since implantation. Lead dislodgement or malposition The most common reason for loss of capture in the hours and days following implantation is either lead dislodgement or malposition. This is manifested as a change in the morphology of the pacemaker-evoked depolarization when capture is present. A change in the anatomic position of the lead on a chest radiograph may also be seen. Identifying these problems is dependent upon comparison with a baseline (ie, when normal capture was present) 12-lead ECG and chest radiograph. Failure to sense native complexes as well as atrial or ventricular ectopy may also be associated with an unstable electrode within one of the cardiac chambers. If identified, treatment may require operative intervention to reposition the lead. Lead dislodgement does not prevent pacemaker output. If a magnet is applied to the pulse generator when the lead is dislodged, even though it may not result in cardiac depolarization, output stimuli will still occur at the magnet rate of the pulse generator. Inflammation and fibrosis at the electrode/myocardial interface The inflammatory reaction and the resulting fibrous tissue that may occur after lead implantation may act as an insulating shield around the electrode, effectively raising the threshold for stimulation and attenuating the amplitude and slew rate of the endocardial signal being sensed. This is a process termed "lead maturation." Although improvements in electrode design and materials (eg, steroid elution) have essentially eliminated a reaction of such severity that a clinically issue is rarely encountered, loss of capture could result if the capture threshold exceeds the programmed output of the pacemaker. In the rare patient in whom this should occur, high thresholds associated with lead maturation may be diagnosed by a stable morphology of the evoked complex when capture is present and https://www.uptodate.com/contents/pacing-system-malfunction-evaluation-and-management/print 3/25 7/6/23, 11:01 AM Pacing system malfunction: Evaluation and management - UpToDate there is a stable anatomic position on chest radiograph. The peak capture thresholds usually occur between several weeks to several months after implantation. Given sufficient time, the inflammatory reaction will subside, and the capture thresholds will improve. Management options include: Increasing the output of the pulse generator Lead repositioning Increase in capture threshold Loss of capture may occur months to years after implantation due to one or more of the following: A late rise in capture thresholds (ie, greater than four weeks post-implant) in the absence of dislodgment, thought to be secondary to excessive fibrosis at the lead/myocardial interface, also called exit block. This, too, is rarely seen given improvements in electrode design and materials (eg, steroid elution). A primary cardiomyopathic process. The addition of various medications or the occurrence of a metabolic abnormality, both of which can transiently elevate the pacemaker capture and sensing thresholds. Management is usually focused on correcting or eliminating the cause and, until this is accomplished, the output of the pacemaker is increased. When the etiology is a primary myocardial process, such as progressive fibrosis associated with a diffuse cardiomyopathy or a focal myocardial infarction, and the loss of capture cannot be managed by increasing the output of the pulse generator, an operative procedure will be required to place a new electrode. Sub-threshold pacemaker output programming Loss of capture may occur when the margin of safety for the programmed output with respect to the measured capture threshold is too low. While the capture threshold is a specific pulse amplitude at a given pulse duration, the measurement is made at only one point in time and usually with the patient in one position. Capture thresholds may change during the day; with body position; and in association with various physiologic stresses, such as exercise, eating, and infection. Programming the output too close to the measured capture threshold may not provide a sufficient margin of safety, resulting in intermittent loss of capture. In this setting, the pacemaker, leads, and patient are all functioning normally, but the pulse generator is programmed inappropriately for the patient. Increasing the output will correct this problem. Lead failure Intrinsic lead failure may result in loss of capture. This is usually a late occurrence (ie, many years after implantation). One or more of the following mechanisms may https://www.uptodate.com/contents/pacing-system-malfunction-evaluation-and-management/print 4/25 7/6/23, 11:01 AM Pacing system malfunction: Evaluation and management - UpToDate be involved: Deterioration of the lead insulation may allow a current leak, resulting in loss of capture. Generation of nonphysiologic electrical transients, causing oversensing or attenuation of the intrinsic cardiac electrogram, can lead to undersensing. Extrinsic stresses on the lead, such as compression between the first rib and clavicle (ie, subclavian crush injury) may result in a conductor fracture or insulation breach due to a point of stress on the lead. These leads must be replaced to reestablish normal pacing system function. A primary lead problem can often be identified by the measurement of lead impedance. This information can be obtained by telemetry of measured data. The normal range for lead impedances typically varies from 300 to 1500 ohms. An insulation failure will result in very low impedances, often less than 250 ohms, while an open circuit associated with a conductor fracture is associated with very high impedances. Given the wide variability in impedance measurements between different lead models and even leads of the same model, trends or changes in serial measurements are often more important than a single isolated assessment. Chest radiograph may help diagnose a fracture of the conductor coil. The lead insulation is radiolucent and a defect in the insulation will generally not be visible radiographically. Compression or distortion of the conductor coil may identify a point of increased external stress on the lead. In a bipolar coaxial lead, capture may be restored in the presence of an internal insulation failure or a fracture of the outer conductor coil by programming to the unipolar (tip to case) output configuration. However, this should be considered a temporary measure. Battery depletion All pacemakers require a power source to function. They are powered by a battery, which is usually comprised of a lithium iodine power cell. The longevity of these systems is often six to eight years, but the longevity of any specific pulse generator depends upon the proportion of paced and sensed events, the programmed output and rate parameters, and the stimulation impedance. Each pacemaker incorporates one or more special features that will identify the recommended replacement time (RRT) or elective replacement indicator (ERI). When this occurs, one can expect the system to function properly for approximately three months, allowing sufficient time to electively replace the pulse generator. If these indicators are ignored or the patient is lost to follow-up, the battery may deplete to a point where the effective output falls below the capture threshold, resulting in loss of capture. https://www.uptodate.com/contents/pacing-system-malfunction-evaluation-and-management/print 5/25 7/6/23, 11:01 AM Pacing system malfunction: Evaluation and management - UpToDate Battery depletion requires replacement of the pulse generator. If battery depletion occurs sooner than anticipated, based upon the programmed parameters of the pacemaker and the projections of the manufacturer, the integrity of the pacing lead should be carefully evaluated at the time of the replacement procedure. An insulation failure will result in a low-stimulation impedance that can accelerate battery depletion. Unless this is identified and the lead replaced, the new pulse generator will encounter a similar problem of rapid battery depletion. Recording system artifact Recording system artifacts may raise concerns about a system malfunction when they occur in a patient who has a pacemaker. Digital recording systems can either create a pacing stimulus in response to a high-frequency signal or initiate a blocking period in response to a very high-frequency signal to protect its own circuitry. This has the undesirable effect of either eliminating true pacing artifacts or creating a false artifact on the recording in response to a high-frequency transient of another etiology, mimicking either noncapture or undersensing. Another relatively common artifact occurs in systems that have special circuitry to protect the recorder from too large an incoming signal by blocking the circuit for a variable period of time. This may have the effect of showing the stimulus but eliminating the resultant evoked potential, leading to concerns about loss of capture. In this situation, there will be a visible T wave reflecting repolarization. Regardless of the cause, the likelihood of a recording system artifact is minimized by recording the rhythm in multiple simultaneous leads or sequentially in different leads. PACING STIMULUS PRESENT WITH FAILURE TO SENSE The second major functional capability of the modern pacemaker is the ability to sense or recognize intrinsic cardiac depolarizations. The pacemaker should be able to sense intrinsic atrial activity when the lead is located in the atrium, and intrinsic ventricular activity when the lead is located in the ventricle. Sensing is a complex phenomenon, being dependent upon the sense amplifier within the pacemaker and the size and characteristics of the signal inside the heart at the location of the pacing electrode. Although sensitivity is commonly reported as a function of the amplitude or size of the signal, it is a more complex process. Lack of sensing may occur with a QRS with very low amplitude. A signal with sufficient amplitude, but whose dominant frequencies fall outside the constraints imposed by the pacemaker's filters, will also not be sensed. Unless one is able to examine the intracardiac signal, which is what is actually sensed by the pacemaker (ie, the intrinsic deflection of the endocardial electrogram [EGM]), it is not feasible to ascertain from the surface recording of either the https://www.uptodate.com/contents/pacing-system-malfunction-evaluation-and-management/print 6/25 7/6/23, 11:01 AM Pacing system malfunction: Evaluation and management - UpToDate isolated P wave or QRS complex why a specific signal is or is not sensed and where sensing occurs within the complex. To examine the intrinsic deflection as seen by the pacemaker, electrograms can either be recorded via the pacing lead at the time of implantation or be telemetered from the functioning pacemaker at any time postimplantation. The basic pacing interval associated with either single-chamber pacing or either channel of a dual-chamber system is divided into two subintervals. Immediately following a paced or sensed event, the sense amplifier is rendered refractory. The purpose of the refractory period is to prevent the pacemaker from sensing and responding to known but inappropriate electrical or physiologic signals, such as ringing on the sense amplifier from the output pulse, the T wave, or far-field signals, such as R waves detected on the atrial channel, which are known to occur in close proximity to the paced or sensed event. However, appropriate signals that would normally be sensed at other times will not be sensed if they coincide with the refractory period. This becomes more common during dual-chamber pacing, and is also more likely to occur if the refractory period is programmed to a long interval. If there is a failure to sense or recognize a signal that should otherwise be sensed because the signal coincided with the refractory period, this is not a true system malfunction and should be termed "functional undersensing." Both true and functional undersensing are commonly associated with functional noncapture because the timing intervals are not reset, allowing the pacing stimulus to be delivered at a time when the myocardium is physiologically refractory. The other portion of the single-channel timing cycle is the alert period. An event occurring during the alert period should be sensed to trigger or inhibit the output of the pacemaker, depending upon the design of the system. The failure to sense an event in the alert period is true "undersensing." Causes of undersensing The causes of true undersensing include: Inadequate signal A signal is inadequate when it does not fulfill the pacemaker's requirements for an appropriate signal; there is too low an amplitude, slew rate, or inappropriate frequency content. Pacemaker programmed to a value insufficient to sense intrinsic activity Undersensing occurs if the sensitivity parameter of the pacemaker is set inappropriately for the intrinsic signal characteristics. https://www.uptodate.com/contents/pacing-system-malfunction-evaluation-and-management/print 7/25 7/6/23, 11:01 AM Pacing system malfunction: Evaluation and management - UpToDate Change in native signal A change in the native signal from the time of implant due to a primary myocardial process, such as an infarction, or transient changes as with drugs or metabolic abnormalities will cause undersensing. Ectopic beats Not uncommonly, ectopic beats that arise from a location different from that of the dominant intrinsic complex, whether atrial or ventricular, may not be sensed. Although a ventricular ectopic beat is frequently larger than the QRS on the surface ECG, the intrinsic deflection of the QRS on the intracardiac electrogram, what is actually sensed, may be of very low amplitude. Lead maturation Rarely, the inflammatory reaction associated with lead maturation may attenuate the amplitude and slew rate of the native complex by as much as 50 percent, so that if an original signal was only borderline in amplitude for appropriate sensing, undersensing may occur during this time. Lead failure Primary lead malfunctions, most typically an insulation failure, will effectively attenuate the signal coming into the sense amplifier, resulting in undersensing problems. Pulse generator failure There may be a true component malfunction involving the sense amplifier that results in undersensing. If this is the case, either the telemetered electrogram or the invasively recorded electrogram will be an appropriate signal for sensing, and other parameters of lead function will be normal. As a result, the pulse generator should be replaced and the leads reutilized. Although clinically important pulse generator failure is uncommon, if there is evidence of an unacceptable incidence of failures or potential failures, the US Food and Drug Administration (FDA) may issue a recall or advisory because of potentially harmful consequences resulting from device malfunction [5]. However, an advisory or a recall is not a guarantee that a problem will occur; rather these are usually issued when there is an increased incidence of a given problem about which the clinician should be made aware, at a minimum. The increased level of communication from the manufacturer to the clinical community facilitates the clinician's ability to care for the patient, although that same level of communication may increase the clinician's anxiety. Environmental electrical fields The pacemaker may detect and respond to strong electrical fields in the environment. There are very few sources of clinically significant electromagnetic interference found in the non-hospital environment. However, in the hospital a variety of diagnostic and therapeutic equipment, such as electrocautery, may interfere with normal pacing system function. (See 'Electromagnetic interference' below and "Cardiac implantable electronic device interactions with electromagnetic fields in the nonhospital environment".) https://www.uptodate.com/contents/pacing-system-malfunction-evaluation-and-management/print 8/25 7/6/23, 11:01 AM Pacing system malfunction: Evaluation and management - UpToDate Magnet application During magnet application, a mechanism occurs within the pacemaker, whereby the sense amplifier is bypassed, inducing asynchronous function (eg, a pacemaker programmed to the DDD mode will function as DOO when the magnet is applied). Magnet application is essential to confirm the integrity of the output circuit when the pacemaker is otherwise being inhibited. A change in the magnet-induced behavior, usually rate but sometimes AV delay and mode, is also used by many manufacturers as an indicator of battery depletion. When the magnet behavior is unique and different from the programmed rates and intervals, it is relatively easy to identify the expected loss of sensing due to magnet application. If the magnet application results in rates and intervals that are identical to the programmed parameters of the pacemaker, even though in an asynchronous manner, and the individual interpreting the ECG is not informed that a magnet was placed over the pulse generator, there will probably be concern about loss of sensing as indicative of a true malfunction. Noise detection The noise mode response is another situation in which asynchronous behavior is consistent with normal system behavior. Electrical noise is defined by the pulse generator as a series of electrical signals occurring at a very rapid rate, frequently greater than 6 Hz or 360 cycles or signals per minute. This is above the physiologic range for heart rate, and if the pacemaker "sees" signals coming in this rapidly, it interprets these as not being true physiologic signals. Rather than inhibit the output in response to these signals, which leaves the patient at potential jeopardy from asystole, most systems are designed to function in an asynchronous manner as long as electrical noise is being detected. Noise detection typically occurs during the terminal portion of the refractory period. The beginning of the refractory period is the equivalent of the effective refractory period in the heart; at this time, there is absolutely no sensing. The last portion of the refractory period is the noise sampling period (NSP), which would be the equivalent of the functional refractory period. If an event is sensed during the NSP, another, and often shorter, refractory period is initiated. It is also comprised of an absolute refractory period and an NSP. If a signal is sensed in this next NSP, the refractory period is again reset. Assuming this continues to occur, the basic rate timing will complete and, since nothing that the pacemaker recognized as a true or appropriate signal occurred, a pacemaker pulse will be released. If the patient's native rhythm is asystole and the system is detecting electrical noise, pacing will be maintained and the patient will be protected. If, on the other hand, the patient has https://www.uptodate.com/contents/pacing-system-malfunction-evaluation-and-management/print 9/25 7/6/23, 11:01 AM Pacing system malfunction: Evaluation and management - UpToDate an intrinsic rhythm, the noise mode response will result in asynchronous pacing, with potential competition between the pacemaker and native rhythm appearing as "undersensing" on the ECG. Management of undersensing The vast majority of contemporary pacemakers have the capability for electrogram telemetry with a broad band-pass filter, which provides an excellent means to evaluate the characteristics of the native signals to determine why they are not being sensed. Alternatively, management may involve programming the sensitivity of the device to a more sensitive value, which makes it responsive to progressively smaller amplitude signals. If the device was programmed to an overly sensitive value, it could result in inappropriate sensing of noncardiac signals, such as electrical potentials arising from either the skeletal muscle contiguous to the pulse generator or that associated with diaphragmatic contractions. ELECTROMAGNETIC INTERFERENCE Electrical noise from external sources in the hospital or non-hospital environment may result in a series of rapid and/or erratic electrical signals that, when sensed by the pacemaker, may result in a variety of responses. For example, if electromagnetic interference (EMI) is persistent, it could result in asynchronous pacing via noise reversion; if sensed on the ventricular lead, it could result in inhibition of the pacemaker; and if sensed on the atrial lead of a dual-chamber pacing, it could result in rapid ventricular tracking of the EMI. While some household- and work-based electrical equipment had the potential to inhibit early-generation pacemakers, improvements in circuit design and shielding have eliminated many causes of EMI. Strong electrical fields applied either in very close proximity or directly to the patient are usually required for there to be a major effect on the implanted pacing system. This most commonly occurs in the medical environment, when the implanted system is exposed to electrocautery at the time of surgery, internal or external cardioversion or defibrillation, and magnetic resonance imaging (MRI) ( table 1). (See "Perioperative management of patients with a pacemaker or implantable cardioverter-defibrillator".) Other non-medical causes of EMI are discussed separately. (See "Cardiac implantable electronic device interactions with electromagnetic fields in the nonhospital environment".) Therapeutic radiation Therapeutic radiation can have adverse effects on cardiac implantable electronic devices (CIEDs). Radiation equipment can temporarily or permanently alter CIED function in a variety of ways. (See 'Magnetic resonance imaging' below.) https://www.uptodate.com/contents/pacing-system-malfunction-evaluation-and-management/print 10/25 7/6/23, 11:01 AM Pacing system malfunction: Evaluation and management - UpToDate Ionizing radiation may result in electrical reset of a device, reprogramming with errant behavior, or can permanently damage a pacemaker by causing defects in semiconductor insulation [6-10]. If there is permanent damage of components of the CIED, clinical manifestations may include sudden no-output of the pacemaker or runaway pacemaker. Manufacturers provide information regarding the amount of "scatter" radiation that is felt would be safe (ie, would not result in abnormalities of the implanted device). The acceptable dose of scatter radiation will not always be available, and the manufacturer would need to be contacted on a case by case basis. The following recommendations for the management of patients with pacemakers undergoing radiation therapy are adopted from published experience [11-14]: The patient's implantable device status should be evaluated by someone with pacemaker expertise prior to therapy. Before treatment, estimate and record the dose (from scatter) to be received by the CIED with the assistance of the radiation oncologist. Subsequent management is to be determined by patient characteristics, "beam type," beam energy and type of CIED. A professional society consensus document on the management of CIED patients when exposed to therapeutic radiation extensively details the steps to be taken in a patient with a CIED who will receive therapeutic radiation ( algorithm 1) [15]. External cardioversion/defibrillation and electrocautery The mechanism by which electrocautery, cardioversion, and defibrillation, all of which deliver a large amount of energy to the body, can damage the implanted pacing system is virtually identical. If a voltage surge is inadvertently delivered to the pulse generator, which can occur even when all appropriate clinical steps have been taken, there is potential to temporarily or permanently alter device function. Manufacturers have incorporated a variety of circuits designed to protect the complex electronics of the pacemaker from a large voltage surge. These circuits shunt the energy acquired by the housing of the pulse generator to the lead, which results in a large amount of energy being delivered to the heart via the electrode. Such a high level of energy concentrated at the electrode-myocardial interface can induce endocardial burns, causing an elevation of the capture and sensing threshold, which may be transient or permanent. https://www.uptodate.com/contents/pacing-system-malfunction-evaluation-and-management/print 11/25 7/6/23, 11:01 AM Pacing system malfunction: Evaluation and management - UpToDate In addition, sustained electrical signals associated with electrocautery may induce ventricular fibrillation in the electrically unstable patient; this occurs in a manner analogous to the intentional induction of ventricular fibrillation for defibrillation threshold testing at the time of implantable cardioverter-defibrillator (ICD) insertion. Lower doses of electrical current that may not cause damage to the pacing system may nonetheless cause transient pacemaker malfunction. The pacemaker may sense the externally applied electrical field, causing inappropriate inhibition of pacing, or in a DDD pacemaker, the electrical field could be sensed on the atrial sensing circuit with "tracking" of the noise by the ventricular channel to result in inappropriately rapid ventricular pacing. When external cardioversion or defibrillation is performed, the paddles should be as far from the pulse generator as possible without compromising the efficacy of the procedure. Where feasible, the current path should be perpendicular to the plane of the pacing system; this would usually mean using anterior-posterior positioning. With appropriate positioning of the paddle or pads, cardioversion or defibrillation can be safely performed with minimal risk to the implanted device in most patients. This was illustrated in a series of 44 patients with implanted devices, including pacemakers and ICDs [16]. Patients were randomly assigned to either monophasic or biphasic shocks. Cardioversion paddles were placed in an anterior-posterior position, at least 8 cm from the device. Serial device interrogations demonstrated small changes in pacing impedances and ventricular sensing immediately after cardioversion that returned to baseline within one week. There were no differences between the effects of monophasic or biphasic shocks. When the patient undergoes surgery requiring electrocautery, the use of bipolar cautery will minimize the electrical field affecting the pacing system. At a minimum, every effort should be made to assure that the current path for the electrocautery will not encompass the pacing system. A full discussion regarding the perioperative management of patients with a permanent pacemaker, including optimal monitoring and pacemaker programming, is presented separately. (See "Perioperative management of patients with a pacemaker or implantable cardioverter-defibrillator".) Magnetic resonance imaging There has been significant debate about when patients with a CIED should be allowed to undergo an MRI study. The risks of scanning patients with permanent pacemakers or ICDs are related to programming changes, asynchronous pacing, activation of tachyarrhythmia therapies, inhibition of pacing output, and induced currents in lead wires leading to heating and/or cardiac stimulation. All aspects of MRI imaging in the CIED patient is https://www.uptodate.com/contents/pacing-system-malfunction-evaluation-and-management/print 12/25 7/6/23, 11:01 AM Pacing system malfunction: Evaluation and management - UpToDate detailed in a professional society consensus document [15]. This subject is discussed in detail elsewhere. Computed tomography Although most patients with implanted cardiac devices can undergo computed tomography (CT) scans without any adverse consequences, direct exposure to high radiograph dose rates during CT examinations can potentially cause transient changes in pacemaker output pulse rate [17]. However, at this time, the evidence for device interference by CT is very limited and should rarely, if ever, limit device patients from undergoing standard CT imaging. Exceptions to this are detailed in a professional society consensus document [15]. Extracorporeal shock wave lithotripsy Extracorporeal shock wave lithotripsy is occasionally performed as a treatment for urinary tract calculi and cholelithiasis [18]. An older concern was that in patients with pacemakers or ICDs that incorporate a piezoelectric crystal for rate- adaptive pacing, the piezoelectric crystal can be shattered by the shock wave. Therefore, if such a sensor is incorporated in a device placed in the abdomen, extracorporeal shock wave lithotripsy is probably best avoided. However, there are now very few devices that incorporate a piezoelectric crystal. There have also been concerns that extracorporeal shock wave lithotripsy may cause significant mechanical forces that can shatter piezoelectric elements, circuitry, or lead connections but this is less well established. For patients who have any significant degree of pacemaker dependence, pacing mode should be reprogrammed to VOO or DOO because of the potential for inhibition of the ventricular pacing circuit due to interference from extracorporeal shock wave lithotripsy. However, in patients with an ICD, one study found that lithotripsy can be performed safely in those with tiered-therapy ICDs; however, it is recommended that antitachycardia therapies be turned "off" and the patient continuously monitored electrocardiographically during the procedure. Following the procedure the device should be interrogated, antitachycardia therapies turned back "on" and confirmation that final programming matches the programmed values prior to the procedure [19]. (See "Kidney stones in adults: Surgical management of kidney and ureteral stones".) PACING STIMULI ABSENT The absence of pacing stimuli is certainly appropriate when there is an intact native rhythm which inhibits pacemaker output. However, in the patient who has a pacemaker, one should usually not observe an absence of pacing stimuli with pauses in the rhythm that are longer than the programmed base rate of the pacemaker. If these pauses occur in the presence of pacing https://www.uptodate.com/contents/pacing-system-malfunction-evaluation-and-management/print 13/25 7/6/23, 11:01 AM Pacing system malfunction: Evaluation and management - UpToDate stimuli that do not capture, this is a noncapture problem. If there are no visible pacing stimuli, then one of several possible system malfunctions have occurred: The malfunction may arise from oversensing by the pacemaker of an inappropriate signal that is not visible on the surface ECG. Failure to deliver a pacing stimuli at the appropriate time may result from an open circuit, which may be due to inadequate lead fixation to pacemaker generator at the time of implantation (loose set screw) or lead fracture. A component in the pulse generator may have failed. This is exceedingly rare, and other possibilities should be considered first. Hysteresis and ventricular pacing avoidance algorithms may result in intervals longer than the programmed base rate. Confusion may arise if it is not recognized that one of these features is programmed "on." Applying a magnet over the pacemaker will usually allow one to quickly differentiate an oversensing problem from an open circuit or pulse generator component malfunction: If pacing resumes with a stimulus being present, whether or not there is intact capture, the etiology of the absent pulses is likely oversensing. In unipolar pacing systems, particularly when programmed to a very sensitive setting, oversensing of myopotentials may be more likely to occur. If magnet application fails to eliminate the pauses, the problem is not one of oversensing, but is either a component failure within the pulse generator, an open circuit, or, in the setting of a bipolar coaxial lead, an internal insulation failure. Oversensing Historically, oversensing has been felt to be somewhat more common in the unipolar sensing configuration because the antenna for signal detection is much larger, extending from the housing of the pulse generator to the electrode located within the heart. Bipolar systems, in which both active electrodes are inside the heart, have a much smaller antenna effect and a better signal/noise ratio with respect to electrical noise originating from outside the heart. As a result, bipolar systems can often be programmed to very sensitive settings without encountering oversensing. Internal insulation failure in bipolar coaxial leads has produced repeated make-break contacts between the two conductor coils. Large nonphysiologic electrical transients result, which can be seen by the pacing system and inhibit it, but are not visible on the surface ECG. When this https://www.uptodate.com/contents/pacing-system-malfunction-evaluation-and-management/print 14/25 7/6/23, 11:01 AM Pacing system malfunction: Evaluation and management - UpToDate occurs, reducing the sensitivity will probably not correct the system malfunction, and management will require replacement of the malfunctioning pacing lead. Management Oversensing can usually be corrected by reducing the pacemaker sensitivity (ie, programming the pacemaker to a higher sensitivity number that defines the smallest millivolt amplitude of the signal that the pacemaker can sense). Reducing the sensitivity predisposes to episodes of undersensing if the amplitude of the intrinsic signal is too small, a