Continuing Education Activity

Pacemakers are implanted in patients with rhythmic cardiac problems. These pacemakers can often malfunction and produce a set of symptoms that require timely assessment and rectification. This activity outlines different ways a pacemaker can malfunction and highlights the role of the interprofessional team in managing the patient with pacemaker malfunction.

Objectives:

• Describe the basic functioning of a pacemaker.
• Outline different ways in which a pacemaker typically malfunctions.
• Review the underlying mechanisms of malfunction of the pacemaker.
• Summarize the importance of the interprofessional team in the management of the patient with pacemaker malfunction and the preoperative assessment of patients with pacemakers.

Introduction

The human heart is a pivotal organ in the circulatory system, and it beats more than 2 billion times during normal life. This functioning of the heart depends on the cardiac conduction system, which includes impulse generators (e.g., sino-atrial node) and the impulse propagating (His-Purkinje) system. [1] The sinoatrial node acts as the natural pacemaker of the heart. The cells present in the sinus node have innate automaticity, which starts the electrical activity in the heart. This innate electrical potential moves from the sinoatrial node to the atrioventricular node and finally into the His-Purkinje system.[2] This movement of electric potential in an orderly manner controls the rhythmic contraction of the heart's chambers. The failure of this intrinsic electrical conduction in the heart can result in different arrhythmic problems. Several diseases and conditions affect the conduction system by involving impulse generation, impulse propagation, or both. Acquired conditions such as myocardial infarction, age-related degeneration, procedural complications, and drug toxicity are the major causes of the native conduction system malfunction.[3] 

The current standard of care for symptomatic bradyarrhythmias due to conduction system diseases is the implantation of a cardiac implantable electronic device.[4] These pacing devices provide an external electrical stimulus that leads to depolarization of myocytes and helps maintain the electrical excitability of the heart tissue. This process leads to excitation-contraction coupling resulting in the contraction of myocardial tissue.[5] 

Despite their success, electronic pacemakers have limitations, including complications related to implantation, limited battery life, the potential for infection, lack of physiologic autonomic responsiveness, and size restriction in younger patients.[6] The periodic evaluation of an implanted pacemaker is necessary to optimize programming and to identify correctable problems. This review will discuss the common pacing system problems of a cardiac implantable electronic device (pacemaker).

Function

Pacemakers consist of two main components: a pulse generator and the leads. Pulse generator houses the battery and other electronics which control the modes of the pacemaker. Pacemaker leads conduct the depolarizing potential to the myocardium. The sensing of the innate activity of the heart is also a function of the leads.[7]

Pacemaker Timing Cycle

A pacemaker has two primary functions, pacing (an electrical stimulus for myocardial depolarization) and sensing (detecting intrinsic electrical activity and wave of depolarization). Different timing cycles are programmed in a pacemaker for its functioning.[8]

• Lower Rate Limit: Base rate or slowest rate that the pacemaker will allow the patient's heart to go. It is programmable.
• Upper Rate Limit: The maximum rate at which a pacemaker can pace in the absence of intrinsic activity.
• Maximum Tracking Rate (MTR): Maximum atrial rate at which a dual-chamber pacemaker tracks the sensed atrial activity and paces the ventricle.
• Sensed AV Delay: It is a time duration after sensed atrial activity at which the pacemaker will pace the ventricle if it does not sense the intrinsic activity.
• Paced AV Delay: It is a time duration after atrial pacing at which the pacemaker will pace the ventricle if it does not sense the intrinsic activity.
• Post Ventricular Atrial Refractory Period (PVARP): It is a time duration after the ventricular event when a pacemaker does not react to a sensed atrial activity. It sees the activity but does not reset the timing cycle.
• Post Ventricular Atrial Blanking (PVAB): This is the time duration after the ventricular event when a pacemaker does not sense any atrial event. Pacemaker becomes blind for atrial events.
• Total Atrial Refractory Period (TARP): The sum of AV delay and post-ventricular atrial refractory period.

Issues of Concern

Pacemakers are electronic devices programmed to pace (deliver the depolarizing current) the specified cardiac chamber and sense the intrinsic cardiac activity in the respected chamber.[9] So, the patients with pacemakers generally face problems related to either sensing or pacing, and these problems can be grouped into the following categories.[10]

• Problems related to pacing.

• Output failure
• Failure to capture

• Problem related to sensing

• Under sensing
• Over sensing

• Pseudo-malfunction

• Safety pacing
• Pacemaker-mediated tachycardia
• Upper rate behavior
• Runaway pacemaker

Problems Related to Pacing

Output Failure

It is defined as the inability of the pacemaker to generate an impulse resulting in a heart rate lower than the programmed lower rate limit. It is characterized by the absence of a pacing spike on an electrocardiogram, and device interrogation confirms the diagnosis. (Figure.1) Causes of output failure include lead fracture,  generator failure, and inhibition of pacing due to over-sensing and crosstalk.[11]

Failure to Capture

Failure to capture is defined as the inability of pacing impulse to produce an evoked potential. It is characterized by a pacing spike on the surface electrocardiogram at programmed heart rate, which is not followed by an evoked potential (P or a QRS). (Figure.2) Causes of failure to capture include lead dislodgment and elevated thresholds due to fibrosis or exit block at the site of lead implantation.[12] Acidosis and hyperkalemia may also lead to capture failure.[13]

These pacing problems could lead to life-threatening bradyarrhythmias as well as asystole in pacemaker-dependent patients.

Problems Related to Sensing

Under-sensing

Under-sensing is defined by a failure of the pacemaker to see the spontaneous intrinsic activity, which results in asynchronous pacing. On a surface ECG, it is characterized by pacing spikes regardless of P waves or QRS complex. (Figure.3) The main causes of under-sensing include an improperly programmed sensing threshold (high sensing threshold), insufficient myocardial voltage signal,  lead displacement, or pacemaker failure.[14]

Over-sensing

Over-sensing happens when the pacemaker detects an electrical signal which is not expected to be sensed. Over-sensing results in an inappropriate inhibition of the pacing stimulus leading to potentially life-threatening consequences. In addition to the native cardiac depolarization signals (P or R waves), any electrical activity with sufficient amplitude can be sensed by a pacemaker, inhibiting the pacing when required. Over-sensing can be caused either by a physiologic signal like T waves or by a non-physiologic signal like electromagnetic interference or a lead failure (an insulation break or a lead fracture.[15][16] Over-sensing is characterized by fewer pacing spikes than expected on a surface electrocardiogram. (Figure.4)

Sensing plays a major role in pacemakers, in patients with implantable cardioverter defibrillators (ICDs), sensing problems lead to inappropriate shocks.

Pseudomalformations

Fusion and Pseudofusion

Fusion is an electrical summation of an intrinsic beat and a depolarization from a pacing stimulus. The hallmark of the fusion phenomenon is that its morphology lies between a fully paced beat and an intrinsic beat. (Figure.5)

Pseudofusion occurs when the pacemaker spikes coincide with an intrinsic; however, it does not contribute to the actual depolarization.  It is characterized by a morphology similar to an intrinsic beat. Fusion and pseudo-fusion beats are considered normal pacemaker behavior.[17]

Pacemaker Crosstalk

Pacemaker crosstalk is a feature of a dual-chamber pacemaker, characterized by detecting a paced signal in one chamber by the lead in another chamber and by the misrepresentation of the paced signal as a cardiac depolarization signal. This, in turn, results in inappropriate inhibition of pacing in the 2nd chamber.[18]

Ventricular Safety Pacing

During ventricular safety pacing, the pacemaker delivers a ventricular pacing stimulus after detecting a ventricular sensed event shortly after an atrial paced event. It is typical characterized by the appearance of two very closely spaced atrial and ventricular paced events on ECG. Safety pacing (SP) algorithms differ among pacemaker manufacturers.[19]

Pacemaker Mediated Tachycardia

Pacemaker-mediated tachycardia is a feature dual-chamber pacemaker with tracking mode (DDD, VDD). It is also called an endless-loop tachycardia characterized by atrial sensing followed by ventricular pacing at an upper tracking rate.[20] Pacemaker-mediated tachycardia requires the presence of retrograde (ventriculoatrial) conduction and a triggering event like premature ventricular contraction or loss of AV synchrony. A retrograde P wave produced by a premature ventricular complex is sensed by a pacemaker when it falls beyond the PVARP. This sensed atrial activity triggers AV delay, and the ventricle is paced at the end of programmed AV delay. This paced event again conducts retrograde and sensed as an atrial activity and triggers an AV delay again. (Figure.6) This endless loop tachycardia continues similar to a re-ent

rant tachycardia, except that the pacemaker forms part of the re-entrant circuit.[21] Pacemaker-mediated tachycardia could therefore be avoided by programming a sufficiently long post ventricular atrial refractory period (PVARP). Placing a magnet on the device during the PMT will change the pacemaker's mode to asynchronous dual-chamber pacing mode (in DOO, intrinsic P waves and R waves are ignored), which results in the termination of tachycardia by suspending the pacemaker's sensing function. This tachycardia is rare in the contemporary era due to advanced PMT algorithms programmed in the newer pacemakers.[22][23]

Upper Rate Behavior

Upper-rate behavior is also a feature of dual-chamber pacemakers with atrial tracking mode. In dual-chambered pacemakers, it is necessary to limit the atrial rate at which the device paces the ventricle. This limit is called the maximum tracking rate (MTR), and it is a programmable value. Upper rate behavior occurs when the atrial rate increases and approaches the maximum tracking rate.[24] When the atrial rate exceeds MTR, it results in pacemaker Wenckebach. If the atrial rate keeps increasing and exceeds the TARP, it will result in a pacemaker 2:1 AV block.

Runaway Pacemaker

Runaway pacemaker is a rare, life-threatening phenomenon caused by generator dysfunction, usually related to pacemaker battery depletion.[25] Other than the runaway phenomenon, the low battery voltage can provoke low rate stimuli, capture and sensing failures, and mode changes. Runaway pacemaker typically shows an ECG with captured beats alternating with non-captured high rate spikes. (Figure.7)

Clinical Significance

Advances in pacing device technology have led to its widespread use in treating patients with bradyarrhythmia and tachyarrhythmias.[26] It is imperative to have a comprehensive knowledge of normal pacemaker function to understand the pacemaker malfunction. Pacemaker malfunction can lead to potentially life-threatening situations, including syncope and even cardiac arrest.[27] Regular follow-up and programming of pacing devices are required for the basic understanding of their function, troubleshooting, and management of pacemaker malfunction.  Comprehensive knowledge of pacemaker function and its management at the time of surgery or imaging enhances patient care.[28]

Other Issues

The topic of magnetic resonance imaging (MRI) in patients with cardiac implantable electronic devices (CIED) is still debatable. More than three-fourths of the patients require MRI at some point in time after implanting a pacing device.[29] The list of anticipated risks with MRI includes aberrant changes in the pacing output, changes in the programmed mode, and generation of current in the lead wires leading to heat-induced thermal damage at contact points and causing unintended cardiac stimulation.[30] A research study in patients with non-MRI-conditional devices concluded that there was no failure of the device or lead in these patients when undergoing non-thoracic MRI of approximately 1.5 teslas.[31]

MRI-conditional pacemakers are better able to handle the interference due to magnetic resonance imaging. The term MRI-conditional refers to devices with no known hazards or risks under specific magnetic resonance conditions. Notably, there are no “MRI-safe” devices, which are devices that have no known hazards or risks under all conditions. MRI-conditional devices have minimal ferromagnetic material, altered filtering, as well as specially designed lead conductors, which minimize current induction and heating of the tissue. MRI conditional leads are also required for a device to be labeled as MRI conditional.[32]

CT scan of the patient does not usually cause problems in the pacemakers. Therefore, the presence of an implanted pacemaker should not hinder such investigative imaging modality. In rare cases, it might lead to transient changes in the output of the pacemaker.[33]

Some CIEDs make use of piezoelectric crystal components in the circuitry or lead connections. Extracorporeal shock wave lithotripsy can damage such components due to its effect on those components leading to device malfunction.[34]

Therapeutic radiation can produce undesirable outcomes in patients with pacemakers. These outcomes include reprogramming resulting in aberrant behavior, resetting the device, or permanent malfunction due to damage to the semiconductor insulation. There will be precipitous output failure of the pacemaker in case of permanent damage to its components.[35] The volume of “scatter radiation” deemed safe for an implanted pacemaker is often provided by the manufacturer. However, in case of lack of that information, contacting the manufacturer for that information is the best next step.[36]

Enhancing Healthcare Team Outcomes

When planning therapeutic radiation for a patient with an implanted pacemaker, the status of the device requires monitoring by a healthcare provider who specializes in monitoring the pacemakers. Moreover, the radiation oncologist should assess the radiation dose to be received by the device in that particular case.[37]

If the malfunction of the device is due to lead dislodgement, the management depends on the timing of the event related to the timing of the implantation of the device, the severity of the dysfunction, the clinical situation of the patient,  and the location of the displaced lead (atrial or ventricular). Reopening and reinstating the lead is a good management option in early displacements as the chances of fixation of the lead by the fibrous endocardial reaction are very low. Thus, allowing the manipulation of the lead.[38]

On the other hand, if the displacement of the lead is late, lead manipulation might not be an option. In such cases, introducing the lead in the heart chamber where the displacement has occurred is a good management plan if lead extraction is not possible. This new lead cancels the effect of previously displaced lead in that chamber.[39]

The best treatment of pacemaker dysfunction involves an interprofessional team of primary care clinicians, emergency medicine clinicians, cardiologists, cardiac surgeons, and cardiac nurses. Each of these disciplines needs to understand the function of pacemakers, be able to identify potential issues with pacemaker function, and engage in open information sharing with other team members to preclude adverse events and improve patient outcomes in those patients with pacemakers. [Level 5]

Different problems can arise during anesthesia, surgery, or ICU management of patients with cardiac implantable electrical devices (CIED). These problems include ventricular tachyarrhythmias, asystole, hypotension, and bradycardia.[40] Proper preoperative management is crucial to avoid such undesirable outcomes. The first step is to identify patients with CIED. The identification of such patients is followed by the clinical assessment, analysis of the functioning of the device. Hospitals should have pacemaker clinics or trained electrophysiology specialists who can properly assess the pacemakers before the surgery. In most cases, the modes of the pacemaker will require changing for undergoing surgical procedures.[11]