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. This movement of electric potential in an orderly manner controls the rhythmic contraction of the chambers of the heart. The failure of this intrinsic electrical conduction in the heart can result in different arrhythmic problems. However, in this situation, an external electrical stimulus can lead to depolarization of myocytes due to the electrical excitability of the heart tissue. This process leads to excitation-contraction coupling resulting in the contraction of myocardial tissue. The external electrical stimuli that initiate the cascade of events, in this case, is provided by an external pacemaker.[1]

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. And the leads conduct the depolarizing potential to the myocardium. The sensing of the innate activity of the heart is also a function of the leads.[1]

Patients who will benefit the most from the cardiac pacemakers often present with the symptoms related to bradyarrhythmias. Decreased tolerance to exercise, syncope, easy fatiguability, and lightheadedness are a few symptoms that might be present. Dysfunction of the atrioventricular node or sinoatrial node is the underlying pathology in most of these cases. Building a solid causal relationship between the symptoms and bradyarrhythmias requires intricate history taking and thorough examination with support of electrocardiographic findings. The stronger the causal relationship, the stronger will be the benefit of cardiac pacemaking.[2]

Even though pacemakers are a reliable piece of technology, they can malfunction, resulting in adverse outcomes. Pacemakers can have problems with pacing the intended heart chamber. These kinds of malfunctions further divide into an output failure or a capture failure. 

In the case of output failure of a pacemaker, the heart rate falls below the programmed lower cut off of the pacemaker. Moreover, the lack of pacing spikes on the electrocardiogram depicts the absence of pacer electrical output. Crosstalk inhibition, oversensing, failure of the battery, lead, or pulse generator are a few causes that can lead to output failure[3]. Malfunction due to the battery can occur with its depletion. The pacemakers usually show an “elective replacement indicator,” which points to 90 days of functional battery remaining. The “end of life” indication represents the depletion of the battery to the point that its function is now unreliable.[4]

Lead displacement is one of the most common occurrences with pacemakers that can cause its malfunction. If the dislodgement of the lead occurs within six weeks of implantation, it is known as early displacement. And if it happens after six weeks, it is delayed displacement.[5]

Capture failure occurs when the pacing stimulus is present, but it fails to produce depolarization in the myocardium. The electrocardiogram will show the presence of pacing spikes, but there will be no QRS complex or P wave following these spikes. Fibrosis of the tissue in contact with the leads, dislodgement of the leads, low output of the pacemaker generator, or the maturation of lead are a few underlying mechanisms that can lead to capture failure of a pacemaker.[6]

Other than failure to pace the intended part of the heart, pacemakers can malfunction when they are unable to detect innate signals produced in the heart. These problems can be either undersensing or oversensing by the pacemaker. Undersensing can cause asynchronous pacing of the heart when the pacemaker fails to detect spontaneous depolarization in the myocardium. The pacing spikes of the atria or ventricles are generated without any coordination with the P waves or the QRS complex. The programming problems associated with the pacemakers, failure of lead or the pulse generator, the low voltage signal of the myocardium, any derangements in the electrolyte levels, etc. can lead to undersensing.

The process of oversensing occurs when the pacemaker detects the electric signals; it should not sense. This oversensing results in the pacemaker inhibiting the pacing stimulus. Pacemakers usually sense the signals of cardiac depolarization like P or R waves, but a repetitive signal of sufficient amplitude can be detected by the pacemakers, which results in inhibition of the pacing when this action is not required. The interference due to electromagnetic waves, failure of lead, or even T waves can cause oversensing in a pacemaker.[7]

The topic of thoracic MRI in patients with cardiac implanted electronic devices (CIED) is a debatable one. The list of anticipated risks includes aberrant changes in the pacing output, changes in the programmed mode or current might be generated in the lead wires leading to heat-induced thermal damage at contact points or causing unintended cardiac stimulation[8]. MRI-conditional pacemakers are better able to handle the interference due to magnetic resonance imaging. 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 tesla.[9]

CT scan of the patient does not usually cause any 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.[10]

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.[11]

Therapeutic radiation can produce undesirable outcomes in patients with pacemakers. These outcomes include reprogramming resulting in aberrant behavior, resetting of the device, or permanent malfunction due to damage to the semiconductor insulation. There will be precipitous no-output of the pacemaker in case of permanent damage to its components.[12] The volume of “scatter radiation” deemed to be 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.[13]