How to: a practical guide to cardiac conduction devices on chest radiograph

Abstract With the increasing number of cardiac conduction devices (CCDs) insertions with various complexities in recent decades, it is crucial for clinicians (especially internal medicine residents and cardiologists) to have an up-to-date review of the current devices on chest radiograph. Chest X-ray remains the most cost-effective and accessible imaging modality to assess the device position and its associated complications, not only immediately after insertion but also during the follow-up visit as outpatient. Various types of CCDs such as permanent pacemaker, implantable cardioverter defibrillator, and cardiac resynchronization therapy (CRT, CRT with defibrillation or pacing) with their appearances on chest radiograph and possible complications with a step-by-step guide to how to assess are discussed in this article.


Introduction
There are increasing numbers of cardiac conduction devices (CCDs) insertions with rising complexity for a broad range of indications in modern cardiology.The British Heart Society estimates that 555 pacemakers were implanted per million people (pmp) in UK in 2016, including 182 pmp of high-energy devices [implantable cardioverter defibrillator (ICD) and cardiac resynchronization therapy-defibrillator (CRT-D)]. 1Chest X-ray remains one of the most utilized tools for assessing CCDs, both acutely after implantation and on follow-up.Pacemakers, whilst felt to be a safe procedure, have a significant complication rate.Implantable cardiac defibrillators have an even higher rate of complications associated with acute insertion and chronic placement. 2,3The aim of this 'how to' article is to provide clinicians with an up-to-date review of the X-ray appearance of current devices and their associated complications.

Anatomy of a conventional CCD
Classically, CCDs could be divided into three main components: the pulse generator, the leads, and the electrodes.Recently, we have also seen the introduction of wireless pacing systems (also known as leadless pacemakers) enters clinical practice, which are equally important for the clinicians to be able to identify. 4

Pulse generator
Pulse generator contains an electrical circuitry with a small computer and a battery.It is designed to detect the heart's intrinsic rhythm and send an electrical impulse to the heart, if necessary, within its programmed settings.In ICD or CRT devices, they may not only be able to store data about significant events of cardiac arrhythmia.Pulse generators are typically inserted infra-clavicularly on the left or right side in a subcutaneous pocket but can also be placed in the axilla, sub mammary, or under the pectoralis major muscle.

Leads and electrodes
Pacemaker lead is an insulated wire which connects an electrode to a generator.In contrast to this, the electrode is the uninsulated termination of the lead which is connected to the heart.Defibrillator electrodes can be identified on chest radiograph as having coils compared with usual electrodes.Electrodes can be screwed into the myocardium by active fixation or passively fixated by fibrosis.
Typically, there will be a single working lead in the ventricle and/or atrium.Multiple leads may be seen in the same chamber as old/damaged leads are often not removed due to high risks of complications associated with the removal procedure.It is vital to identify the number of transvenous leads as it can increase the patient's risk of superior vena cava obstruction and importantly complicate future attempts at endocardial access.In patients who have undergone removal of their pacing system, an X-ray can allow to identify any lead remnants, which is imperative to be aware of when considering a magnetic resonance imaging (MRI).Any redundant lead fragment is a contraindication to an MRI scan (see Figure 1).
Single chamber pacemaker has one lead, which can be inserted into either the right atrium or right ventricle (RV) (see Figure 2).
A dual-chamber pacemaker has two leads, which are connected to right atrium and RV with electrodes (see Figure 3).When assessing the positions of atrial and ventricular leads, both a frontal and lateral chest X-ray (CXR) views are recommended.
The right atrium (RA) lead is usually inserted in the RA appendage but can be positioned in atrial free wall, high, and lower atrial septum.RA lead follows a slight medical course on a posterior-anterior (PA) radiograph whilst it is anterior subtending an angle < 90 and forming a 'J' on a lateral view.
RV lead is typically inserted in the apex i.e. the tip is pointing towards the cardiac apex and should be to the left of the spine on PA CXR and the lead curves along the course of the right atrial lateral wall, passing the tricuspid valve and reaching the apex, pointing anteriorly and slightly superiorly (or inferiorly) on lateral CXR.The other placement sites of RV lead may be in RV outflow tract or mid-septum.
CRT devices can be identified on chest X-ray by a left ventricular (LV) lead.This lead is sited via the coronary sinus and fed into an epicardial venous system.The LV lead is commonly placed at the lateral or postero-lateral wall of the LV (see Figure 5).

Device types Pacemaker
Generally, pacemakers are inserted into the endocardium, but they can also be inserted epicardially.Endocardial pacemakers are inserted via the venous system typically by subclavian or cephalic approach, but the internal veins may also be used.Epicardial systems can be inserted via a supradiaphragmatic abdominal incision or by thoracotomy.The generator for a pacemaker can be inserted subcutaneously or below the pectoralis muscle.
Wireless models have emerged recently, which can provide backup ventricular pacing.The pacemaker is about the size of a large vitamin, and it avoids the need for any wires to be inserted.It is inserted via the femoral venous system into the RV.A lateral radiograph is still essential when assessing this system as wireless pacemakers can easily be mistaken for intermittent loop recorders (see Figure 4).

Cardiac resynchronization therapy device
These devices are typically inserted for patients with significant heart failure on maximal medical therapy.They deliver electrical energy to both RV and LVs to improve ventricular co-ordination.They are identified by the presence of leads in both the RV and LV.They may have pacing electrode in the case of CRT-pacing or a shock coil in CRT-D (see Figure 5).
Newer devices such as wireless cardiac stimulation (WiCS) devices are being inserted in specialist centres (see Figure 6).The LV lead is replaced by a wireless endocardial LV electrode which communicates with a pulse generator that delivers ultrasound energy to co-ordinate pacing which is placed subcutaneously in the left fourth to sixth  intercostal space to achieve an optimal acoustic window.The LV endocardial device is inserted retrogradely via the aortic valve.These devices are considered when traditional CRT lead placement is not possible or does not provide adequate resynchronization.

High-energy devices
High-energy devices are a group of devices encompassing both ICD and CRT-D.They are designed to deliver a large amount of electrical energy to the heart in response to a tachyarrhythmia.They may be inserted for the patients who have previously had a life-threatening tachyarrhythmia or are deemed to be at high risk of one.High-energy devices can be identified on chest radiograph by the presence of coiled electrodes (see Figure 5).The generator is activated based on pre-determined algorithms to deliver energy via a certain vector which involves the coil on the lead.There can be a single or dual coils.The leads can be implanted into the endocardium, epicardium, or subcutaneously; however, epicardial and subcutaneous ICDs are not capable of pacing.

Complications of CCDs
Complications of the implantation of CCDs can be generally divided into acute (occurs at the time of device insertion) or chronic (occurs

Figure 4 ILR (white arrow) at left sternal edge inserted superior to leadless pacemaker (black arrow) on frontal view (A). ILR is positioned subcutaneously and leadless pacemaker in RV on lateral view (B).
How to: a practical guide to cardiac conduction devices on chest radiograph on follow-up).Leads-related complications appear to be more prevalent than those associated with generator.The implantation of pacemakers can also damage surrounding structures leading to myocardial perforation, pneumothorax, etc. 3

Perforation
Myocardial perforation occurs more commonly after ICD placement than pacemaker insertion.Two-viewed CXR offers a useful screening tool for myocardial perforation (see Figure 7), but computed tomography scan remains a more sensitive and specific imaging modality.

Pneumothorax
Pneumothorax usually happens during the subclavian puncture phase of the procedure.This is usually detected on chest X-ray immediately after CCD insertion (see Figure 8).It can occur with or without pneumomediastinum and surgical emphysema.

Lead fracture
Lead fracture is one of the most common chronic pacemaker complications.Lead fracture may lead to a defibrillator delivering inappropriate shocks or failure of a pacemaker to sense.This is likely related to the  mechanical stress that pacemaker leads undergo in their lifetime.Common sites of lead stress are at stationary points of the lead such as the subclavian vein-first rib junction and at the site of excessively tight fixation sutures.

Lead dislodgement and inappropriate lead placement
Atrial lead dislodgement has higher prevalence compared with ventricular one.A lateral view is essential to provide 3D when it is required to describe lead location accurately (see Figure 9). 5,6

Twiddler syndrome
Twiddler syndrome is a rare complication caused by conscious or unconscious manipulation at the implantation site by the patient resulting in lead dislodgement.It can cause an extreme lead dislodgement, diaphragmatic stimulation, and loss of capture. 7 Step-by-step guide on how to assess chest radiograph after device implantation a) Immediate after device implantation • Look for immediate post-procedural complications, e.g.pneumothorax, haemothorax, or myocardial perforation (where ventricular lead positioned in abnormal location), lead dislodgement, and air/fluid in the pericardium.• Differentiate between a pacemaker and an ICD.
• Ensure proximal end of electrode, which has an insertion port that attaches the lead to the generator, is beyond the pins in the generator.• Check for lead damage by tracing the entire course-normally the lead should follow a linear pathway without any loops or breakage.• PA and lateral chest X-rays are required to evaluate the lead position.
• Evaluate the correct position of pacemaker casing inside the pocket and look for complications such as haematoma, infection, or presence of airfluid level.• Pacemaker check should be done after each new device implantation.
• Clinical correlation with any X-ray findings or device check result is a key.

Reporting on a chest X-ray should include:
• the number of leads present; • are the leads all connected to the generator; • where is the generator (left/right infraclavicular, mid-axillary, abdominal generator, inframammary); • where are the leads inserted in the myocardium (RA/RV/CS/LV; appendage/lateral wall/apex/septum/extracardiac); • are they in the same position as previous imaging; • dose the lead have a coil (1 or 2) and if present this is likely a defibrillator; • are there any other cardiac devices present; • are there any complications evident such as pneumothorax, perforation, haemothorax, cardiac enlargement; • does the lead look intact, or any obvious fracture/uncoiling of the lead.

Conclusion
Chest radiograph offers the simple and cost-effective way to assess the position and possible complications after insertion of CCDs.This article will help the clinicians who deal with the patients with CCDs especially cardiologists, internal medicine residents, and emergency medicine physicians to be familiarized with the common appearances of CCDs and their complications on chest radiograph for early detection of potential complications not only for those patients who have immediate pacemaker insertion but also who present with new cardiac symptom and previous history of CCD insertion, to reduce potential morbidity and even the mortality.

Figure 1
Figure 1 Radio-opaque pacemaker electrode remnant is seen intravascularly in the left subclavian region post-device removal (white arrow).

Figure 2
Figure 2 Right-sided single chamber pacemaker with lead positioned in RV, apex (black arrow).

Figure 3
Figure 3Left-sided infraclavicular dual-chamber pacemaker with implantable loop recorder (ILR) (A).ILR is subcutaneously situated with atrial lead makes J-loop in atrial appendage, while RV lead pointing towards apex on lateral view (B).White arrow indicates ILR, black arrow, atrial lead, and red arrow, ventricular lead.

Figure 5
Figure 5 Left-sided CRT-D with coiled shock lead (red arrow) at left sternal edge (A), atrial lead (black arrow) makes J loop in atrial appendage and ventricular lead (red arrow) positioned in posterolateral wall of LV on lateral view (B).

Figure 6
Figure 6 Frontal view of WiCS device (A): electrode in endocardial LV (white arrow) with co-implant left-sided pacemaker (red arrow, in this case, a dual coil implantable defibrillator) and transmitter positioned just lateral to LV apex (black arrow) and a battery subcutaneously (white star).Lateral view confirms electrode in LV lateral wall and battery positioned in mid-axillary line (B).

Figure 7
Figure 7 Left-sided dual chamber pacemaker with RV lead (black arrow) perforation (A).Post-RV lead repositioning in RV septum (B).

Figure 8
Figure 8 Left-sided dual chamber pacemaker (black arrow indicates atrial lead, and red arrow, ventricular lead), complicated by tension pneumothorax on left lung.
b) During follow-up, when there is new cardiac symptom or device malfunction is detected • Always compare with the previous CXRs, if available, to confirm the type of device inserted and any change in lead positions.• PA and lateral chest X-rays are required to evaluate the lead position.•If previous chest X-ray is not available, follow the above steps mentioned for immediate post-device implantation to assess the possible complications.• Possible longer term complications include lead perforation, lead dislodgement, lead fracture (at the tip of the electrode or the point of access to the axillary/subclavian as a result of clavicular crush), and insulation break relating to specific manufactured leads.• Correlation with patient history and device check is important.