How to Differentiate Between a Pacemaker and ICD on CXR

The single most reliable feature for telling a pacemaker from an implantable cardioverter-defibrillator (ICD) on a chest X-ray is the shock coil, a thick radiopaque spring visible along one of the leads inside the heart. A pacemaker lead has only thin pacing electrodes at its tip, while an ICD lead carries one or two chunky metallic coils designed to deliver high-energy shocks. Once you know what that coil looks like, identification becomes straightforward even at a glance, but a systematic approach covering the generator, the leads, and their trajectories catches the less obvious cases too.

Why the Shock Coil Settles the Question

Both pacemakers and ICDs route their leads through the venous system and into the right side of the heart, so the overall path of the wires on a frontal chest film can look similar. The difference is in the hardware on the lead itself. An ICD lead has at least one shock coil, a densely wound metallic spring that sits in the right ventricle, usually near the tip. Many ICD leads also have a second, more proximal coil positioned in or near the superior vena cava. On a well-penetrated PA chest X-ray, these coils appear as thick, bright, cylindrical segments a few centimeters long, clearly wider than the rest of the lead body. A standard pacemaker lead, by contrast, tapers to a small electrode tip without any such thickening. If you see a thick coil segment on any lead, the device is an ICD (or a defibrillator-capable variant like a CRT-D), not a simple pacemaker.

A dual-coil ICD lead is especially easy to spot because the two bright springs, one distal and one proximal, are separated by a gap of several centimeters along the lead. A single-coil ICD lead has only the distal coil in the right ventricle but still looks distinctly different from a pacemaker lead. The coils are radiopaque enough that they stand out even on a suboptimal portable film in an emergency department.

Generator Size and Location

The pulse generator, the metal can implanted under the skin, also offers clues, though it is less definitive on its own. ICD generators are generally larger and thicker than pacemaker generators because they house a high-voltage capacitor and battery big enough to deliver a shock. On X-ray, an ICD generator often appears as a wider, more rectangular or rounded box compared with the smaller, flatter profile of a pacemaker. That said, modern ICD generators have been shrinking, and some older pacemaker models were bulky, so generator size alone is not a reliable differentiator. Use it as supporting evidence, not as the deciding feature.

The generator is most commonly implanted in a subcutaneous pocket over the pectoralis muscle, typically on the left side. In pediatric patients, the generator is sometimes placed in the abdomen because the child’s chest wall is too small to accommodate it comfortably, and the leads may be routed epicardially rather than transvenously.¹ If you see a generator in the abdomen with leads sutured directly onto the heart surface, the patient likely had surgery in childhood or has venous access issues, and identifying whether the device is a pacemaker or ICD still comes down to whether those leads carry shock coils.

Counting and Tracing the Leads

The number of leads does not by itself tell you whether a device is a pacemaker or an ICD, because both come in single-chamber and dual-chamber configurations. A single-chamber pacemaker has one lead, usually in the right ventricle. A dual-chamber pacemaker has two leads, one in the right atrium and one in the right ventricle. An ICD can also be single- or dual-chamber, with the same lead positions. The distinguishing factor remains the shock coil on the ventricular lead, not the lead count.

Tracing each lead from the generator to its tip is still valuable, though, because it tells you the device’s pacing mode and can flag problems. On a frontal chest film, a right atrial lead typically curves into the right atrial appendage and points upward and slightly to the right. A right ventricular lead follows a path through the tricuspid valve and ends at the right ventricular apex, pointing downward and to the left. If the tip of a ventricular lead points too far leftward or posteriorly, it may have perforated through the ventricular wall or been inadvertently placed in the coronary sinus, both situations worth catching early.

Cardiac Resynchronization Therapy Devices

Cardiac resynchronization therapy (CRT) devices add another layer. These devices have a third lead that threads through the coronary sinus and sits in a vein along the lateral or posterolateral wall of the left ventricle. On a PA chest film, this left ventricular lead follows a distinctive path: instead of heading into the right ventricular chamber, it curves posteriorly and laterally around the heart, often visible overlying the left cardiac silhouette. The lateral view confirms it sitting posteriorly. The presence of leads in both the right ventricle and the left ventricle is the hallmark of a CRT device.²

The critical follow-up question is whether you are looking at a CRT-P (pacing only) or a CRT-D (pacing plus defibrillation). Both have three leads, and both have the characteristic left ventricular lead. The distinction, once again, comes down to the shock coil: a CRT-D has a shock coil on the right ventricular lead, while a CRT-P does not.² If you see three leads and a shock coil, it is a CRT-D. Three leads with no shock coil means CRT-P. This matters clinically because a CRT-D can deliver shocks for life-threatening arrhythmias, while a CRT-P cannot.

Subcutaneous ICDs Look Completely Different

The subcutaneous ICD (S-ICD) breaks all the rules above because it has no transvenous leads at all. Instead, a single electrode tunnels under the skin along the left sternal border, running from the generator (usually positioned over the left lateral ribcage near the mid-axillary line) up to a point near the manubrium. On a chest X-ray, the S-ICD looks strikingly different from a transvenous system: the generator sits lower and more laterally than a conventional pacemaker or ICD, and the lead runs parallel to the sternum rather than following the venous anatomy into the heart chambers. The lead has a shock coil, but it lies outside the heart entirely, visible as a thick segment running vertically along the left parasternal area.

Because the S-ICD electrode does not enter the heart, you will not see any lead tips inside the cardiac silhouette. If you spot a generator on the lateral chest wall with a lead tracking along the sternum and no intracardiac leads, you are looking at a subcutaneous ICD. These devices cannot provide anti-bradycardia pacing (except briefly after a shock), so they serve a purely defibrillation role. Recognizing this configuration matters because management decisions differ from transvenous systems, and MRI compatibility rules may also differ depending on the generation of the device.

Radiopaque Manufacturer Identification Codes

Most modern generators have tiny radiopaque identification markers etched or embedded in the header of the pulse generator. Under magnification or digital zoom on the chest X-ray, these markers appear as small alphanumeric characters or geometric shapes. Each manufacturer uses a distinct coding system, and cross-referencing the marker against published databases can tell you the device manufacturer and sometimes the specific model. Knowing the manufacturer helps the clinical team reprogram or interrogate the device, especially in emergencies when the patient cannot provide their device card.

These markers are useful but secondary to the shock-coil method for basic pacemaker-versus-ICD differentiation. They require magnification and familiarity with manufacturer-specific symbols, which may not be practical at three in the morning. For rapid triage purposes, the shock coil remains the fastest and most reliable distinguishing feature.

A Practical Step-by-Step Approach

If you want a reliable mental checklist for any chest X-ray with an implanted cardiac device, here is one that works:

Find the generator: Note its size, shape, and position. Left pectoral is most common; right pectoral or abdominal placements occur. A lateral position near the ribcage suggests a subcutaneous ICD.
Count the leads: One lead suggests a single-chamber device, two leads suggest dual-chamber, and three leads suggest a CRT system. No intracardiac leads with a parasternal electrode means subcutaneous ICD.
Trace each lead to its tip: Identify whether each lead terminates in the right atrium, right ventricle, left ventricle (via coronary sinus), or runs along the sternum subcutaneously.
Look for the shock coil: A thick, bright, cylindrical segment on any lead means the device has defibrillation capability. No shock coil means pacemaker only (or CRT-P).
Check for manufacturer codes: If you need to identify the device model, zoom into the generator header for radiopaque markers.

Following these steps in order efficiently sorts nearly all devices you will encounter. Chest radiography remains the only widely available imaging modality that allows direct visual evaluation of the physical integrity of cardiac device leads, making it the first-line tool for device assessment.³

Complications Worth Spotting While You Are Looking

Since you are already studying the chest X-ray closely enough to identify the device type, it is worth knowing the common complications that show up on the same film. Acute complications after device implantation include pneumothorax, lead malpositioning, and perforation of the heart wall or a vein. Chronic complications include lead fracture, damage to lead insulation, and lead displacement from its original position.³

Lead fracture can be subtle. The break may appear as a discontinuity or kink in the lead, sometimes visible only on the lateral projection or with careful digital magnification. A loose connection between the lead and the generator header, where the terminal connector pin has not been fully seated, can mimic a fracture at the proximal end of the lead. In pediatric patients, failure to leave an adequate redundant loop of lead in the right atrium to accommodate growth can lead to lead tension and eventual dislodgement as the child grows.⁴

Twiddler syndrome is an uncommon but dramatic complication in which the patient, consciously or unconsciously, rotates the generator in its subcutaneous pocket, winding the leads around it and retracting them from their cardiac positions. On chest X-ray, you see a rotated generator and coiled, retracted leads.⁵ The lead tips may end up nowhere near the heart. This is more common in elderly patients with loose subcutaneous tissue and is an indication for surgical revision.

Devices That Mimic Cardiac Implants

Not every small metallic object on a chest X-ray is a pacemaker or ICD. Implantable loop recorders (ILRs) are small, leadless rectangular devices inserted subcutaneously for long-term heart rhythm monitoring. They are much smaller than a pacemaker generator and have no leads at all. On a chest film, an ILR appears as a tiny metallic sliver, usually positioned to the left of the sternum in the subcutaneous tissue, and can be easily mistaken for a foreign body or artifact if you are not aware of its existence.⁶ Unlike pacemakers and ICDs, ILRs do not pace or shock; they simply record electrical activity.

Other metallic objects that occasionally cause confusion include retained surgical wires from sternotomy, sternal closure devices, port-a-cath reservoirs, and certain types of spinal cord stimulators. The key differentiator for cardiac devices is always the combination of a generator-sized box connected to one or more leads that track through the venous system toward the heart. If you see a metallic object without leads heading into the cardiac chambers, it is almost certainly not a pacemaker or ICD.

Leadless Pacemakers on Imaging

Leadless pacemakers represent a newer challenge. These are self-contained capsules about the size of a large vitamin pill, implanted directly into the right ventricle via a catheter. On a chest X-ray, a leadless pacemaker appears as a small, dense, cylindrical object sitting inside the right ventricular chamber with no visible lead and no subcutaneous generator. Because these devices have no shock coil and no external generator, they are clearly distinct from both conventional pacemakers and ICDs, but they can be confusing if you have never seen one before. They are purely pacemakers; no leadless ICD exists on the market at the time of writing.

If you see a small metallic capsule inside the right ventricle with no leads, a leadless pacemaker is the most likely explanation. Rarely, an embolized device fragment or foreign body could mimic this appearance, but the capsule shape and position at the right ventricular apex are characteristic.

When CT Adds Information the Chest X-Ray Cannot

Chest X-ray is the workhorse for cardiac device evaluation, but CT scanning can be useful in specific situations, such as evaluating suspected lead perforation, assessing the relationship of leads to cardiac structures, or planning lead extraction procedures. The main limitation of CT in patients with cardiac implants is metal artifact: the generator and leads create bright streaks and dark voids on the images that can obscure surrounding anatomy.

Newer CT techniques have improved this. Combining virtual monoenergetic images with metal artifact reduction algorithms substantially reduces the streak artifacts from cardiac devices, making the surrounding soft tissue and vessels more visible. However, pushing to higher energy levels for artifact suppression comes at the cost of losing some soft tissue and vessel contrast along the leads, so there is a practical tradeoff.⁷ In routine clinical practice, CT is a second-line tool for device evaluation; the plain chest X-ray remains the standard first step.

Conduction System Pacing and Newer Lead Positions

Over the past several years, conduction system pacing, which includes His-bundle pacing and left bundle branch area pacing, has become increasingly popular as an alternative to conventional right ventricular pacing. These leads are placed in the interventricular septum rather than at the right ventricular apex, so their tip position on X-ray differs from the traditional picture. On a frontal chest film, a left bundle branch area pacing lead appears to terminate higher in the septum, sometimes giving the impression of being in an unusual or even incorrect position if you are expecting the standard apical location.

These pacing leads are used in pacemaker systems, not ICDs, so they will not have shock coils. If you see a lead with a tip in the high septum and no shock coil, conduction system pacing is a likely explanation, not lead malposition. Awareness of this newer lead trajectory prevents unnecessary concern about displacement and avoids triggering unnecessary additional workups. As these techniques continue to spread, this variant will become a more common sight on routine chest films.