A Pacemaker (PM) and an Implantable Cardioverter Defibrillator (ICD) are electronic devices surgically placed to manage abnormal heart rhythms (arrhythmias). A PM delivers low-energy electrical pulses to regulate a slow heart rate. An ICD is more complex; it performs this pacing function but also delivers a high-energy electrical shock to stop dangerously fast rhythms. Since these devices look similar on a patient’s chest, a chest X-ray (CXR) is the primary tool for visually distinguishing them.
Generator Size and Location
The main body of the device, often called the generator, appears as a dense, rectangular or oval-shaped metallic object on a CXR, typically situated in the upper chest. This component houses the battery and the computer circuitry necessary for the device’s function. A general observation is that the ICD generator is usually larger and bulkier than a standard PM generator.
This size difference is primarily due to the ICD’s requirement for large, specialized capacitors, which store the high energy needed for a defibrillation shock. In contrast, a PM only requires a smaller battery and circuitry to generate low-energy pacing impulses. While both devices are most commonly placed under the skin near the collarbone, the larger size of some ICDs may occasionally necessitate an abdominal placement. However, generator size and location are not definitive features and serve only as initial clues for identification.
Key Differences in Lead Structure
The most reliable way to differentiate between a PM and an ICD on a CXR is by examining the structure of the leads, which are the insulated wires extending from the generator into the heart chambers. Pacemaker leads appear as thin, uniform, and smooth lines extending from the generator to the right atrium and/or right ventricle. These wires are designed only to carry the low-voltage electrical signals for pacing and sensing the heart’s natural rhythm.
ICD leads, however, have a distinctive appearance because they must be able to deliver a high-energy defibrillation shock. The absolute radiographic hallmark of an ICD is the presence of thick, radiopaque segments along the lead called “shock coils” or “defibrillation coils.” These coils appear much denser and thicker than the rest of the lead wire on the X-ray, looking less like a thin wire and more like a braided or spring-like segment.
A typical ICD lead system will have one or two of these coils, which act as the electrodes for delivering the shock. One coil is often positioned near the superior vena cava (SVC), and the other is usually located near the tip of the lead in the right ventricle (RV). The presence of these visibly thick, dense coil segments on a lead is the definitive visual evidence that the device is an ICD, regardless of the generator’s size or placement.
Clinical Importance of Identification
Accurate identification of the device type on a CXR carries significant weight in a clinical setting, especially in emergency situations or when planning a medical procedure. The fundamental difference lies in the energy delivered: a pacemaker provides only low-energy pacing, while an ICD can deliver both low-energy pacing and a high-energy shock. Knowing this distinction immediately guides management and safety protocols for the patient.
For example, a patient with an ICD who requires an MRI or surgery involving electrocautery needs specific device programming or deactivation to prevent the unintended delivery of a shock. An immediate shock from an ICD during a procedure could be harmful to the patient and the medical team. In contrast, while a pacemaker also requires management, the risk of a high-voltage discharge is absent.
In an emergency room setting, knowing the device is an ICD can explain the cause of a patient’s symptoms if they report receiving a “shock.” Furthermore, the ability to identify the manufacturer from the generator’s radiopaque markers allows clinicians to access the correct device-specific programmer for interrogation and adjustment. This visual diagnosis is a fundamental step in providing appropriate and safe patient care.