PET-Guided Ablation Planning in Cardiac Sarcoidosis
Cardiac sarcoidosis is an inflammatory myocardial disease with a broad and often unpredictable clinical spectrum. Patients may present with atrioventricular block, ventricular tachycardia, heart failure, or incidental abnormalities on advanced imaging. The disease can evolve from active granulomatous inflammation to permanent fibrosis, creating a substrate for monomorphic or polymorphic ventricular arrhythmias.
For electrophysiologists, the central challenge is distinguishing active inflammation from established scar and then translating that information into a safe, effective ablation strategy. Fluorodeoxyglucose positron emission tomography, usually combined with myocardial perfusion imaging and cardiac magnetic resonance, provides valuable biological and anatomical information that is not available from the surface electrocardiogram or electroanatomic voltage map alone.
PET does not replace clinical judgment, endomyocardial biopsy, magnetic resonance imaging, or invasive mapping. Its role is to clarify disease activity, identify potentially arrhythmogenic regions, guide procedural planning, and support longitudinal decisions about immunosuppression and repeat intervention.
Why Sarcoidosis Produces Ventricular Arrhythmias
Sarcoid granulomas may involve the basal septum, ventricular free wall, papillary muscles, or conduction system. Active inflammation can cause edema, cellular injury, and electrical instability. As inflammation resolves, patchy replacement fibrosis and scar create areas of slow conduction and conduction block. These regions can support re-entry, the dominant mechanism of many sustained monomorphic ventricular tachycardias.
The arrhythmic substrate is frequently heterogeneous. A patient may have a large scar burden on late gadolinium enhancement cardiac MRI but only limited active inflammation on FDG-PET. Another patient may show intense focal or multifocal FDG uptake with relatively preserved ventricular function, signaling an active phase in which inflammation may be contributing directly to electrical instability.
This distinction affects treatment priorities. Active inflammation may warrant corticosteroids or steroid-sparing immunosuppression, while recurrent scar-mediated ventricular tachycardia may require implantable cardioverter-defibrillator therapy and catheter ablation. In many cases, both processes coexist and must be managed in parallel.
What FDG-PET Adds to the Diagnostic Picture
The principal value of FDG-PET is the detection of metabolically active myocardial inflammation. A properly performed study suppresses normal myocardial glucose uptake so that focal or patchy inflammatory uptake becomes visible. Perfusion imaging adds complementary information: a matched perfusion defect and FDG-avid region may indicate inflammatory injury or scar, whereas abnormal uptake without a corresponding defect can suggest earlier or more active disease.
Interpretation should account for image quality and preparation. A high-fat, low-carbohydrate diet, prolonged fasting, and appropriate medication review are commonly used to reduce physiologic myocardial uptake. Inadequate suppression can produce diffuse or heterogeneous activity that is difficult to distinguish from disease. Reports should describe the distribution, intensity, focality, and relationship between inflammation and perfusion defects rather than simply labeling a scan positive or negative.
PET findings are most useful when integrated with symptoms, biomarkers, ventricular function, rhythm history, cardiac MRI, and extracardiac evidence of sarcoidosis. Serial PET examinations may help assess treatment response, although changes in uptake should not be interpreted in isolation. Persistent ventricular tachycardia can reflect fixed scar even after inflammatory activity has subsided.
Translating PET Findings Into Ablation Targets
Before ablation, PET can identify regions that deserve particular attention during electroanatomic mapping. Focal FDG uptake may point toward active inflammatory tissue, while perfusion defects and MRI enhancement can identify areas of structural injury. When these modalities overlap with low-voltage regions, abnormal electrograms, or clinical tachycardia exit sites, confidence increases that the region is relevant to the arrhythmogenic substrate.
Ablation planning should remain flexible because the clinical ventricular tachycardia may not be inducible, may be poorly tolerated, or may arise from more than one circuit. Substrate mapping in sinus rhythm can reveal bipolar and unipolar voltage abnormalities, late potentials, fractionated signals, and local conduction delay. PET and MRI help define where those abnormalities may extend beyond the visible endocardial scar.
The route of ablation depends on the substrate. Endocardial ablation may be sufficient for septal or inner-wall circuits, but sarcoid scar often has an intramural or epicardial component. Failure of an endocardial procedure does not necessarily mean that the diagnosis or target was incorrect; it may indicate that the critical isthmus lies deeper in the myocardium or on the epicardial surface. Preprocedural imaging can help anticipate this possibility and inform discussions about procedural risk.
Imaging-guided procedural workflows are also evolving. Experience with zero-fluoroscopy ablation demonstrates how three-dimensional mapping and intracardiac imaging can reduce radiation exposure in selected procedures, although complex ventricular ablation still requires individualized safety planning.
Combining PET With MRI and Electroanatomic Mapping
Cardiac MRI is particularly valuable for showing late gadolinium enhancement, wall thinning, edema, ventricular aneurysm formation, and the distribution of replacement fibrosis. PET supplies metabolic information, whereas MRI supplies high-resolution tissue characterization. Their combination can separate active inflammation from inactive scar more effectively than either test alone.
The most useful target is often the intersection of abnormalities across modalities. For example, a PET-avid region that corresponds to MRI enhancement and low voltage on mapping is more compelling than an isolated imaging abnormality. Conversely, discordant findings should prompt caution. FDG uptake may reflect inflammation without a mature re-entry circuit, while MRI enhancement may represent old scar with no current metabolic activity.
A practical planning map can include PET inflammation, perfusion defects, MRI scar, ventricular aneurysms, coronary anatomy, phrenic nerve relationships, and prior ablation lesions. These data can be imported or visually registered with the electroanatomic system when available. The goal is not to ablate every abnormal pixel, but to identify clinically meaningful channels, border zones, and potential exit sites while preserving viable myocardium.
Disease distribution may also influence the choice between endocardial, epicardial, or combined access. Septal involvement requires particular attention to conduction tissue and the risk of complete heart block. Lesions near the coronary arteries, phrenic nerve, or thin ventricular wall require careful risk assessment, especially when an epicardial approach is considered.
Reading Imaging Alongside Clinical Outcomes
Recurrent ventricular arrhythmia after ablation can result from incomplete lesion delivery, an unrecognized epicardial or intramural circuit, progression of scar, or renewed inflammation. PET can help distinguish some of these possibilities, but it cannot independently determine the cause of recurrence. A new cluster of FDG uptake may support renewed inflammatory activity, whereas stable non-avid scar with recurrent monomorphic tachycardia may favor a residual or newly accessible re-entry pathway.
Left ventricular dysfunction also requires careful attribution. Tachycardia-mediated decline can coexist with inflammatory cardiomyopathy and scar. The broader literature on tachycardia-induced cardiomyopathy recovery illustrates why ventricular recovery and arrhythmia control should be followed over time rather than judged from a single early assessment.
Ablation success should therefore include more than acute noninducibility. Relevant outcomes include ventricular tachycardia recurrence, appropriate ICD therapies, heart failure hospitalization, ventricular function, exercise capacity, inflammatory activity, and adverse effects of immunosuppression. Long-term monitoring is essential because cardiac sarcoidosis may relapse or progress after an apparently successful intervention.
Comparing the Main Imaging Roles
| Imaging or mapping method | Principal information | Strength in ablation planning | Important limitation |
|---|---|---|---|
| FDG-PET | Active myocardial inflammation | Identifies metabolically active regions and supports treatment-response assessment | Sensitive to preparation quality and nonspecific uptake |
| Perfusion PET or SPECT | Regional myocardial blood-flow abnormalities | Helps distinguish perfusion defects from active inflammatory uptake | Limited spatial and tissue characterization compared with MRI |
| Cardiac MRI | Fibrosis, edema, wall thinning, aneurysm, ventricular anatomy | Defines scar architecture and possible intramural substrate | May be limited by device compatibility, renal function, or image quality |
| Electroanatomic mapping | Voltage, activation, conduction, electrogram features | Delivers real-time substrate definition and guides lesion placement | Findings can be altered by rhythm, prior ablation, and catheter contact |
| Surface ECG and rhythm monitoring | QRS morphology, burden, temporal pattern | Helps identify likely ventricular origin and treatment response | Provides limited spatial information |
The table highlights why multimodality assessment is preferable to reliance on a single scan. PET is particularly influential when the clinical decision depends on whether inflammation remains active, while MRI and invasive mapping are often more informative for defining fixed scar and re-entry channels.
Building a Multidisciplinary Treatment Strategy
Patients with suspected or confirmed cardiac sarcoidosis benefit from collaboration among electrophysiologists, imaging specialists, heart failure clinicians, rheumatologists, and, when needed, pulmonologists. The team should establish the diagnosis, assess extracardiac involvement, review immunosuppression, evaluate ventricular function, and determine the indication for an ICD independently of the ablation plan.
The ICD remains central for many patients at meaningful risk of sudden cardiac death, even when ventricular tachycardia is suppressed. Catheter ablation can reduce recurrent sustained VT and ICD shocks, but it does not eliminate the underlying inflammatory or fibrotic disease. Inflammation control, heart failure therapy, device follow-up, and arrhythmia surveillance remain important after the procedure.
Other electrophysiology conditions may complicate the overall rhythm-management pathway. For example, clinicians reviewing anticoagulation and atrial arrhythmias can consult discussion of atrial fibrillation with PFO, while keeping those decisions separate from the imaging and ablation principles specific to ventricular sarcoidosis.
Practical Recommendations for PET-Guided Planning
- Confirm adequate dietary and fasting preparation before interpreting myocardial FDG uptake.
- Combine PET with cardiac MRI, perfusion imaging, ECG findings, ventricular function, and clinical history.
- Treat active inflammation and scar-mediated ventricular tachycardia as related but distinct therapeutic problems.
- Use PET and MRI to prioritize mapping regions, while allowing invasive electroanatomic findings to define the final ablation strategy.
- Reassess recurrent arrhythmia with attention to inflammatory relapse, residual scar, and possible intramural or epicardial circuits.
PET-guided planning is most effective when it changes a specific clinical decision: whether inflammation remains active, where mapping should begin, whether an epicardial strategy may be needed, or how recurrence should be investigated. Used in that focused manner, metabolic imaging can connect disease biology with procedural anatomy and make ventricular arrhythmia care more precise.
For clinicians and researchers, ongoing study of multimodality imaging, immunosuppression timing, substrate-based ablation, and long-term device outcomes will refine care for this complex population. The Journal of Arrhythmia provides a forum for that evolving evidence across cardiac electrophysiology, imaging, ablation, and implantable device practice.