Does an implanted defibrillator (implantable cardioverter-defibrillator, ICD) cause scarring and further fibrosis of the myocardium every time it discharges?

Does an Implanted Defibrillator Cause Scarring and Further Fibrosis of the Myocardium Every Time It Discharges?

No, there is no direct evidence that implantable cardioverter-defibrillator (ICD) discharges cause significant myocardial scarring or fibrosis with each shock, but ICD shocks have been associated with increased heart failure events and potential myocardial injury.

Relationship Between ICD Shocks and Myocardial Injury

• ICD shocks deliver high-energy electrical current to terminate life-threatening ventricular arrhythmias, but this electrical energy can potentially affect the myocardium 1
• When an ICD shock is delivered, electricity conducted along the implanted electrode to the endocardium may cause temporary myocardial injury associated with a temporary or permanent increase in stimulation threshold 1
• This myocardial injury can potentially lead to exit block (failure of ventricular capture) when pronounced, but this is typically a temporary phenomenon rather than permanent scarring 1

Evidence for Potential Myocardial Effects

• Analysis from the MADIT-II trial showed that patients randomized to ICD therapy had a significant increase in the risk of first heart failure events (HR: 1.39; p = 0.02) that was more pronounced in patients who received shocks (HR: 1.9; p = 0.01) 2
• The study authors postulated that defibrillator shocks may result in injury to the myocardium, and ventricular function can be further impaired as a consequence 1, 2
• Patients who received life-prolonging therapy from ICDs had an increased risk for first and recurrent heart failure events (HR: 1.90; p = 0.01 and 1.74; p < 0.001, respectively) 2

Relationship Between Existing Scars and ICD Shocks

• Myocardial scarring is a significant predictor of appropriate ICD therapy and arrhythmic events, rather than being caused by ICD shocks 1
• Patients with significant scarring (>5% of left ventricular mass) have a substantially higher risk of ventricular arrhythmias requiring ICD therapy compared to those with minimal or no scarring 1
• Scar tissue in the myocardium forms the underlying substrate for ventricular tachycardia by creating areas of slow conduction that facilitate reentrant circuits 3

Clinical Implications

• ICD shocks are associated with higher mortality and impaired quality of life, though this may be due to the underlying arrhythmia rather than direct effects of the shock itself 3, 4
• Inappropriate ICD shocks (those delivered for non-life-threatening arrhythmias or due to oversensing) occur at a rate of approximately 5.3% per year and can further impact quality of life 5
• Congestive heart failure remains the leading cause of death (30%) in patients with ICDs, while sudden death still accounts for 20% of deaths despite ICD therapy 4

Minimizing Potential Myocardial Impact

• Proper positioning of external cardioversion paddles as distant as possible from the implanted device (preferably in anterior-posterior configuration) can reduce the risk of myocardial injury during cardioversion 1
• Device interrogation before and after cardioversion is recommended to verify appropriate function and reprogram if necessary 1
• Programming strategies to reduce unnecessary shocks may help minimize potential myocardial effects 3

Special Considerations

• In patients with cardiac sarcoidosis, almost one-third receive appropriate ICD therapies, which may be due to the myocardial inflammatory process leading to increased triggered activity and subsequent scarring 6
• The presence of myocardial scarring (detected by delayed enhancement cardiac MRI) is a strong predictor of adverse outcomes regardless of left ventricular ejection fraction 1

While there is evidence that ICD shocks may be associated with myocardial injury and increased heart failure events, there is no definitive evidence that each ICD discharge directly causes significant additional scarring or fibrosis of the myocardium.