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Introduction: Immune checkpoint inhibitor (ICI)-myocarditis is a new syndrome with estimated 50% mortality. Similar to acute cellular rejection (ACR), it is pathologically characterized by lymphocytic infiltration. We aimed to characterize the electrocardiograph features of ICI-myocarditis, compare them to ACR, and evaluate their association with adverse outcome. Methods: Presenting ECG of 130 cases of ICI-myocarditis were collected from a multicenter network spanning 12 countries and compared to 50 cases of ACR. ECG were quantified and interpreted by two blinded cardiologists. 53 patients with ICI-myocarditis had baseline ECG available for comparison via paired univariate analysis. Cox models correcting for age and sex determined association with a composite outcome of life-threatening arrhythmia or myocarditis-related death. Results: ICI-myocarditis patients had average age of 68(58-76), were 61.2% male, and 64.8% had prior cardiovascular disease. QRS prolongation (26% vs 13%, p=0.008), conduction disorders (67% vs 44%, p=0.007) such as left bundle branch block (LBBB) (18% vs 4% p=0.008), ST/T wave changes (50% vs 24%, p=0.004), and PVCs (16% vs 6%, p=0.020) were more prevalent on presenting ECG compared to baseline. ICI-myocarditis showed more PVCs (16% vs 2%, p=0.011) and less ST/T wave changes (41% vs 66%, p=0.002) when compared to ACR. On multivariate analysis, the combined outcome of life-threatening arrhythmia or myocarditis-related death was associated with pathological Q waves (HR=3.60 (1.78-7.27) p<0.001), QRS prolongation (HR=3.35 [1.00-11.21] p=0.05), LBBB (HR=2.24 [1.13-4.45] p=0.021), and supraventricular arrhythmia (HR=2.03 [1.05-3.91] p=0.035) on presenting EKG. Conclusions: ICI-myocarditis manifests as new conduction delays, ST/T-wave changes, and PVCs. QRS prolongation, LBBB, pathological Q waves, and supraventricular arrhythmias were associated with subsequent adverse outcomes.

Importance: Immune-checkpoint inhibitor (ICI)-myocarditis often presents with arrhythmias, but electrocardiographic (ECG) findings have not been well described. ICI-myocarditis and acute cellular rejection (ACR) following cardiac transplantation share similarities on histopathology; however, whether they differ in arrhythmogenicity is unclear. Objectives: To describe ECG findings in ICI-myocarditis, compare them to ACR, and evaluate their prognostic significance. Design: Cases of ICI-myocarditis were retrospectively identified through a multicenter network. Grade 2R or 3R ACR was retrospectively identified within one center. Two blinded cardiologists interpreted ECGs. Setting: 49 medical centers spanning 11 countries. Participants: 147 adults with ICI-myocarditis, 50 adults with ACR. Exposure: Myocarditis after ICI exposure per European Society of Cardiology criteria for clinically suspected myocarditis, grade 2R or 3R ACR per the International Society for Heart and Lung Transplantation working formulation for biopsy diagnosis of rejection. Outcomes: All-cause mortality, myocarditis-related mortality; and composite endpoint (defined as myocarditis-related mortality and life-threatening ventricular arrhythmia). Results: Of 147 patients, the median age was 67 years (58-77) with 92 (62.6%) men. At 30 days, ICI-myocarditis had an all-cause mortality of 39/146(26.7%), myocarditis-related mortality of 24/146(16.4%), and composite endpoint of 37/146(25.3%). All-cause mortality was more common in patients who developed complete heart block (12/25[48%] vs 27/121[22.3%], hazard ratio (HR)=2.62, 95% confidence interval [1.33-5.18],p=0.01) or life-threatening ventricular arrhythmias (12/22[55%] vs 27/124[21.8%], HR=3.10 [1.57-6.12],p=0.001) within 30 days after presentation. Compared to ACR, patients with ICI-myocarditis were more likely to experience life-threatening ventricular arrhythmias (22/147 [16.3%] vs 1/50 [2%];p=0.01) or third-degree heart block (25/147 [17.0%] vs 0/50 [0%];p=0.002). In ICI-myocarditis, overall mortality, myocarditis-related mortality, and composite outcome adjusted for age and sex were associated with pathological Q-waves on presenting ECG (hazard ratio by subdistribution model [HR(sh)]=5.98[2.8-12.79],p<.001; 3.40[1.38-8.33],p=0.008; 2.20[0.95-5.12],p=0.07; respectively) but inversely associated with Sokolow-Lyon Index (HR(sh)/mV=0.57[0.34-0.94],p=0.03; HR(sh)=0.54[0.30-0.97],p=0.04; 0.50[0.30-0.85],p=0.01; respectively). The composite outcome was also associated with conduction disorders on presenting ECG (HR(sh)=3.27[1.29-8.34],p=0.01). Conclusions: ICI-myocarditis has more life-threatening arrhythmias than ACR and manifests as decreased voltage, conduction disorders, and repolarization abnormalities . Ventricular tachycardias, complete heart block, low-voltage, and pathological Q-waves were associated with adverse outcomes.

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