ECG Findings in Toxic Myocarditis
The ECG in toxic myocarditis shows nonspecific abnormalities including ST-segment elevation, T-wave changes, conduction abnormalities, and arrhythmias, but these findings are neither sensitive nor specific enough for definitive diagnosis as a stand-alone test. 1
Common ECG Abnormalities
The most frequently encountered ECG findings in myocarditis (including toxic etiology) include:
ST-Segment and T-Wave Changes
- ST-segment elevation is common and typically occurs without reciprocal ST depression, which helps differentiate it from acute myocardial infarction 1, 2
- T-wave inversion may develop and can persist for weeks to months 1
- ST-segment and T-wave changes are often accompanied by sinus tachycardia, which is the most common rhythm abnormality 3
Conduction Abnormalities
- AV block (first-degree through third-degree) is a characteristic finding 1
- Bundle branch block (both right and left) can occur 1
- Intraventricular conduction delay is frequently present 1
- Sinus arrest may occur in severe cases 1
QRS Complex Abnormalities
- New Q waves can develop but typically disappear within a short period, unlike myocardial infarction 1, 2
- Low voltage QRS complexes may be present, particularly when associated with myocardial edema and fluid retention 1, 4
- Decreased total QRS amplitudes in the acute stage compared to baseline 2
Arrhythmias
- Extrasystoles (both atrial and ventricular) 1
- Supraventricular tachycardia and atrial fibrillation 1
- Ventricular tachycardia, ventricular fibrillation, and asystole in severe cases 1
Features Distinguishing Myocarditis from Myocardial Infarction
Several ECG patterns favor myocarditis over acute coronary syndrome:
- PR segment depression in both precordial and limb leads 3
- PR segment elevation in lead aVR 3
- ST elevation without reciprocal ST depression (unlike typical MI patterns) 2
- Absence of pathological Q waves or Q waves that resolve quickly 2
Prognostic ECG Indicators
Poor prognostic ECG features in myocarditis include:
- Pathological Q waves 3
- Wide QRS complex 3
- QRS/T angle ≥100° 3
- Prolonged QT interval 3
- High-degree atrioventricular block 3
- Malignant ventricular tachyarrhythmias 3
Critical Clinical Caveats
The ECG alone cannot definitively diagnose or exclude myocarditis. 1 The clinical history is essential—specifically, recent exposure to toxic or allergenic agents should raise suspicion for toxic myocarditis 1
When evaluating suspected toxic myocarditis:
- Obtain serial ECGs to track evolution of changes 1
- Correlate ECG findings with elevated troponin and creatine kinase levels 1
- Use cardiac MRI as the definitive diagnostic test, as it can directly visualize myocardial inflammation, edema, and necrosis 1
- Perform echocardiography to assess ventricular function and exclude other causes of heart failure 1, 5
The ESC Working Group criteria require at least one clinical symptom (chest pain, dyspnea, fatigue, palpitations, or syncope) plus at least one diagnostic criterion from different categories (ECG, biomarkers, or imaging) to suspect myocarditis clinically 1