Myocarditis produces a range of ECG abnormalities, but the most common finding is sinus tachycardia with nonspecific ST-segment and T-wave changes. ST-segment elevation is the most frequently seen ST change, appearing in 24% to 73% of acute myocarditis cases depending on the study. Because these changes closely mimic a heart attack, distinguishing myocarditis on ECG requires looking at the pattern of changes across leads, the clinical context, and how the tracing evolves over time.
ST-Segment Elevation
ST elevation is often the first thing that catches attention on a myocarditis ECG. Unlike a heart attack, where ST elevation typically maps to a single coronary artery territory (a cluster of leads fed by one blood vessel), myocarditis tends to cause diffuse ST elevation across multiple lead groups that don’t follow a single vascular territory. This diffuse pattern reflects widespread inflammation rather than a blocked artery.
The ST elevation in myocarditis is often concave (curving upward like a smile), similar to what you’d see with pericarditis. ST depression is uncommon in myocarditis. When it does appear alongside ST elevation, it usually shows up as a reciprocal (mirror image) change in lead aVR and may actually point toward a true heart attack rather than myocarditis.
T-Wave Inversion
T-wave inversion is the second hallmark ECG change, reported in 9% to 48% of acute myocarditis patients. T waves may initially appear tall or peaked in the earliest hours, then flip negative as inflammation progresses. Like the ST changes, T-wave inversions in myocarditis tend to be widespread rather than confined to a few leads. In many cases, the T-wave changes outlast the ST elevation and can persist into the subacute phase (beyond four weeks) even as symptoms improve.
Sinus Tachycardia and Arrhythmias
A fast resting heart rate is the single most common ECG abnormality in myocarditis, reflecting the body’s response to inflammation, fever, or declining heart function. But the rhythm disturbances can go well beyond a fast sinus rate.
Premature ventricular contractions (extra beats originating in the ventricles) are especially common, found in up to 63% of patients in some series. Ventricular tachycardia, both sustained and nonsustained, also occurs. During the acute phase, these ventricular arrhythmias tend to be polymorphic, meaning they shift in shape from beat to beat, reflecting active, unstable inflammation. In patients who develop chronic myocarditis or lasting scarring, the arrhythmias typically become monomorphic and regular, a pattern linked to fixed scar tissue rather than active inflammation.
Conduction disturbances are another key finding. Myocarditis can cause first-, second-, or third-degree atrioventricular block (where electrical signals between the upper and lower chambers slow or stop entirely). Bundle branch blocks, where conduction through one of the main electrical pathways in the ventricles is delayed, also appear. Third-degree heart block in a young person with recent viral symptoms is a classic red flag for myocarditis.
Low Voltage and QRS Widening
These findings are particularly important because they signal more severe disease. In a study of 150 patients with fulminant myocarditis (the most dangerous form), low voltage was the most common ECG abnormality, present in 55% of cases. The QRS complexes, which represent ventricular contraction, appear noticeably smaller than normal across all leads. This happens because inflamed, edematous heart muscle conducts electrical signals poorly. Pericardial effusion (fluid around the heart), which often accompanies myocarditis, can further dampen voltage.
QRS widening is even more clinically significant. A prolonged QRS duration is an independent predictor of cardiac death or need for heart transplantation in myocarditis patients. In fulminant myocarditis, QRS broadening (including left or right bundle branch block patterns and nonspecific intraventricular conduction delays) was strongly associated with reduced heart function. Sinus tachycardia, low voltage, and QRS broadening together were the strongest ECG predictors of how well the heart was pumping in these critically ill patients.
QT Prolongation
The corrected QT interval (QTc) is often prolonged in myocarditis. Fulminant myocarditis patients in one study had a median QTc of 452 milliseconds compared to 399 milliseconds in healthy controls. A prolonged QT interval reflects delayed electrical recovery of the ventricles and increases the risk of dangerous arrhythmias. This is one reason patients with myocarditis are monitored on continuous telemetry during hospitalization.
How These Changes Mimic a Heart Attack
Myocarditis is one of the most common causes of a “false STEMI.” A young person arrives with chest pain, elevated troponin levels, and ST elevation on the ECG, and the initial picture looks identical to a heart attack. Several clues can help distinguish the two:
- Age: Myocarditis patients are typically under 40.
- Recent illness: A viral infection in the preceding one to four weeks is a classic trigger.
- Lead distribution: ECG changes span multiple vascular territories rather than one.
- Absence of reciprocal ST depression: True heart attacks usually produce ST depression in leads opposite the elevation; myocarditis generally does not.
- Wall motion abnormalities: On echocardiogram, myocarditis causes diffuse or patchy dysfunction rather than a single territory of abnormal wall motion.
Despite these patterns, no single ECG feature reliably separates myocarditis from a heart attack. When ST elevation and troponin are both present, most emergency protocols treat the situation as a potential heart attack until coronary angiography or cardiac MRI can clarify the diagnosis.
How ECG Changes Evolve Over Time
Acute myocarditis (symptoms lasting four weeks or less) typically produces the most dramatic ECG changes: ST elevation, sinus tachycardia, and polymorphic ventricular arrhythmias. As inflammation subsides into the subacute phase (four weeks to three months), ST segments usually normalize first, while T-wave inversions and conduction delays may persist longer.
In patients who progress to chronic myocarditis (beyond three months) or inflammatory cardiomyopathy, the ECG may show persistent low voltage, bundle branch block patterns, or monomorphic ventricular arrhythmias related to myocardial fibrosis. Full remission is defined not just by symptom resolution but also by normalization of ECG, biomarkers, and imaging findings. Patients whose ECG abnormalities persist despite feeling better are classified as being in “remission with residuals,” which carries a different follow-up and risk profile.
ECG Findings in Children
Pediatric myocarditis produces similar ECG abnormalities to the adult form: sinus tachycardia, low-voltage QRS complexes, ST-T wave changes, prolonged QT intervals, and atrioventricular block. However, the ECG is almost always abnormal in children with myocarditis, making it a particularly useful screening tool in this age group. Ventricular tachycardia and complete heart block can occur even in young children and represent the most dangerous rhythm disturbances.
Limitations of ECG Alone
The ECG is sensitive enough to detect abnormalities in most myocarditis cases, but it lacks specificity. A normal ECG does not rule out myocarditis when clinical suspicion is high. The 2024 American College of Cardiology expert consensus document emphasizes that ECG, echocardiography, and troponin testing are all standard initial tests, but normal results on any of them cannot exclude the diagnosis. Cardiac MRI remains the gold standard for noninvasive confirmation, and ongoing monitoring with 24-hour ECG recording and exercise stress testing is recommended before returning to strenuous physical activity.