ECG criteria
- The electrical axis of the heart is often deviated to the right, frequently greater than +100 to +110°; the degree and direction of deviation are variable and depend on age, the anatomy of the defect, and prior surgery.
- Right ventricular hypertrophy: R or qR in V1, deep S in V5–V6.
- Tall R or qR in V1 supports the diagnosis of RVH; quantitative amplitude thresholds are age-dependent and are not used to assess the severity of obstruction without echocardiography.
- P pulmonale: in adults, the reference threshold is a P wave greater than 2.5 mm in lead II; in children, age-specific amplitude and duration norms for P are evaluated, and the fixed adult threshold is not used. This sign may be prominent in inferior leads (II, III, aVF), but is not required to be present in all three simultaneously, and is generally inconstant.
- Before surgery, the QRS is often narrow; QRS widening or bundle branch block can occur even before surgery and requires separate evaluation.
- Following surgical repair, right bundle branch block frequently appears; marked and progressive QRS widening (historical threshold around 180 ms) is one of the risk factors for ventricular arrhythmias, evaluated in conjunction with other data.
What it looks like on ECG
The basis of the picture is right ventricular hypertrophy (RVH). Right ventricular outflow tract obstruction contributes to its development, but the severity of ECG signs does not reliably indicate the severity of the defect and is not always proportional to the obstruction gradient: the electrical pattern is also influenced by age, the level of obstruction, the size of the VSD, associated anomalies, and prior surgeries.
| Element | Where | What to look for |
|---|---|---|
| QRS Axis | frontal plane | often deviated to the right, severity is variable |
| QRS in V1 | V1 | tall R or qR; quantitative amplitude thresholds are age-dependent |
| QRS in V5–V6 | V5–V6 | deep S reflecting right ventricular overload |
| P wave | II, III, aVF | pointed, tall (P pulmonale) with right atrial enlargement, an inconstant sign |
| Transition zone | chest leads | may be shifted, R wave progression is altered |
In newborns and infants, the initially physiological dominance of the right ventricle hinders early diagnosis—threshold criteria for RVH in children are different and age-dependent, detailed in pediatric ECG normal value tables. This also applies to P pulmonale: a threshold greater than 2.5 mm in lead II is a reference criterion for adults, whereas in children right atrial enlargement is assessed using age-specific P wave amplitude and duration norms rather than a fixed adult threshold.
Following surgical repair (VSD closure, relief of outflow tract obstruction, and in some patients with a transannular patch across the pulmonary valve ring), right bundle branch block frequently appears. It is associated with intervention in the region of the VSD and the RV outflow tract, but does not prove ventriculotomy was performed: modern surgeries are often performed transatrially and transpulmonally without classical ventriculotomy, and RBBB is not an obligatory outcome of repair.
Mechanism
In tetralogy of Fallot, infundibular, valvular, or supravalvular pulmonary stenosis creates an obstacle to blood ejection from the RV. The ventricular septal defect is typically large and non-restrictive, so systolic pressures in the ventricles largely equalize; dextroposition (anterior displacement) of the aortic root allows it to "override" the defect, receiving blood from both ventricles. With significant obstruction and a large non-restrictive VSD, systolic pressure in the RV can reach systemic or near-systemic levels, contributing to its hypertrophy; this is not universal for all anatomical and hemodynamic variants of the defect, particularly with moderate obstruction and in newborns. Following repair, hemodynamics and the ECG are determined by residual lesions rather than the initial obstruction. The increased electrical mass of the RV shifts the net depolarization vector to the right—this produces rightward electrical axis deviation and signs of RVH in the precordial leads. P pulmonale may be detected with right atrial enlargement or overload, but is not an obligatory or specific sign of tetralogy of Fallot; its cause is clarified by echocardiography.
Causes
- Isolated congenital defect (most frequent cyanotic CHD)
- Association with genetic syndromes: 22q11.2 deletion (DiGeorge syndrome), trisomy 21
- Variant with pulmonary atresia (extreme degree of obstruction)
- Status post palliative shunts (Blalock-Taussig and equivalents)—the ECG pattern may additionally change due to volume overload
Differential diagnosis
- Isolated right ventricular hypertrophy of another etiology (cor pulmonale, pulmonary stenosis without tetralogy)—differentiated by echocardiography and a history of cyanosis from childhood, the ECG is similar
- Dextrocardia—on the ECG, inverted P wave and QRS in lead I, decreasing amplitude of complexes from V1 to V6; in tetralogy of Fallot, the heart axis is not mirror-imaged
- Atrial septal defect—also causes RVH and can cause P pulmonale, but usually without such pronounced rightward electrical axis deviation and without cyanosis prior to the development of Eisenmenger syndrome
- Transposition of the great arteries—different clinical picture, ECG pattern depends on associated anomalies
Clinical significance and management
The ECG in tetralogy of Fallot does not make the diagnosis on its own—the defect is confirmed by echocardiography, and if necessary, cardiac CT/MRI and catheterization. The task of the functional diagnostics physician is to describe the degree of RVH, the axis, rhythm, and, most importantly, the dynamics following surgery. In operated patients with residual RBBB, marked and progressive QRS widening, including the historically used threshold of around 180 ms, is one of the risk factors for increased ventricular arrhythmias and sudden cardiac death. This sign is not interpreted in isolation: modern risk stratification by a CHD specialist also considers RV function and dimensions, scar tissue volume, ventricular dysfunction, episodes of nonsustained VT, syncope, and other clinical data. The appearance of new ventricular premature beats, runs of tachycardia, or progressive QRS widening is an indication for referral to an adult congenital heart disease (ACHD) cardiologist and for more frequent ECG and Holter monitoring.
Common interpretation errors
- Mistaking physiological rightward electrical axis deviation in newborns for pathology without accounting for age norms
- Failing to correlate the ECG with clinical findings (cyanosis, murmur, SpO2) and making a diagnosis based solely on the tracing
- Considering the R wave amplitude in V1 as an independent indicator of the severity of RV outflow tract obstruction
- Interpreting QRS widening around 180 ms as an isolated and unambiguous predictor of arrhythmias without considering RV function and other risk factors
- Forgetting that P pulmonale is an inconstant sign, and in children it is assessed by age-specific norms rather than the adult threshold of 2.5 mm, and interpreting the absence of the sign as ruling out the defect
1. What main ECG sign is characteristic of tetralogy of Fallot?
Right ventricular outflow tract obstruction contributes to its hypertrophy, which shifts the electrical axis to the right and produces signs of RVH in V1–V2.
2. What frequently appears on the ECG following surgical repair of tetralogy of Fallot?
RBBB frequently appears after repair due to surgical intervention in the area of the VSD and the RV outflow tract, but is not obligatory.
3. Which ECG parameter in operated patients with tetralogy of Fallot is considered one of the markers of increased risk for ventricular arrhythmias?
QRS ≥180 ms or its progressive widening with residual RBBB is one of the risk markers, interpreted together with echocardiography/MRI data, RV function, and clinical factors, rather than in isolation.
Frequently asked questions
Can a diagnosis of Tetralogy of Fallot be made based on ECG alone?
No. The ECG shows only the consequences of the defect—right ventricular hypertrophy, electrical axis deviation, and sometimes P pulmonale. The diagnosis is made based on echocardiography and clinical presentation (cyanosis, murmur), while the ECG is used to assess chamber overload and monitor changes over time.
Why do children with Tetralogy of Fallot have right axis deviation?
The right ventricle works against systemic or near-systemic pressure due to outflow tract obstruction and undergoes hypertrophy. The increased electrical mass of the RV shifts the net depolarization vector to the right. The severity and exact range of deviation are individual and depend on age and the anatomy of the defect.
Why is the post-surgical ECG pattern in Tetralogy of Fallot dangerous?
Following surgical correction, right bundle branch block frequently develops. Pronounced and progressive QRS widening, including the historical threshold of approximately 180 ms, is one of the markers of increased risk for ventricular arrhythmias and sudden cardiac death, which is assessed alongside data on RV function and other factors; therefore, such patients are monitored with regular ECG follow-up.
Does the ECG in a newborn with Tetralogy of Fallot differ from that of an adult with this defect?
Yes. In newborns, the right ventricle is physiologically dominant, and criteria for hypertrophy and P pulmonale are age-dependent, unlike the fixed thresholds for adults. In adults, especially those who have undergone surgery, the pattern is determined by residual changes and the surgical intervention rather than the underlying defect alone.
Is P pulmonale always present in Tetralogy of Fallot?
No, this depends on the severity of right atrial enlargement or overload and does not necessarily appear simultaneously in all inferior leads. The sign is not mandatory for diagnosis, and its absence does not rule out the defect.
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Related terms
More in the section "Pericardium, Lungs, Brain and Others"
Sources
- Surawicz B., Knilans T. Chou's Electrocardiography in Clinical Practice
- Orlov V.N. Guide to Electrocardiography
- Hampton J.R. The ECG in Practice
- Goldberger A.L. Clinical Electrocardiography: A Simplified Approach
Educational material for medical students and physicians. Does not replace clinical guidelines and physical examination; the ECG must always be interpreted in conjunction with the clinical picture.