Embryology · Heart and vessels
Tetralogy of Fallot !
- Malformation
One error in the outflow tract gives four findings: pulmonary obstruction, a ventricular hole, an overriding aorta and a thick right ventricle.
Updated 2026-11-01
See it in 3D: Right ventricle · Interventricular septum · Pulmonary trunk · Ascending aorta · Pulmonary anterior cusp · Pulmonary left cusp · Pulmonary right cusp · Left ventricle
What it is
Tetralogy of Fallot arises from anterosuperior displacement of the infundibular (conal) septum, which divides the conus unequally. The pulmonary infundibulum ends up narrow, the aorta wide and overriding an outlet VSD, and the right ventricle hypertrophies. It is cyanotic and the most common cyanotic heart defect beyond the newborn period.
Embryological cause
- Weeks 5–7: the conal septum forms displaced forward and upward instead of lining up with the muscular septum.
- The pulmonary conus is small: infundibular and often valvular stenosis.
- The malaligned septum cannot close the interventricular foramen: an outlet VSD.
- The wider aortic root sits over the VSD, receiving blood from both ventricles.
- Right ventricular hypertrophy is secondary to the obstruction.
- Linked to faults in cardiac neural crest and the second heart field.
Markers
- Cardiac neural crest
- TBX1 (22q11.2)
- NKX2-5
- JAG1
How it presents
- Systolic ejection murmur at the upper left sternal border (from the stenosis, not the VSD).
- Variable cyanosis from birth or in the first months.
- Hypercyanotic ('tet') spells with irritability, deep fast breathing and worsening cyanosis.
- Squatting in older unrepaired children; clubbing with long-standing cyanosis.
Diagnosis
- Prenatal or postnatal echocardiography: outlet VSD with an overriding aorta and a narrow pulmonary outflow.
- Newborn pulse oximetry screening.
- Chest X-ray: boot-shaped heart with small pulmonary hila.
Test yourself
Think of the answer, then tap the question to see it.
What is the embryological cause of tetralogy of Fallot?
Anterosuperior displacement of the infundibular (conal) septum, which divides the conus unequally.
What are the four findings of tetralogy of Fallot?
Infundibular pulmonary stenosis, VSD, overriding aorta and right ventricular hypertrophy.
Why does squatting relieve cyanosis in tetralogy?
It raises systemic vascular resistance, cuts right-to-left flow across the VSD and sends more blood to the lungs.
What shape is the heart on a chest X-ray in tetralogy?
Boot-shaped, with the apex lifted by right ventricular hypertrophy.
See it in 3D
The structures of Durgical's 3D anatomy where it is found.
- Right ventricle
- Interventricular septum
- Pulmonary trunk
- Ascending aorta
- Pulmonary anterior cusp
- Pulmonary left cusp
- Pulmonary right cusp
- Left ventricle
Related
- Outflow tract: the spiral aorticopulmonary septum and neural crest
- Ventricular septal defect
- Persistent truncus arteriosus
- Transposition of the great arteries
- Neural crest cells and their derivatives
- 22q11.2 deletion (DiGeorge): the third and fourth pouches
Related conditions
Educational content on embryonic development; not a guide to prenatal diagnosis or to medicines in pregnancy and no substitute for a health professional's advice. No doses are given.
Sources: Langman's Medical Embryology, 13th ed., ch. 13: Cardiovascular System (Wolters Kluwer, 2015); Tetralogy of Fallot | Congenital Heart Defects (CHDs) | CDC (Centers for Disease Control and Prevention, 2026).
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Representative model, for educational purposes only.