Anatomical structure of the defect
As the name implies, classical tetralogy of Fallot includes exactly four interrelated anatomical components. Understanding these structural anomalies is essential for explaining the subsequent clinical picture and hemodynamic changes.
- Large ventricular septal defect (VSD). The presence of a large opening between the right and left ventricles creates conditions for pathological blood mixing.
- Pulmonary stenosis. A mechanical obstacle forms, making it difficult for venous blood to adequately outflow from the right ventricle into the pulmonary circulation (lungs).
- Dextroposition of the aorta. The aorta is shifted to the right and positioned directly over the ventricular septal defect. In clinical practice, this position is often referred to as an "overriding aorta".
- Compensatory hypertrophy of the right ventricle (RV). Due to the need to push blood through the narrowed pulmonary artery, the muscular wall of the right ventricle thickens in an attempt to compensate for the increased workload.
Mechanism of cyanosis development and clinical presentation
The main hemodynamic feature of this defect is the shunt of oxygen-depleted venous blood from the right ventricle directly into the aorta via the VSD. Blood literally bypasses the lungs due to pulmonary stenosis, leading to systemic oxygen deprivation.
Main complaints and patient appearance:
- Dyspnea and fatigue. Patients complain of exercise intolerance and rapid fatigue.
- Pronounced cyanosis. The skin acquires a bluish tint due to the circulation of venous blood in the systemic circulation.
- Specific posture. Patients instinctively assume a "squatting" position. This provides noticeable relief because this posture reduces venous return to the heart and alters total vascular resistance.
- Clubbing of the fingers (Hippocratic fingers). Due to prolonged oxygen starvation (chronic hypoxia), the terminal phalanges of the fingers become bulbously thickened.
- Developmental delay. Children often show a lag in physical development.
- "Cardiac hump". In some patients, a bulging chest deformity can be observed upon visual inspection. It forms as a direct consequence of pronounced right ventricular hypertrophy.
Diagnostic criteria
To confirm the diagnosis in propedeutics, a set of physical, laboratory, and instrumental methods is used.
- Heart auscultation: Auscultation reveals a loud systolic murmur. The epicenter of the murmur is located to the left of the sternum, in the II–III intercostal space. This sound phenomenon is caused by blood passing through the narrowed pulmonary artery.
- Complete blood count (CBC): Secondary erythrocytosis develops—a compensatory body reaction to constant hypoxia (the bone marrow produces more red blood cells to carry oxygen). A decrease in the erythrocyte sedimentation rate (ESR) is also characteristic.
- Instrumental studies: The main method for verifying the defect is echocardiography (EchoCG) with Doppler mode. It allows visualization of the anatomy of all defects and accurate assessment of the degree of pulmonary stenosis.
Disease course and management tactics
In late stages, the disease leads to severe hemodynamic disturbances. The most formidable complications are the development of severe right ventricular failure and thromboembolism (including due to blood thickening against the background of erythrocytosis).
Therapeutic methods cannot eliminate anatomical defects. Treatment of tetralogy of Fallot is exclusively surgical, aimed at radical correction of intracardiac anatomy.
Briefly: answers to topical questions
What is the pathogenesis of hypercyanotic spells in tetralogy of Fallot?
Hypercyanotic spells occur due to a sudden spasm of the right ventricular outflow tract. This leads to acute cerebral ischemia, clinically manifested by a sudden onset, psychomotor agitation, and increasing tachypnea and cyanosis.
What changes are detected on the ECG in patients with tetralogy of Fallot?
Given that tetralogy of Fallot features right ventricular hypertrophy, the ECG may show signs of right ventricular hypertrophy. Right ventricular hypertrophy is characterized by a rightward deviation of the electrical axis of the heart (rightogram); visual signs include a tall R wave in lead III and a deep S wave in lead I.
What shape does the cardiac silhouette acquire on a chest X-ray in tetralogy of Fallot?
The cardiac silhouette acquires an aortic configuration shaped like a "wooden shoe" ("cœur en sabot").
How does the second sound (S2) change over the pulmonary artery during auscultation?
The second sound over the pulmonary artery is locally diminished. In sources, this is associated with pulmonary stenosis; pulmonary stenosis is one of the components of tetralogy of Fallot.
To remember the 4 components, use the acronym V-A-H-P: Ventricular septal defect, Overriding Aorta, Hypertrophy of RV, Pulmonary stenosis.
Class and exam questions
Why is tetralogy of Fallot called a "blue" defect?
Due to the shunting of venous blood from the right ventricle into the aorta (bypassing the pulmonary circulation), the blood in the arterial bed is depleted of oxygen, which visually manifests as pronounced skin cyanosis.
What is the reason for relief in the "squatting" position?
This position mechanically reduces venous blood return to the right side of the heart and alters vascular resistance, which temporarily improves hemodynamics and reduces dyspnea.
What is the cause of the formation of the "cardiac hump"?
It forms as a result of pronounced compensatory hypertrophy and enlargement of the right ventricle, which deforms the compliant chest wall.
Why does ESR decrease and erythrocytosis occur in this defect?
The body tries to compensate for chronic oxygen deficiency (hypoxia) by producing an excess of red blood cells (secondary erythrocytosis). Due to blood thickening, the ESR naturally decreases.
What else to study on this topic
- Hemodynamic differences between "blue" and "pale" congenital heart defects
- Mechanism of digital clubbing development in chronic hypoxia
- EchoCG criteria for the severity of pulmonary stenosis
- Mechanisms of thromboembolic complications against the background of secondary erythrocytosis
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