Physiology of congenital shunts !
- Mechanism
- Exam priority: medium
A hole between the two halves of the heart or the great vessels lets blood flow from the high-pressure side to the low-pressure side.
Updated 2026-11-01
See it in 3D: Interatrial septum · Interventricular septum · Ligamentum arteriosum · Pulmonary trunk · Right ventricle
What it is
After birth the left heart works at higher pressure than the right, so septal defects and a patent ductus send blood left to right and overload the lungs. If pulmonary pressure rises enough, flow reverses and cyanosis appears (Eisenmenger). In tetralogy of Fallot, right ventricular outflow obstruction makes the shunt right to left from the start.
How it works, step by step
- At birth pulmonary resistance falls and systemic resistance rises: the left side ends up at higher pressure.
- Through any opening blood passes from left to right and recirculates through the lungs.
- The chamber that receives the extra volume dilates: right side in ASD; left side in VSD and ductus (the blood returns via the pulmonary veins).
- Excess flow and pressure in the pulmonary bed thicken the intima and media of the arterioles.
- Once pulmonary resistance exceeds systemic, the shunt reverses and venous blood reaches the aorta: cyanosis.
- In tetralogy of Fallot the narrow right outflow diverts RV blood into the aorta across the VSD from the start.
Equations
- SaO₂
- systemic arterial saturation
- SvO₂
- mixed venous saturation (vena cava or RA before mixing)
- SpvO₂
- pulmonary vein saturation
- SpaO₂
- pulmonary artery saturation
It follows from the Fick principle; a Qp/Qs of 2 means the lungs get twice the body's flow.
The shunts that get asked
| Defect | Initial direction | Overloaded chambers | Classic sign |
|---|---|---|---|
| Atrial septal defect | Left to right | RA, RV and lungs | Fixed split S2; pulmonary flow murmur |
| Ventricular septal defect | Left to right | Lungs, LA and LV | Holosystolic murmur at the lower left sternal border |
| Patent ductus arteriosus | Aorta to pulmonary artery | Lungs, LA and LV | Continuous "machinery" murmur; bounding pulses |
| Tetralogy of Fallot | Right to left | RV (pressure overload) | Cyanosis, hypercyanotic spells, "boot-shaped" heart |
Clinical notes
- When a ductus reverses, cyanosis shows in the feet but not the hands (differential cyanosis), because the ductus enters beyond the left subclavian artery.
- Patent ductus arteriosus has been reported more often in children born at high altitude, as in much of the Andean region.
- Right-to-left shunts let a venous clot reach the brain (paradoxical embolism).
Test yourself
Think of the answer, then tap the question to see it.
Why does an atrial septal defect dilate the right ventricle and not the left?
Blood crosses from LA to RA, then to the RV and lungs; it returns to the LA and loops again without passing the LV, which carries only the systemic output.
What change turns a left-to-right shunt into a right-to-left one?
High pulmonary flow remodels the arterioles and raises pulmonary resistance until it exceeds systemic resistance (Eisenmenger syndrome): flow reverses and cyanosis appears.
Why does a child with tetralogy of Fallot squat during a cyanotic spell?
Squatting kinks the leg arteries and raises systemic resistance, so less RV blood goes into the aorta and more goes to the lungs.
In a reversed patent ductus, why are the feet cyanotic and the hands not?
The ductus joins the aorta beyond the left subclavian artery: desaturated blood goes down the descending aorta to the legs.
See it in 3D
The structures of Durgical's 3D anatomy where it happens.
Related topics
- Murmurs and the hemodynamics of valve disease
- Heart sounds and where they come from
- Flow, pressure and resistance: Poiseuille and turbulent flow
Related conditions
- Atrial septal defect
- Ventricular septal defect
- Patent ductus arteriosus
- Tetralogy of Fallot
- Coarctation of the aorta
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: 19.5 Development of the Heart (OpenStax (Rice University), 2022); Ventricular Septal Defect (StatPearls (NCBI Bookshelf), 2024); Patent Ductus Arteriosus (StatPearls (NCBI Bookshelf), 2023); Tetralogy of Fallot (StatPearls (NCBI Bookshelf), 2024); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 23). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).
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Representative model, for educational purposes only.