Rapid Review·Cardiovascular
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PATHOLOGY
T1Must knowCongenital Heart Disease: Left-To-Right Shunts
P301
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Three acyanotic lesions that can reverse, and the syndrome that names the reversal.
Key takeaways
The acyanotic shunts
Left-to-right shunts are acyanotic at presentation
cyanosis appears years later if uncorrected. Frequency: VSD > ASD > PDA.VSD
an abnormal communication between the ventricles, most often through the membranous part of the septum.- Asymptomatic at birth, may declare itself weeks later or stay silent for life. Most smaller defects close on their own.
- Larger untreated defects raise pulmonary blood flow and overload the LV, leading to Eisenmenger syndrome and heart failure.
- Holosystolic, harsh murmur loudest at the tricuspid area; it may be inaudible right after birth, because high pulmonary vascular resistance limits the left-to-right flow.
- Oxygen saturation steps up in the RV and pulmonary artery.
ASD
a defect in the interatrial septum, i.e. a patent foramen ovale that never fused. Symptoms range from none to heart failure.- Systolic ejection murmur with a wide, fixed split S2. The ejection murmur is early-peaking and pulmonic, from the relative pulmonary stenosis of the extra flow.
- Ostium secundum defects are the most common and usually isolated; ostium primum is rarer and usually accompanies other anomalies.
- Oxygen steps up in the RA, and onward through the RV and pulmonary artery.
- Associated with Down syndrome, and can cause paradoxical emboli during transient shunt reversal, classically while lifting weights.
Atrioventricular septal defect (endocardial cushion defect)
the endocardial cushions fail to fuse, leaving an ostium primum ASD, a VSD, and a single common AV valve. Strongly associated with Down syndrome. Auscultation gives AV valve regurgitation (holosystolic, best at the apex) and a mid-diastolic rumble from the increased pulmonary venous return.PDA
normal in utero, and normally closes only after birth.- In the fetus the shunt is right to left, which is normal. After birth pulmonary vascular resistance falls and the shunt becomes left to right, driving progressive RVH and/or LVH and heart failure.
- Continuous machine-like murmur, best at the left infraclavicular area and loudest at S2. Patency is maintained by prostaglandin synthesis and low oxygen tension.
- Risk factors: congenital rubella and prematurity.
- Uncorrected, it causes late differential cyanosis of the lower extremities only, because the duct delivers deoxygenated blood distal to the left subclavian.
| Feature | VSD | ASD | PDA |
|---|---|---|---|
| Frequency | Most common | Middle | Least |
| Murmur | Holosystolic, harsh at the left lower sternal border | Systolic ejection with a wide, FIXED split S2 | Continuous machine-like |
| Oxygen step-up at | RV | RA | Pulmonary artery |
| Association | 22q11, fetal alcohol syndrome | Down syndrome | Congenital rubella, prematurity |
| Late finding | Eisenmenger | Paradoxical emboli | Differential cyanosis of the lower limbs |
Eisenmenger syndrome, the endpoint all three share
Uncorrected left-to-right shunt
VSD, ASD, or PDA.Raised pulmonary blood flow
causes irreversible pathologic remodeling of the pulmonary vasculature.Pulmonary arterial hypertension.
Compensatory RVH.
RV pressure exceeds LV pressure, so the shunt reverses to right-to-left.
The result, late cyanosis, clubbing, polycythemia
the age of onset depends on the size and severity of the original shunt.Mehlman's questions
| Oxygen increases between | Diagnosis | Why |
|---|---|---|
| SVC and RA | ASD | Oxygenated blood crosses LA to RA |
| RA and RV | VSD | Oxygenated blood crosses LV to RV |
| RV and pulmonary artery | PDA | Oxygenated blood crosses aorta to pulmonary artery |
Chamber saturations rise between the RA and the RV. Which lesion, and what murmur goes with it?
A VSD: oxygenated blood crosses from LV to RV. Expect a harsh holosystolic murmur at the left lower sternal border, louder once pulmonary vascular resistance falls.
How it's tested
Mehlman notes the exam repeatedly gives an oxygen saturation diagram and asks you to infer the lesion, and the logic is unambiguous: a step-up in the right atrium can only be an ASD, in the right ventricle only a VSD, and in the pulmonary artery only a PDA. The VSD murmur growing louder at one week is a second favourite, and the answer is decreased pulmonary vascular resistance as the lungs open, which widens the left-to-right gradient. A newborn's quiet murmur is therefore not reassuring.
Go deeper
First Aid 2026 — CV/Pathology (p.301) · B&B — Left-to-right shunts & Eisenmenger · Mehlman — HY Cardio (oxygen step-up, VSD murmur timing, repair indications)
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