Septal Defects & Shunt Quantification
Key Takeaways
- Secundum ASD (fossa ovalis, mid-septum) accounts for roughly 75% of atrial septal defects and is the type most amenable to percutaneous device closure.
- Perimembranous VSD, located adjacent to the tricuspid/aortic valves, is the most common ventricular septal defect, accounting for roughly 80% of cases.
- Qp/Qs is calculated as (RVOT CSA x RVOT VTI) divided by (LVOT CSA x LVOT VTI), using diameters and pulsed-wave Doppler velocity-time integrals from each outflow tract.
- A Qp/Qs ratio of 1.5 or greater defines a hemodynamically significant left-to-right shunt; a ratio of 2.0 or greater is a common threshold favoring intervention.
- In an agitated-saline bubble study, microbubbles crossing into the left atrium within 3 cardiac cycles of right atrial opacification indicate an intracardiac shunt (e.g., PFO or ASD), while later appearance (roughly 4-8 cycles) suggests an intrapulmonary shunt.
Atrial Septal Defects (ASD)
An ASD is a communication in the interatrial septum that permits shunting, almost always left-to-right at rest because left atrial (LA) pressure normally exceeds right atrial (RA) pressure. ASDs are classified by embryologic location, which determines both their echocardiographic appearance and their common associations.
| ASD Type | Location | Approx. Frequency | Key Association |
|---|---|---|---|
| Secundum | Fossa ovalis, mid-septum | ~75% | Most common; best candidate for device closure |
| Primum | Inferior septum, adjacent to AV valves | ~15-20% | Cleft anterior mitral leaflet; part of the AV canal spectrum |
| Sinus venosus (superior/inferior) | Superior or inferior to the fossa, near SVC/IVC entry | ~5-10% | Partial anomalous pulmonary venous return (PAPVR); poorly seen on standard TTE, often needs TEE or CMR |
| Unroofed coronary sinus | Deficiency of the wall separating the coronary sinus from the LA | Rare | Persistent left superior vena cava |
Secundum ASDs are imaged best in the subcostal four-chamber and short-axis views, where the septum is perpendicular to the ultrasound beam and dropout artifact is minimized -- a parasternal or apical four-chamber view alone can create false septal dropout and should never be used in isolation to diagnose an ASD.
Ventricular Septal Defects (VSD)
VSDs are classified by their location within the interventricular septum.
| VSD Type | Location | Approx. Frequency | Notes |
|---|---|---|---|
| Perimembranous | Membranous septum, beneath the aortic and tricuspid valves | ~80% | Most common type; may develop an aneurysm of surrounding septal tissue that partially seals the defect |
| Muscular (trabecular) | Muscular septum, any level | ~5-20% | Can be multiple ("Swiss cheese" septum); many close spontaneously |
| Inlet | Posterior septum, AV-canal type | Less common | Associated with complete AV canal defects and Down syndrome |
| Outlet (supracristal/conal) | Immediately below the pulmonic valve | Less common overall | Risk of aortic cusp (usually right coronary cusp) prolapse and progressive aortic regurgitation |
Unlike an ASD, a VSD shunts a higher-pressure LV into a lower-pressure RV, so even a small defect generates a high-velocity systolic jet on color and spectral Doppler; defect size on 2D imaging, not just Doppler velocity, determines the physiologic significance.
Quantifying the Shunt: Qp/Qs
The pulmonary-to-systemic flow ratio (Qp/Qs) is the standard echocardiographic method for sizing a shunt. Because flow through the right ventricular outflow tract (RVOT) represents pulmonary blood flow (Qp) and flow through the left ventricular outflow tract (LVOT) represents systemic blood flow (Qs), the ratio is:
Qp/Qs = (RVOT CSA x RVOT VTI) / (LVOT CSA x LVOT VTI)
Each outflow tract's cross-sectional area (CSA) is derived from its diameter (CSA = pi x (diameter/2)^2), measured in mid-systole from a parasternal view, and paired with its own pulsed-wave Doppler velocity-time integral (VTI) traced from the corresponding outflow view. Using matched, outflow-tract-specific diameter and VTI measurements (rather than mixing sites) is essential for an accurate ratio.
| Qp/Qs | Interpretation |
|---|---|
| ~1.0 | No significant shunt |
| <1.5 | Trivial to small shunt |
| 1.5-1.9 | Hemodynamically significant shunt |
| >=2.0 | Large shunt; common threshold favoring closure |
A Qp/Qs of 1.5 or higher, especially when paired with echocardiographic evidence of right heart volume overload (RV/RA enlargement, paradoxical septal motion), is the standard trigger for referral to consider ASD or VSD closure.
Eisenmenger Physiology
A large, long-standing, uncorrected left-to-right shunt exposes the pulmonary vasculature to chronically elevated flow and pressure. Over years, this drives progressive pulmonary vascular remodeling and rising pulmonary vascular resistance (PVR). Once PVR rises to meet or exceed systemic vascular resistance (SVR), the pressure gradient across the defect reverses and the shunt becomes right-to-left -- Eisenmenger syndrome. Clinically this produces cyanosis, clubbing, and erythrocytosis. Critically, once Eisenmenger physiology is established, closing the defect is contraindicated: the defect has become the RV's only pop-off valve against suprasystemic pulmonary pressures, and closure precipitates acute right heart failure.
Agitated-Saline (Bubble) Study for PFO
When a patent foramen ovale (PFO) is suspected but color Doppler shunt flow is not visualized (common, since resting LA pressure keeps the flap-valve closed), agitated saline contrast is used. Saline is agitated to create microbubbles, injected through a peripheral IV, and tracked on 2D imaging (typically apical four-chamber) as it opacifies the right heart. A Valsalva maneuver, released just as contrast fills the RA, transiently raises RA pressure above LA pressure and forces the PFO flap open. Timing of left-heart bubble appearance differentiates shunt type: bubbles appearing in the LA within 3 cardiac cycles of RA opacification indicate an intracardiac shunt (PFO or ASD); bubbles appearing later (roughly 4-8 cycles, beyond 3-5) indicate an intrapulmonary shunt, such as a pulmonary arteriovenous malformation, where blood must transit the pulmonary capillary bed before reaching the left heart. Imaging should begin just before contrast reaches the RA and continue for at least several cardiac cycles afterward so early bubble arrival is not missed; a poorly timed or incomplete Valsalva release is the most common technical reason for a false-negative bubble study.
Putting Quantification Together
A comprehensive septal-defect assessment combines three tools: 2D/color Doppler to localize and size the anatomic defect, Doppler-derived Qp/Qs to quantify shunt burden, and (when the defect is not directly visualized) agitated saline contrast to confirm an atrial-level shunt. Right heart size and septal motion should always be checked against the calculated Qp/Qs -- a significant ratio without RV/RA enlargement should prompt re-measurement of the outflow tract diameters, since diameter is squared in the CSA calculation and is the most error-sensitive term.
Which formula correctly calculates the pulmonary-to-systemic flow ratio (Qp/Qs) by echocardiography?
A 45-year-old with a secundum ASD has a calculated Qp/Qs of 2.3. What does this value represent?