A shunt is proved by FLOW, not by a hole: measure Qp:Qs with phase contrast, look for the right-heart volume load, then name the defect — and in a sinus venosus defect, find the anomalous pulmonary vein.
Orient first
- Qp:Qs = pulmonary flow (main PA) ÷ systemic flow (ascending aorta); ≥ 1.5 is haemodynamically significant (verify the local threshold).
- Atrial-level shunts (ASD, PAPVR) dilate the RA and RV; ventricular-level shunts (VSD) and PDA load the LA and LV.
- Secundum ASD is central; sinus venosus defects sit at the SVC–RA junction and almost always come with right upper pulmonary vein drainage to the SVC.
Acquire the study
- Cine stack; phase contrast in the main PA and ascending aorta (and each branch PA / pulmonary vein when needed); contrast MRA or CTA for the pulmonary veins; atrial septum cine stack.
The manoeuvre
- Phase contrast: forward flow in the main PA and ascending aorta → Qp:Qs; check consistency with the stroke volumes from cine.
- Volumes: RV (atrial shunt) or LV and LA (ventricular shunt/PDA) dilatation, indexed to BSA.
- Atrial septum cine stack: defect size and rims (secundum) or the SVC–RA junction (sinus venosus); coronary sinus unroofing.
- MRA/CTA: trace every pulmonary vein to the left atrium; an anomalous vein to the SVC, RA, IVC (scimitar) or innominate vein.
- Ventricular septum and the arterial duct: perimembranous or muscular VSD jet; PDA from the proximal descending aorta to the PA.
What confirms it
- A measured Qp:Qs > 1 with a named defect and matching chamber volume load.
What licenses you to exclude it
- Qp:Qs of 1.0 with normal right and left chamber volumes excludes a haemodynamically significant shunt; a small defect may still exist.
The classic misread
- Missing a sinus venosus defect and its anomalous vein because the septal stack stopped below the SVC.