Pulmonary-to-systemic flow ratio

Qp:Qs · RVOT diameter and VTI (PSAX) for Qp; LVOT diameter and VTI (PLAX/A5C) for Qs — sites depend on the shunt level

Normal range & thresholds

Qp:Qs 1.0 normal. ≥1.5 with RV volume overload indicates a haemodynamically significant left-to-right shunt; closure of a secundum ASD is indicated (class I) when pulmonary vascular resistance is not prohibitive ^[25,175].

Pathophysiology

In a shunt one circuit carries blood twice. For an ASD or VSD, Qp is measured across the RVOT and Qs across the LVOT; for a PDA the extra flow passes through the left heart, so Qp is measured at the LVOT and Qs at the RVOT. The ratio quantifies the volume load the receiving chambers bear ^[175].

Raised by

• Atrial septal defect (secundum, sinus venosus, primum), partial anomalous pulmonary venous return
• VSD, PDA, ruptured sinus of Valsalva aneurysm, coronary fistula

Lowered by

• Right-to-left shunting (Eisenmenger physiology, severe PH, cyanotic heart disease) gives Qp:Qs <1

Technique & pitfalls

• RVOT diameter in mid-systole just proximal to the pulmonary valve, with the PW sample at the same level.
• Measure both flows in the same rhythm and heart rate; average several beats.
• Confirm with agitated saline, colour Doppler and subcostal imaging; CMR phase contrast or catheter oximetry adjudicate ^[175].

Pseudo-change & artefact

• Diameter errors are squared twice — a 2 mm RVOT error alone changes Qp by ≈20%.
• Coexisting regurgitation (PR, AR) inflates the flow at that site.
• Sampling position mismatch between diameter and VTI.

Treatment thresholds

• ASD: RV dilatation with Qp:Qs ≥1.5 → closure (class I) if PVR <3 WU; closure is contraindicated in Eisenmenger physiology ^[25].
• VSD, PDA: left-heart volume overload and Qp:Qs guide closure decisions ^[25].

Next step

• Next: subcostal and TOE imaging of the septum and pulmonary veins; estimate PA pressure and PVR.
• Then: CMR or catheterisation when echo and clinical picture disagree.

Drugs

None change the shunt; pulmonary vasodilators in shunt-related PAH are specialist therapy.

Reversibility

RV volume overload regresses after closure, most completely when closure is early ^[25].