Shunt fraction (Qp/Qs)

Clinical background

The decision to close an intracardiac shunt — for example an atrial septal defect, a ventricular septal defect or a patent ductus arteriosus — rests in practice on a single question: is the shunt large enough to justify the procedure? Clinical symptoms are unreliable, particularly in adults, in whom the volume overload may be long-standing and well compensated before heart failure or pulmonary hypertension appears. Echocardiographic quantification of the shunt fraction (Qp/Qs) provides a non-invasive measure of the haemodynamic significance of the shunt and, together with assessment of pulmonary vascular resistance, forms the basis for the indication for closure under current guidelines [1].

Calculating the shunt fraction

The shunt fraction is calculated as the ratio of pulmonary flow (Qp) to systemic flow (Qs), where each flow is the product of the cross-sectional area of the outflow tract and its velocity–time integral (VTI):

$$\text{Qp/Qs} = \frac{\pi \times \left(\frac{D_{\text{RVOT}}}{2}\right)^{2} \times \text{VTI}$${\text{RVOT}}}{\pi \times \left(\frac{D{\text{LVOT}}}{2}\right)^{2} \times \text{VTI}_{\text{LVOT}}}

Since $\pi$ cancels, the formula can be simplified to:

$$\text{Qp/Qs} = \frac{D_{\text{RVOT}}^{2} \times \text{VTI}$${\text{RVOT}}}{D{\text{LVOT}}^{2} \times \text{VTI}_{\text{LVOT}}}

The variables are:

• RVOT diameter: the diameter of the right ventricular outflow tract, measured in the parasternal short-axis view at the pulmonary valve insertion, in millimetres.
• RVOT VTI: the velocity–time integral at the pulmonary valve, measured with pulsed-wave Doppler from the parasternal short axis, in centimetres.
• LVOT diameter: the diameter of the left ventricular outflow tract, measured in the apical five-chamber view at the aortic valve insertion, in millimetres.
• LVOT VTI: the velocity–time integral at the aortic valve, measured with pulsed-wave Doppler from the apical five-chamber view, in centimetres.

The method was developed in the 1980s by Sanders and colleagues as a non-invasive technique for estimating Qp/Qs with combined two-dimensional echocardiography and Doppler, and was validated against the Fick method at cardiac catheterisation [2]. Vargas Barron and colleagues applied the method in children with atrial septal defect, ventricular septal defect and patent ductus arteriosus and found good agreement with catheterisation and radionuclide angiography [3].

Interpretation in practice

The clinically decisive threshold is a Qp/Qs ≥ 1.5, which is generally taken to denote a haemodynamically significant left-to-right shunt and hence an indication for closure, provided that pulmonary vascular resistance is not excessively raised [1]. The interpretation in practice is summarised below.

A shunt fraction below 1.5 does not exclude symptoms from another cause, and the decision on follow-up should be guided by symptoms and cardiac chamber size rather than by the ratio alone. In older patients, or patients with signs of left ventricular dysfunction, measurement of left-sided filling pressure during test balloon occlusion is additionally recommended before definitive closure, since closure is not always tolerated [4].

Validation and performance

The Doppler-based method for Qp/Qs was originally validated against invasive Fick oximetry at cardiac catheterisation. In the early validation cohort of Sanders and colleagues, which comprised 33 paediatric and adult patients with various congenital heart defects, good correlation was reported between Doppler- and Fick-based Qp/Qs measurements [2]. Vargas Barron and colleagues studied 21 children with atrial septal defect, ventricular septal defect or patent ductus arteriosus and found corresponding agreement between Doppler-estimated Qp/Qs and catheterisation or radionuclide angiography [3].

In clinical practice, echocardiography has largely replaced routine diagnostic catheterisation for shunt quantification. Torres notes that diagnostic catheterisation is usually not indicated in atrial septal defect unless the echocardiogram suggests pulmonary hypertension, and that in most cases the haemodynamics can be estimated by echocardiography alone [4]. Galzerano and colleagues emphasise the role of echocardiography as the primary imaging modality in simple congenital heart disease in adults, where quantification of shunt dynamics and ventricular function allows an individualised therapeutic strategy [5].

An important methodological weakness is that the outflow tract diameter enters the formula squared, which means that a 10 per cent measurement error in diameter produces an error of approximately 20 per cent in the flow calculation. This makes measurement precision in the RVOT and LVOT the single most important determinant of the reliability of the calculation.

Limitations

The method presupposes that flow in the RVOT and LVOT represents total pulmonary and systemic flow respectively. This does not hold in the following situations:

• Outflow tract obstruction: stenosis of the pulmonary or aortic valve produces accelerated flow profiles that do not correctly reflect volumetric flow, and the VTI becomes misleading [2].
• Semilunar valve regurgitation: significant pulmonary or aortic regurgitation means that flow at the valve does not correspond to net flow distally, so the ratio becomes inaccurate [2].
• Multiple shunts: if the patient has both an atrial and a ventricular septal defect, or a ventricular septal defect combined with a duct, a Qp/Qs measured at a single site cannot separate the sizes of the individual shunts.
• Complex congenital heart disease: in transposition of the great arteries, hypoplastic left heart syndrome or other complex lesions the flow pathways are not anatomically standard, and the method is not applicable without adaptation.

The most important clinical pitfall is to use Qp/Qs alone as the basis for decision without assessing pulmonary vascular resistance. A significant shunt ratio in a patient with severe pulmonary hypertension may mean that the shunt is partly right-to-left, and closure may then be contraindicated. The 2020 ESC guidelines recommend that pulmonary vascular resistance be evaluated, usually by catheterisation, before definitive closure in patients with suspected pulmonary hypertension [1].

References

1. Baumgartner H, De Backer J, Babu-Narayan SV, et al. 2020 ESC Guidelines for the management of adult congenital heart disease. Eur Heart J. 2021;42(6):563–645. PMID: 32860028
2. Sanders SP, Yeager S, Williams RG. Measurement of systemic and pulmonary blood flow and QP/QS ratio using Doppler and two-dimensional echocardiography. Am J Cardiol. 1983;51(6):952–956. PMID: 6829471
3. Vargas Barron J, Sahn DJ, Valdes-Cruz LM, et al. Clinical utility of two-dimensional doppler echocardiographic techniques for estimating pulmonary to systemic blood flow ratios in children with left to right shunting atrial septal defect, ventricular septal defect or patent ductus arteriosus. J Am Coll Cardiol. 1984;3(1):169–178. PMID: 6690547
4. Torres AJ. Hemodynamic assessment of atrial septal defects. J Thorac Dis. 2018;10(Suppl 24):S2882–S2889. PMID: 30305948
5. Galzerano D, Pergola V, Eltayeb A, et al. Echocardiography in simple congenital heart diseases: Guiding adult patient management. J Cardiovasc Echogr. 2023;33(4):171–182. PMID: 38486692