Mixing and Streaming in Single-Ventricle Physiology

In single-ventricle hearts, intracavitary blood is often described as if it “fully mixes.” In reality, anatomic relationships among atrial inlets, the ventricular cavity, and the great arteries create preferential flow paths—“streaming.” Classic series showed complete mixing in only ~16% of patients without severe pulmonary stenosis, with favorable streaming in ~58% directing oxygenated blood preferentially to the systemic circulation [1]. Thus, systemic oxygenation depends as much on where each inflow stream goes as on total Qp:Qs.

Determinants of Streaming

• Great-artery position & outlet chamber location. Proximity of the aortic outlet to a given atrial inflow biases that stream toward the aorta; proximity to the pulmonary outlet favors the pulmonary circuit. Favorable streaming is most common in L-transposition and when the outlet chamber is lateral rather than anterior [2, 3].
• AV-valve morphology/orientation. Orifice angle, offset, and chordal geometry steer jets toward one outlet.
• Septal remnants & trabeculations. Cristae and bulboventricular ridges deflect inflow across the inlet–outlet interface, accentuating preferential paths.
• Physiologic loading. PVR/SVR or shunt changes alter the quantity in each stream but usually do not reverse the direction set by anatomy; saturations may plateau despite major PVR shifts if geometry continues to send systemic venous blood toward the aorta.

Unfavorable vs Favorable Streaming

• Unfavorable (more cyanosis). Aorta close to right-atrial inflow → systemic venous blood enters the aorta → desaturation (e.g., DILV with D-loop, D-TGA with VSD) [2, 3].
• Favorable (less cyanosis). Aorta close to left-atrial inflow → pulmonary venous blood enters the aorta → higher systemic saturation (e.g., DILV with L-loop, L-TGA with VSD) [2, 3].

Key point. In single-ventricle physiology it is insufficient to assume “complete mixing”; streaming materially affects oxygenation and decisions [1–3].

Imaging & Quantification

Beyond standard Qp:Qs estimation, 4D-flow MRI and CMR oximetry can visualize inflow jets, map streamlines, and quantify oxygenation distributions across outlets in vivo [4]. These data reinforce that streaming can uncouple saturation from Qp:Qs, explaining why patients with similar flow ratios may present with different systemic saturations [5].

Clinical Implications

1. Interpreting oxygen saturation. Two patients with the same Qp:Qs can differ in SpO₂ because of streaming; “refractory cyanosis” may reflect unfavorable geometry, not inadequate pulmonary flow [5].
2. Choice and tuning of palliation.
• Systemic-to-pulmonary shunts / PAB. If geometry directs systemic venous blood toward the aorta, larger increases in Qp may yield smaller-than-expected SpO₂ gains; with favorable streaming, modest Qp increases can give disproportionate improvement [1–3, 5].
• Atrial septal intervention. Enlarging or repositioning the interatrial communication can re-vector inflow toward the pulmonary outlet when atrial mixing is restrictive [2–5].
3. Post-Glenn/Fontan planning. Preferential streaming persists after cavopulmonary connections and influences hepatic factor delivery and pulmonary arteriovenous malformation (PAVM) burden; bubble contrast echocardiography correlates with systemic oxygenation in Glenn patients and helps phenotype PAVMs/streaming consequences [6].
4. Assessment strategy. Combine echo (including contrast), 3D/4D echo, and phase-contrast/4D-flow MRI to determine whether a saturation problem is geometric (streaming) vs hemodynamic (flow magnitude) [4, 5].

Summary

Single-ventricle circulation is governed not only by how much blood flows but also where each inflow stream goes. Outlet-inlet geometry defines unfavorable vs favorable streaming and thereby the degree of systemic desaturation. Recognizing—and when feasible, modifying—streaming sharpens diagnosis, sets realistic SpO₂ targets, and optimizes palliation timing and type [1–6].

References

[1] Rahimtoola SH, Ongley PA, Swan HJC. The hemodynamics of common (or single) ventricle. Circulation. 1966;34(1):14-23.

[2] Macartney FJ, Partridge JB, Scott O, Deverall PB. Common or single ventricle. An angiocardiographic and hemodynamic study of 42 patients. Circulation. 1976;53(3):543-554.

[3] Mocellin R, Sauer U. Haemodynamics studies in patients with univentricular hearts. Herz. 1979;4(2):242-247.

[4] Bissell MM, Dyverfeldt P, Carlhäll C-J, et al. 4D flow cardiovascular magnetic resonance consensus statement: 2023 update. J Cardiovasc Magn Reson. 2023;25:40.

[5] Francis DP, Willson K, Thorne SA, Davies LC, Coats AJ. Oxygenation in patients with a functionally univentricular circulation and complete mixing of blood: are saturation and flow interchangeable? Circulation. 1999;100(21):2198-2203.

[6] Phimister A, Bushee C, Merbach M, Challa SA, Pan AY, Spearman AD. Objective quantification of bilateral bubble contrast echocardiography correlates with systemic oxygenation in patients with single ventricle circulation. J Cardiovasc Dev Dis. 2024;11(3):84.