1. Introduction
The expression single-ventricle physiology often evokes a simple model in which pulmonary venous and systemic venous blood enter one ventricular chamber, mix completely, and are then distributed to the systemic and pulmonary arteries. This model is useful for understanding parallel circulation, but it is not a universal description of intracardiac flow. The direction of blood entering the ventricle, the three-dimensional arrangement of the atrioventricular valves, and the position of the arterial outlets may direct relatively oxygen-rich and oxygen-poor blood toward different great arteries.
Functionally univentricular hearts are morphologically heterogeneous. Many contain a dominant pumping chamber and a smaller outlet chamber rather than literally one ventricular cavity. Double-inlet left ventricle (DILV), in which both atrioventricular connections enter a morphologically left ventricle, provides a particularly clear example of how preferential flow can coexist with a common ventricular chamber [1,2].
The central clinical principle is that an arterial oxygen saturation reflects not only pulmonary-to-systemic blood flow but also which venous stream is preferentially directed toward the aorta. Understanding this distinction can change the interpretation of cyanosis and the choice of intervention.
2. Complete Mixing Versus Preferential Streaming
2.1 The complete-mixing model
In the conventional model, pulmonary venous return (Qp) and systemic venous return (Qs) reach a common ventricular reservoir. After uniform admixture, the aortic and pulmonary arterial blood have the same oxygen content, provided that no subsequent shunt or additional oxygenated inflow alters either outlet.
For similar hemoglobin concentrations in both venous streams, the fully mixed ventricular oxygen saturation can be approximated by:
Smix ≈ (Qp × SpvO₂ + Qs × SvO₂) / (Qp + Qs)
Here, SpvO₂ denotes pulmonary venous saturation and SvO₂ denotes systemic mixed venous saturation. This equation is a reference model, not proof that the chamber behaves as a well-stirred reservoir. For example, equal pulmonary and systemic flows with pulmonary venous saturation of 95% and systemic venous saturation of 55% would give a predicted mixed saturation of approximately 75%. The actual aortic saturation could differ if the streams are preferentially routed.
2.2 Why mixing may remain incomplete
Blood entering through two atrioventricular valves is not immediately homogenized. The inflow jets have momentum and direction. During ventricular filling and ejection, they interact with the ventricular wall, trabeculations, vortex structures, the ventricular septal remnant, and the outflow pathways. Relatively coherent streams may therefore maintain different oxygen contents as they approach the semilunar valves.
The actual pattern depends on the orientation and relative size of the inflow jets, ventricular geometry, pressure gradients, timing of contraction, and the geometry of the outlets. In a pathological study of 39 DILV specimens, Meyer and colleagues identified parallel inflow–outflow relationships in 16 hearts and approximately perpendicular relationships in 10. These configurations suggested differing potential for streaming, although anatomical specimens alone cannot establish the in-vivo direction or magnitude of flow [3].
Clinical catheterization studies established long ago that complete mixing should not be assumed. Rahimtoola and colleagues described nonuniform admixture in a historical series of common-ventricle patients, and Macartney and colleagues later demonstrated favorable, unfavorable, and complete-mixing patterns among selected patients with detailed hemodynamic assessment [4,5].
3. Anatomical Determinants of Streaming
3.1 Atrioventricular inflow orientation
The most intuitive determinant is the relationship between each atrioventricular inflow and the aortic outlet. If blood entering from the right atrium travels relatively directly toward the aorta, the systemic arterial output may be enriched in systemic venous blood. Conversely, if the pulmonary venous stream entering through the left atrium is better aligned with the aortic outlet, aortic oxygenation may be relatively preserved.
The term close to an inflow should be interpreted functionally. Spatial proximity on a two-dimensional diagram is less informative than the actual three-dimensional trajectory from the valve orifice through the ventricle toward the outlet. Valve position, inlet angulation, jet direction, ventricular contraction, and intervening septal structures all matter.
3.2 Ventricular looping and great-artery relationships
In DILV, the dominant morphologically left ventricle frequently communicates with a rudimentary morphologically right ventricular outlet chamber through a ventricular septal defect, also termed the bulboventricular foramen. With ventriculoarterial discordance, the aorta often arises from the outlet chamber, so its oxygenation is influenced by the direction of blood passing through that communication [2].
The illustrated D-loop, d-transposition arrangement represents a potential setting for unfavorable streaming: systemic venous inflow is preferentially directed toward the pathway supplying the aorta, producing greater cyanosis. The illustrated L-loop, l-transposition arrangement represents a potential favorable configuration: pulmonary venous inflow is preferentially directed toward the aortic pathway.
These are anatomical teaching examples, not deterministic rules. Ventricular looping, arterial malposition, and atrioventricular connections must be described separately using sequential segmental analysis. In an echocardiographic series of 57 DILV patients, both D-loop and L-loop variants showed important variation in atrioventricular valve morphology, septal defects, and obstruction [6]. Historical hemodynamic studies also found that outlet-chamber orientation, including anterior versus lateral positioning, could be more informative than the simple designation of d- or l-transposition [5,7].
3.3 Outlet chambers and restrictive communications
A restrictive bulboventricular foramen can alter both streaming and effective systemic outflow. When the aorta arises from the outlet chamber, progressive restriction may produce subaortic obstruction, even when the atrioventricular inflow remains unobstructed. Its assessment must therefore encompass anatomical size, Doppler gradients, ventricular loading, and the presence of arch obstruction.
Pulmonary outflow stenosis likewise changes Qp and the momentum of flow directed toward the pulmonary artery. However, an improvement in arterial saturation associated with favorable streaming must not be confused with adequate total systemic oxygen delivery. An anatomical arrangement may favor delivery of oxygen-rich blood to the aorta while a separate lesion limits total pulmonary flow or compromises systemic cardiac output.
4. Favorable and Unfavorable Streaming
4.1 Unfavorable streaming
Unfavorable streaming occurs when systemic venous blood is preferentially ejected into the aorta and relatively oxygenated blood is preferentially directed toward the pulmonary artery. In a setting without confounding ductal or collateral flow, systemic arterial saturation may be lower than pulmonary arterial saturation.
This pattern can produce cyanosis that appears disproportionate to the estimated pulmonary blood flow. It may become clinically conspicuous after a pulmonary artery band or other intervention changes ventricular flow paths, resistance, or the relative contribution of each inlet jet. The important diagnostic error is to assume that severe cyanosis necessarily means that pulmonary blood flow is inadequate.
4.2 Favorable streaming
Favorable streaming is the reverse: pulmonary venous blood preferentially reaches the aorta, while systemic venous blood is more prominently delivered to the pulmonary artery. Systemic arterial saturation may then exceed pulmonary arterial saturation.
This arrangement can partly preserve arterial oxygenation despite a common functional ventricle. It does not eliminate admixture or restore a normal circulation. It also does not prove adequate pulmonary flow, pulmonary arterial pressure, myocardial loading, or long-term physiological stability. A relatively reassuring pulse-oximetry value should not substitute for a full assessment of circulation.
4.3 Observed differences are not equivalent to predicted outcomes
In Macartney and colleagues' series, among patients without atrioventricular valve atresia who underwent complete hemodynamic assessment, the investigators reported complete mixing in 36%, favorable streaming in 52%, and unfavorable streaming in 12% [5]. These figures reflect a selected historical cohort rather than the prevalence of each pattern in contemporary single-ventricle populations.
The major lesson from these studies is not that a particular loop or arterial position always produces one pattern, but that actual arterial oxygenation depends on both flow distribution and preferential routing. Streaming must be demonstrated or strongly supported by patient-specific data.
5. Interaction With Qp/Qs and Oxygen Delivery
In the pre-Glenn circulation, the functional single ventricle commonly supports the pulmonary and systemic vascular beds in parallel. The ratio Qp/Qs is influenced by pulmonary vascular resistance, systemic vascular resistance, and any ductal, shunt, band, or outflow obstruction. A falling pulmonary vascular resistance after birth can cause pulmonary overcirculation, ventricular volume loading, and reduced effective systemic perfusion.
For a completely mixed circulation, the Fick relationship can be expressed approximately using saturations:
Qp/Qs ≈ (SaO₂ − SvO₂) / (SpvO₂ − SpaO₂)
This approximation assumes representative venous samples, comparable oxygen-carrying capacity, negligible dissolved oxygen contribution, and appropriate accounting for shunts. In the presence of streaming, aortic and pulmonary arterial saturations need not be equal; replacing pulmonary arterial saturation with aortic saturation in a simplified complete-mixing equation may give a misleading Qp/Qs estimate.
Systemic oxygen delivery is determined by DO₂ = Qs × CaO₂, where CaO₂ depends mainly on hemoglobin concentration and arterial oxygen saturation. Consequently, an increase in SaO₂ may coexist with deteriorating systemic oxygen delivery if Qs falls. Theoretical modeling of parallel Norwood physiology demonstrates why maximizing arterial saturation alone does not reliably optimize oxygen availability [8].
This distinction is particularly important when interpreting a response to oxygen, ventilation, changes in systemic or pulmonary resistance, or pulmonary artery banding. A change in saturation may reflect altered Qp/Qs, a change in streaming, or both.
6. Diagnostic Assessment
6.1 Echocardiography: first define the pathways
Echocardiography should establish atrial situs, atrioventricular connections, ventricular morphology and looping, the position of the outlet chamber, ventriculoarterial connections, and the relationship of the aorta and pulmonary artery to the inlet streams. The size and location of the bulboventricular foramen, potential systemic and pulmonary outflow obstruction, atrioventricular valve function, and ductal or other shunt pathways should be documented.
Color Doppler can demonstrate flow direction and turbulence, but it does not directly measure the oxygen content of a flow stream. In selected cases, carefully administered contrast echocardiography may help identify preferential systemic venous routing. Miura and colleagues reported an infant with severe desaturation after ductal ligation and pulmonary artery banding in whom contrast echocardiography demonstrated systemic venous streaming toward the aorta. Balloon atrial septostomy was followed by an increase in saturation from 68% to 85%; this was a case report, not evidence that septostomy is routinely indicated for similar patients [9].
6.2 Cross-sectional imaging and catheterization
Cardiac CT or MRI may clarify complex relationships that are difficult to appreciate on two-dimensional imaging, especially the geometry of an outlet chamber, the bulboventricular foramen, the great arteries, and associated vascular obstruction. The choice depends on the clinical question, patient stability, and available expertise [10].
Four-dimensional flow MRI can visualize time-resolved three-dimensional flow pathways and complex inflow directions. In a small study of seven patients after Fontan completion, investigators demonstrated variable atrioventricular inflow orientation and improved agreement of flow quantification using streamline-informed measurements [11]. This supports the broader principle that cardiac flow is three-dimensional, but post-Fontan flow findings should not be directly extrapolated to pre-Glenn intracardiac streaming.
When catheterization is needed, separately obtained aortic and pulmonary arterial saturations, interpreted alongside venous saturations and measured pressures, may provide evidence of streaming. Sampling location is critical; ductal flow, collaterals, pulmonary venous desaturation, and incomplete mixing can complicate interpretation. Direct flow measurements and anatomic findings should inform the final assessment.
7. Management Implications
Management should address the demonstrated cause of impaired oxygen delivery rather than treating pulse oximetry as an isolated target. In an infant with profound desaturation, distinguish insufficient pulmonary blood flow, unfavorable streaming, restricted atrial communication, low cardiac output, pulmonary venous desaturation, and residual or progressive outflow obstruction.
If pulmonary flow is restricted, maintaining ductal patency or providing an appropriate pulmonary blood-flow source may be necessary. If pulmonary overcirculation predominates, measures that rebalance Qp and Qs may be appropriate, including pulmonary artery banding in selected anatomy. These interventions can also change jet velocity and streaming; unexpected postoperative desaturation warrants renewed anatomical and physiological assessment.
Atrial septal enlargement may alter inlet distribution and improve admixture in carefully selected patients, but it is not a universal remedy for unfavorable streaming. In any patient with DILV and transposed great arteries, the clinician must also identify a potentially restrictive systemic outlet before selecting the initial palliation.
After cavopulmonary connection, the circulation is fundamentally altered: a portion, and eventually nearly all, of systemic venous return is directed to the pulmonary arteries without traversing the ventricle. The complete-mixing model and its pre-Glenn predictions therefore should not be carried uncritically into Glenn or Fontan physiology.
8. Key Clinical Principles
- A common ventricular chamber does not guarantee complete mixing. Coherent inflow pathways may preferentially supply different arteries.
- Favorable streaming directs relatively oxygenated pulmonary venous blood toward the aorta; unfavorable streaming preferentially directs systemic venous blood toward it.
- D-loop/d-transposition and L-loop/l-transposition are useful illustrations, not reliable stand-alone predictors. Inflow alignment, outlet-chamber geometry, and septal communications must be assessed individually.
- SaO₂ alone cannot define Qp/Qs or systemic oxygen delivery. The aortic–pulmonary arterial saturation difference may be clinically informative when samples and shunt anatomy permit valid comparison.
- Unexpected cyanosis after palliation requires differential diagnosis. Reduced Qp and adverse streaming are distinct processes and may coexist.
- Treat patient-specific anatomy and physiology. Intervention should optimize systemic perfusion and oxygen delivery, not merely improve the pulse-oximetry number.
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