1. Introduction
A functionally single ventricle is often conceptualized as a complete-mixing chamber in which systemic venous and pulmonary venous blood become uniformly admixed before being distributed to the pulmonary and systemic circulations. This model is useful for understanding parallel circulation, but it is not universally correct. In many single-ventricle anatomies, venous inflow streams retain direction and momentum as they pass through separate atrioventricular valves, traverse the dominant ventricular chamber, and approach different arterial outlets. The result is preferential streaming rather than complete mixing.
Double-inlet left ventricle (DILV) is a particularly useful model. Both atrioventricular valves connect predominantly to a morphologically left ventricular chamber, often with a smaller morphologically right ventricular outlet chamber communicating through a bulboventricular foramen or ventricular septal defect. The relationship among atrioventricular inflows, ventricular loop, outlet chamber, septal communication, and great arteries can strongly influence intracardiac flow pathways. [1,2]
The central clinical principle is that systemic arterial oxygen saturation reflects not only pulmonary blood flow but also which venous stream is preferentially directed toward the aorta. Consequently, SaO₂ alone cannot define Qp/Qs, systemic blood flow, or systemic oxygen delivery.
2. Complete Mixing Versus Preferential Streaming
2.1 The complete-mixing model
If pulmonary venous and systemic venous blood were completely mixed before ejection, both great arteries would receive blood of essentially the same oxygen content, assuming no additional downstream source of flow. A simplified mixing relationship is:
where is pulmonary venous saturation and is systemic mixed venous saturation.
This is a reference model, not proof that the ventricle behaves as a well-stirred reservoir. Classic catheterization studies demonstrated that complete mixing was frequently absent. Rahimtoola and colleagues described preferential streaming in patients with common or single ventricles, and Macartney and colleagues subsequently identified complete mixing, favorable streaming, and unfavorable streaming as distinct hemodynamic patterns. [3,4]
2.2 Why mixing may remain incomplete
Blood entering through an atrioventricular valve has velocity, direction, and momentum. The two inflow jets interact with ventricular walls, trabeculations, septal remnants, vortex structures, and outflow geometry. If one inflow is better aligned with a particular arterial outlet, part of that stream may reach the outlet before complete homogenization occurs.
Morphological work supports this concept. Meyer and colleagues examined 39 postmortem DILV specimens and found marked variability in inflow–outflow relationships. Some hearts had approximately parallel pathways that could facilitate separation of oxygenated and deoxygenated streams, whereas others had more perpendicular relationships likely to promote greater mixing. [2] These observations establish anatomical plausibility, although anatomy alone cannot define the magnitude of streaming in vivo.
3. Anatomical Determinants of Streaming
The most important determinant is three-dimensional inflow–outflow alignment, not simply the diagnostic label of DILV or the designation of d- versus l-transposition.
Relevant variables include the position and orientation of both atrioventricular valve orifices, ventricular looping, the location of the rudimentary outlet chamber, the size and orientation of the bulboventricular foramen, the relationship of the great arteries to the dominant ventricle, and the presence of pulmonary or systemic outflow obstruction.
DILV itself is morphologically heterogeneous. Bevilacqua and colleagues demonstrated substantial variation in ventricular looping, atrioventricular valve morphology, septal communication, pulmonary stenosis, and systemic outflow anatomy in a 57-patient echocardiographic series. [5] Thus, loop orientation is useful descriptively but should not be treated as a stand-alone predictor of streaming.
The configurations shown in this series are therefore teaching examples rather than deterministic rules. A D-loop/d-transposition arrangement may align systemic venous inflow more directly with the aortic pathway and favor unfavorable streaming. An L-loop/l-transposition arrangement may align pulmonary venous inflow more favorably with the aortic pathway. The actual physiology depends on the entire three-dimensional geometry.
4. Favorable and Unfavorable Streaming
4.1 Favorable streaming
Favorable streaming occurs when relatively oxygen-rich pulmonary venous blood is preferentially directed toward the aorta while a larger proportion of systemic venous blood is directed toward the pulmonary artery.
In a simplified circulation without confounding ductal or collateral flow, this may produce:
- systemic arterial saturation higher than predicted by a complete-mixing model;
- aortic saturation higher than pulmonary arterial saturation; and
- less clinically apparent cyanosis at a given Qp/Qs.
In the historical Macartney series, favorable streaming was identified in 52% of patients with complete hemodynamic data, compared with complete mixing in 36% and unfavorable streaming in 12%. [4] These percentages reflect a selected historical cohort and should not be interpreted as contemporary prevalence estimates.
A favorable saturation pattern may nevertheless be misleading. A patient can have a relatively high SaO₂ because pulmonary venous blood is preferentially routed to the aorta while simultaneously having pulmonary overcirculation, reduced systemic output, ventricular dysfunction, or systemic outflow obstruction. Favorable streaming improves the oxygen content of systemic arterial blood; it does not guarantee adequate systemic oxygen delivery.
4.2 Unfavorable streaming
Unfavorable streaming is the reverse pattern: systemic venous blood is preferentially directed toward the aorta while a disproportionate fraction of pulmonary venous blood is recirculated to the pulmonary artery.
This can produce cyanosis that is more severe than expected from pulmonary blood flow alone. Aortic saturation may be lower than pulmonary arterial saturation, and systemic desaturation may persist despite apparently adequate or even excessive Qp.
The major clinical error is to interpret such cyanosis automatically as evidence of inadequate pulmonary blood flow. If Qp is already sufficient, increasing pulmonary flow may fail to improve systemic saturation and may worsen pulmonary overcirculation, ventricular volume loading, and systemic hypoperfusion.
Miura and colleagues reported a neonate with a single left ventricle who developed profound desaturation after ductal ligation and pulmonary artery banding. Contrast echocardiography demonstrated systemic venous blood streaming predominantly toward the aorta. Balloon atrial septostomy improved admixture and increased systemic saturation from 68% to 85%. [8] This is a case report, not evidence for routine septostomy, but it demonstrates that poor mixing and inadequate pulmonary blood flow are mechanistically distinct causes of cyanosis.
5. Streaming, Qp/Qs, and Oxygen Delivery
Before cavopulmonary connection, the pulmonary and systemic circulations are generally arranged in parallel. Total ventricular output is divided between Qp and Qs according to pulmonary and systemic vascular resistance, outflow obstruction, ductal or shunt resistance, and ventricular performance.
The conventional Fick relationship is:
The Fick principle remains valid, but simplified complete-mixing assumptions may not. In a patient with significant streaming, aortic and pulmonary arterial saturations may differ substantially. Substituting aortic saturation for a true pulmonary arterial saturation, or assuming that one arterial sample represents the composition of the entire ventricular output, can therefore produce a misleading estimate.
Systemic oxygen delivery is:
Therefore, SaO₂ and Qs must be interpreted together. Barnea and colleagues showed in mathematical modeling of neonatal univentricular circulation that small increases in SaO₂ may coexist with important reductions in systemic oxygen delivery, and that arterial saturation alone is an unreliable guide to circulatory balance. [6] Francis and colleagues similarly emphasized that oxygen saturation, flow, and tissue oxygenation are related but not interchangeable variables. [7]
Streaming adds another layer of complexity because SaO₂ depends on which stream reaches the aorta. Favorable streaming may increase SaO₂ without increasing Qp, whereas unfavorable streaming may lower SaO₂ despite adequate Qp. Pulse oximetry therefore cannot be used in isolation to infer Qp/Qs or systemic oxygen delivery.
6. Clinical Recognition and Imaging
6.1 Echocardiography
The first step is a complete sequential segmental analysis. Echocardiography should define atrial situs, systemic and pulmonary venous connections, atrioventricular connections, ventricular morphology and looping, the outlet chamber, ventriculoarterial connections, the bulboventricular foramen, atrial communication, atrioventricular valve function, and both pulmonary and systemic outflow pathways.
Color Doppler demonstrates the direction and velocity of jets but does not identify their oxygen content. In selected patients with unexplained cyanosis despite apparently adequate pulmonary flow, contrast echocardiography from a systemic venous injection may directly demonstrate preferential routing. [8]
6.2 Catheterization and advanced imaging
When invasive assessment is clinically indicated, separate aortic and pulmonary arterial saturation measurements may provide evidence of streaming. These data must be interpreted with pulmonary venous and systemic venous saturations, pressure measurements, shunt anatomy, collateral flow, and sampling location.
CT and conventional cardiac MRI are useful when the three-dimensional relationship of the atrioventricular inflows, outlet chamber, septal communication, and great arteries is not adequately defined by echocardiography.
Flow-sensitive CMR can visualize actual intracardiac trajectories. Wong and colleagues used cardiac MRI in a child with transposition, a large VSD, and severe cyanosis to demonstrate that poor systemic oxygenation resulted from unfavorable intracardiac streaming rather than simply inadequate pulmonary flow. [9] This illustrates a fundamental distinction: flow quantity and flow distribution are different physiological variables.
7. Management Implications
Management should address the mechanism of impaired oxygen delivery rather than the pulse-oximetry number alone. Severe cyanosis should prompt consideration of insufficient pulmonary blood flow, unfavorable streaming or inadequate admixture, restrictive atrial communication, pulmonary venous desaturation, low systemic cardiac output, and pulmonary or systemic outflow obstruction.
If pulmonary blood flow is inadequate, ductal support or a catheter- or surgery-based source of pulmonary flow may be required. If pulmonary overcirculation predominates, pulmonary artery banding or another flow-limiting strategy may be appropriate. Importantly, either intervention can alter intracardiac jet geometry and therefore modify streaming.
In selected patients with severe unfavorable streaming, improving atrial-level admixture may be physiologically appropriate, but the published evidence is limited largely to case reports and individualized practice. [8]
In DILV with transposed great arteries, systemic output may traverse the bulboventricular foramen before entering the aorta. Restriction of this pathway can produce clinically important systemic outflow tract obstruction. Long-term multicenter data demonstrate that systemic outflow obstruction is an important determinant of outcome in DILV and related single-ventricle morphologies. [10] A favorable saturation must therefore never distract from evaluation of the entire systemic outflow pathway.
After bidirectional Glenn or Fontan palliation, the circulation is fundamentally altered because systemic venous return is progressively diverted directly to the pulmonary arteries. The pre-Glenn concept of competing atrial inflow streams within a common ventricle therefore becomes much less relevant.
8. Evidence and Limitations
The existence of intracardiac streaming is supported by classic invasive hemodynamic studies and reinforced by modern morphological and imaging work. However, much of the literature is historical, observational, anatomical, or based on individual cases. Large contemporary prospective studies defining the prevalence and magnitude of favorable versus unfavorable streaming across the full spectrum of single-ventricle anatomy are lacking.
Accordingly, streaming should be treated as a patient-specific physiological phenomenon rather than assigned solely from an anatomical label. Ventricular loop, great-artery relationship, or apparent proximity of an inflow to an outlet may suggest a likely pattern, but actual physiology must be interpreted from anatomy, imaging, saturation data, pressure measurements, and the response to intervention.
9. Key Clinical Principles
- A common ventricular chamber does not guarantee complete mixing.
- Favorable streaming preferentially directs pulmonary venous blood toward the aorta, improving systemic saturation without necessarily improving Qp/Qs or systemic oxygen delivery.
- Unfavorable streaming preferentially directs systemic venous blood toward the aorta, producing cyanosis that may be disproportionate to pulmonary blood flow.
- Three-dimensional inflow–outflow alignment matters more than a simple loop or transposition label.
- SaO₂ alone cannot define Qp/Qs or systemic oxygen delivery.
- Unexpected cyanosis should not automatically trigger an increase in pulmonary blood flow. First distinguish low Qp from poor mixing or unfavorable streaming.
- Systemic outflow obstruction and streaming are separate problems and may coexist.
- Treat the demonstrated physiology, not the saturation number.
References
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