Overview
Without an anatomic shunt, the ordinary steady native circulation shares one net forward output. Shunts explicitly separate pulmonary flow Qp from systemic flow Qs; device bypass pathways are also accounted for internally when MCS is active.
Shunt flow is signed: positive represents left-to-right recirculation and negative represents right-to-left flow. Pressure/resistance changes after medications or ventilation can alter direction and magnitude. The model is not an operability or closure assessment.
Modeled connections
The ASD example has reference Qp/Qs above 1.5 and an atrial communication that buffers ventricular filling variation. PAPVR routes a selected 40% of pulmonary venous flow to the right heart and cannot reverse on its own. PDA connects the aorta to PA downstream of the RV. Large VSD connects ventricular systolic flow and represents an unrestrictive defect with elevated pulmonary vascular loading.
Eisenmenger adds advanced pulmonary vascular disease to a reversible communication; oxygen does not erase the anatomy. If necessary, the selector adds a suitable communication rather than labeling isolated PAPVR as a reversing defect.
Conservation and finite reserve
The solver balances the connected branches after rate, vascular loading, therapies and MCS. Qp is used for pulmonary resistance; Qs for systemic resistance. RV outflow is not automatically Qp because PDA joins downstream of the RV; LV aortic ejection is not automatically Qs because recirculation and support can contribute.
Finite pump reserve limits extreme recirculation, reducing useful systemic flow and pressure. Simultaneous PS retains an RV–PA gradient; external constraint remains despite the communication. The branch equations and reserve ceilings are bounded illustrative anatomy.
Saturation run and mixing
Oxygen-content conservation generates the synthetic saturation run and right-to-left admixture. Reversed PDA can lower lower-body saturation while preserving a right-hand sample. PAPVR/ASD/VSD step-ups reflect where oxygenated blood re-enters the right circulation.
Conventional thermodilution is withheld in shunt examples; Fick reports the synthetic systemic Qs where valid. During supported arrest, a steady shunt solution is withheld because transient accumulation, ductal runoff and valve leakage need a different model. P/F and regional MCS/shunt gas mixing are not invented when unresolved.
References & further reading
