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Atrial flow component analysis classifies blood transport through an atrium by the entry, exit and retention histories of represented fluid parcels. It asks a different question from measuring only maximum and minimum left atrium volumes: which blood passed onward, remained within the chamber, or crossed a boundary in another direction?

A primary study used time-resolved computed tomography to reconstruct moving atrial anatomy, computational fluid dynamics to estimate the flow field, and modeled particle trajectories to classify six components. It distinguished atrial conduit flow component, atrial reservoir flow component, atrial delayed-ejection flow component, atrial retained-inflow component, atrial residual-volume component and pulmonary-vein backflow component.^[ref] These are model-derived transport categories, not six separate anatomical chambers.

A typical trajectory calculation followswhere $\mathbf{u}$ is the modeled fluid velocity. Boundary crossings and their timing within a defined cardiac cycle determine the category. For example, the study's conduit definition requires entry and exit within the same ventricular diastolic phase.^[ref] Changing the time window or category definition changes what is counted.

An original weighting example shows why counting markers alone can be misleading. Suppose three modeled markers represent 1, 1 and 8 mL, with the first two assigned to one component. That component contains 2 of 10 mL, or $20\%$, although it contains two-thirds of the markers. The relevant fraction iswith $w_i$ the represented volumes for the chosen analysis. The denominator must be stated; an analysis that includes entering flow is not automatically a partition of one instantaneous chamber volume.

The results depend on reconstructed boundaries, flow assumptions, numerical tracking and phase definitions. Atrial reservoir function and atrial conduit function can be investigated with these categories, but a volumetric proxy and a parcel-based component need not be identical observations.