The atrial retained-inflow component is blood that entered the left atrium during the previous cardiac cycle but has not left by the specified endpoint of the Bäck tracking scheme. Its documented entry distinguishes it from the atrial residual-volume component, whose blood remains throughout both tracked cycles.[ref]
A simple parcel ledger makes the distinction concrete. In a constructed one-second cardiac cycle, let a tracked parcel enter through a pulmonary vein at s during the first of two cycles and remain inside until observation ends at s. Its observed residence time at that endpoint is 1.6 s. If that parcel crosses the mitral valve at s, a later observation reveals an exit; the earlier record was still correctly classified for its own endpoint.
Different amounts of retention need not produce different instantaneous chamber volumes. Consider two ideal histories, each with 40 mL inside the left atrium at the endpoint. In the first, 10 mL has an observed entry during the designated earlier cycle and remains, while 30 mL was present throughout the entire tracking window. In the second, the corresponding volumes are 25 and 15 mL. Both chamber totals are 40 mL, but their atrial retained-inflow component and atrial residual-volume component differ. A chamber outline alone cannot distinguish these histories.
The component is consequently useful for asking about replacement of atrial blood, alongside atrial delayed-ejection flow component and atrial conduit flow component. Its measured or modeled fraction must identify both the tracking interval and denominator. Dividing 10 mL by a 40 mL chamber volume gives ; dividing it by a stipulated 80 mL cycle throughput gives . Those percentages express different comparisons.
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