= Atrial booster-pump function
\b[Atrial booster-pump function] is the active contribution of <atrial contraction> to ventricular filling. In the <left atrium>, contraction late in ventricular <diastole> helps drive <blood> through the open <mitral valve>. It supplements earlier filling associated with <atrial conduit function>, rather than accounting for all filling during the <cardiac cycle>.<Anatomy and Physiology 2e by J. Gordon Betts and others>[§19.3, Cardiac Cycle, atrial systole and passive filling]{ref}
A volumetric descriptor compares the chamber volume just before contraction, $V_{\rm preA}$, with the subsequent minimum, $V_{\min}$. Their difference is the observed active-phase volume loss; the <left atrial active emptying fraction> uses
$$
\mathrm{LAEF}_{\rm active}=\frac{V_{\rm preA}-V_{\min}}{V_{\rm preA}}.
$$
For constructed values 50 and 40 mL, the volume loss is 10 mL and this fraction is $20\%$. Choosing the denominator is part of defining the descriptor.
That loss is not automatically all of the <blood flow> through the <mitral valve> during the interval. Integrating the <left atrium>'s balance gives
$$
V_{\rm mitral}=V_{\rm preA}-V_{\min}+V_{\rm PV},
$$
where $V_{\rm PV}$ is signed net <pulmonary vein> inflow over the same interval. If $V_{\rm PV}=8$ mL in the constructed example, 18 mL leaves through the <mitral valve> despite a chamber-volume decrease of only 10 mL.
A primary <atrial flow component analysis> reconstructed flow from time-resolved <computed tomography> and <computational fluid dynamics>, distinguishing modeled passage and residence histories.<Comprehensive left atrial flow component analysis reveals abnormal flow patterns in paroxysmal atrial fibrillation by Sophia Bäck and others>[Abstract; Methods; Figure 7]{ref} This helps separate a phase-associated volume measurement from the route taken by each parcel. Likewise, an observed contraction-associated volume loss is not itself a controlled measurement of how much less ventricular filling would have occurred without contraction; pressure, timing and continuing inflow can also differ.
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