= Left atrial active emptying fraction
{title2=$\mathrm{LAEF}_{\rm active}$}
\b[Left atrial active emptying fraction] is a volumetric descriptor of the relative decrease in <left atrium> volume during its active emptying phase. With volume $V_{\rm preA}$ immediately before <atrial contraction> and subsequent minimum volume $V_{\min}$,
$$
\mathrm{LAEF}_{\rm active}
=\frac{V_{\rm preA}-V_{\min}}{V_{\rm preA}}.
$$
The phase choice distinguishes this quantity from a fraction formed using the maximum atrial volume over the entire <cardiac cycle>. <Atrial contraction> normally contributes to late ventricular filling after earlier passive filling.<Anatomy and Physiology 2e by J. Gordon Betts and others>[§19.3, Cardiac Cycle, atrial systole]{ref}
An original comparison uses $(V_{\rm preA},V_{\min})=(50,40)$ mL and $(25,15)$ mL. Both chambers lose 10 mL, but their fractions are $20\%$ and $40\%$. The denominator converts an absolute volume decrease into a relative one; it does not add an independent measurement of forward transport.
Timing is part of the measurement. With the first pair, choosing an earlier or later frame that gives $V_{\rm preA}=48$ mL instead of 50 mL changes the calculated fraction to $8/48\approx16.7\%$, even if $V_{\min}$ is unchanged. Consistent boundaries and phase identification therefore matter alongside <image segmentation> precision.
The fraction describes chamber-volume change, not all concurrent <mitral valve> flow. <Pulmonary vein> inflow during the interval contributes to the balance $V_{\rm out}=V_{\rm preA}-V_{\min}+V_{\rm in}$. Nor does the observed volume decrease isolate the causal effect of contraction from every other changing condition. A primary CT-based <atrial flow component analysis> compared volumetric indices with modeled transport components, illustrating the separation between volume measurements and parcel histories.<Comprehensive left atrial flow component analysis reveals abnormal flow patterns in paroxysmal atrial fibrillation by Sophia Bäck and others>[Methods; Figure 7]{ref} <Atrial booster-pump function> is the physiological contribution; this fraction is one specified descriptor of its associated volume behavior.
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