Left atrial “reservoir strain” (LARS) is usually taught as an atrial property. That framing fails physiologically. LARS is best understood as a signal of ventriculo–atrial coupling.
2/ Start with a hard constraint: volume conservation. During LV systole, ventricular volume falls. Total cardiac volume changes little. That displaced volume must go somewhere → atrial expansion.
3/ The physical coupler is the AV plane. LV longitudinal shortening pulls it downward (MAPSE). The LA lengthens passively above it. Same motion. Same driver.
4/ This is why MAPSE and LARS track so closely (R² ≈ 0.95). LARS ≈ MAPSE indexed to LA length. Measuring it from the atrial side does not make it an intrinsic atrial material property.
5/ The <100% correlation is the key insight. If the atrium were a passive balloon, LARS would mirror MAPSE perfectly. The residual variance reflects modulation of coupling, not noise.
6/ LA compliance, geometry (size & shape), fibrosis, and timing determine whether AV-plane motion is stored as reservoir volume or manifests as pressure rise. This defines coupling efficiency.
7/ This resolves the paradoxes. LARS falls in HFpEF and predicts outcomes not because it measures LA distensibility, but because impaired coupling means LV longitudinal work is no longer stored as atrial reservoir volume.
8/ Bottom line: LARS is not LA strain, stiffness, or distensibility. It is a ventriculo–atrial coupling signal dominated by LV systolic longitudinal function and modulated by atrial properties. MAPSE tells you how much the ventricle pulls; LARS tells you how that pull is