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Purpose: The mouse is the most used animal model of cardiovascular diseases, with growing availability of genetic variants. Although there is evidence that the left atrium (LA) plays a central role in early detection of cardiac disease and prognosis, and that murine and human cardiac anatomies are similar, the murine LA has never been described with high frame rate echocardiography.Methods: We studied 30 C57BL6 mice (m:f= 15:15; mean body weight= 21±3 g) with a Vevo 2100 system (30 MHz transducer), during anaesthesia with Isoflurane 0.8-1%. Standard and modified apical 4-chamber views were used to optimize 2D imaging of the LA and mitral valve, and pulsed Doppler of trans-mitral, LA appendage (LAA), and pulmonary venous (PV) flows.Results: Mean heart rate was 484±43 bpm (systole= 76±9 ms; diastole= 49±6 ms). A typical venous "reservoir" was imaged in 28/30 mice as a small spheroid LA chamber communicating laterally with a large, crescent-like shaped LAA and superiorly with a single PV ostium (confirmed by anatomical inspection). There was a 3-fold increase in LA volume (LAV) during reservoir filling and both LA and LAA were smaller in females (Table). The trans-mitral flow pattern was dependent on heart rate: monophasic (atrial systolic wave) in 24 mice and biphasic in 4. The PV inflow was characterized by 3 forward waves, 2 systolic (velocity-time integrals, early: 6±4, late: 5±2 mm) and 1 diastolic (17±5 mm), and the LAA by 2 systolic inflow (early: 9±3, late: 6±2 mm) and 1 diastolic (7±3 mm) outflow waves. As shown in other species, variables related to LA compliance (reservoir function) correlated with left ventricular stroke volume (LVSV): the LAA long axis was linearly related to LVSV (r= .6, p= .01) and at regression analysis both systolic PV inflow and LAA diastolic outflow velocity/time integrals determined LVSV (r= .56, p= .006).Conclusions: This is the first detailed description of murine LA anatomy and function, highly feasible with high frame rate echocardiography. The LAA is a major component of the venous reservoir, which is an independent determinant of LVSV.