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Background Vasovagal syncope results from transient cerebral hypoperfusion, but the precise regional fluid shifts preceding syncope remain unclear.Methods In this prospective observational study, 112 adults referred for syncope evaluation underwent 70° head-up tilt testing with simultaneous continuous multi-segmental bioelectrical impedance analysis (BIA) of thoracic, splanchnic, and thigh compartments, alongside beat-to-beat arterial pressure and ECG monitoring. Nineteen datasets were excluded due to missing signals or electrode displacement, leaving 93 participants with analysable data (56 fainters, 37 non-fainters).Results Systolic blood pressure diverged between groups 3 minutes before syncope (p=0.02). In the final 4 minutes of tilt, non-fainters demonstrated adaptive responses: increased splanchnic impedance (+1.4%±0.3%, p=0.001) with stable thoracic impedance, indicating blood autotransfusion from the splanchnic bed to the central circulation. Fainters showed maladaptive responses: no change in splanchnic impedance (−0.5%±0.4%, p=0.09) with increased thoracic impedance (2.9%±0.3%, p=0.02), indicating splanchnic blood pooling. Both groups similarly accumulated fluid in the lower limbs.Conclusions Vasovagal syncope results from failure of splanchnic circulatory compensation rather than excessive lower limb pooling. The splanchnic circulation represents a critical physiological compartment in the pathogenesis of vasovagal syncope.