Background <p>Reliable prediction of fluid responsiveness continues to be a cornerstone during the resuscitation of the critically ill. While ultrasonographic measurement of inferior vena cava (IVC) dynamics has been widely adopted as a non-invasive tool, its reliability in the emergency department(ED) is increasingly questioned.</p> Objective <p>This narrative review critically appraises the physiological basis, technical limitations, and clinical evidence surrounding IVC diameter variation, with emphasis on its application in the heterogeneous and time-pressured ED environment.</p> Discussion <p>The physiological rationale for IVC collapsibility (cIVC) and distensibility (dIVC) lies in heart–lung interactions and their influence on venous return. However, multiple confounders—including variable inspiratory effort, ventilator settings, cardiopulmonary disease, intra-abdominal pressure, and operator dependence—undermine the reproducibility of these indices. Systematic reviews and meta-analyses demonstrate predominantly moderate specificity but poor to fair sensitivity, which otherwise makes IVC measurements a weak standalone predictor. In particular, cIVC in spontaneously breathing patients and dIVC in mechanically ventilated patients are prone to misclassification outside narrowly defined experimental conditions. These risks are magnified in the ED given the disparate patient cohort, time constraints and variability in operator experience.</p> Conclusion <p>IVC ultrasonography is an attractive bedside test for the rapid assessment of preload responsiveness, although it should be interpreted cautiously in ED. Its greatest value lies as part of a multimodal strategy, integrating dynamic manoeuvres such as passive leg raising, mini-fluid challenges, and echocardiographic stroke volume assessment. Used judiciously, it can complement resuscitation decisions; used in isolation, it risks misguiding care. Utilized judiciously, it may augment resuscitation decisions; however, alone, it risks misdirecting care.</p>