Integrated haemodynamic monitoring of fluid responsiveness and fluid tolerance in critical care: from physiology to bedside applications.

The framework of fluid responsiveness addresses only whether volume expansion will augment cardiac output, not whether the circulation can accept that volume without producing congestive organ injury. The complementary concept of fluid tolerance has accumulated empirical support over recent years and is integral to understanding patients' fluid status. This narrative review synthesises the physiological basis of fluid responsiveness and fluid tolerance, describes the bedside monitoring tools available to assess each, and considers the clinical implications of their joint interpretation in critical care. We performed a literature search restricted to articles published between 2020 and the date of the search. The retrieved abstracts were evaluated for relevance, and their reference lists were subsequently screened to identify additional pertinent publications. Fluid responsiveness and fluid tolerance are dissociable but physiologically interdependent, sharing determinants such as biventricular function, stressed volume and venous capacitance. Dynamic tests remain the preferred approach for assessing preload responsiveness; each has specific applicability conditions, and many require continuous or rapidly repeated cardiac output monitoring. Emerging tools have been developed to assess fluid tolerance, primarily based on vascular sonography; their interpretation requires concurrent assessment of biventricular function and further validation. Reframing fluid stewardship around the interaction between responsiveness and tolerance shifts the focus from forward-flow alone to net physiological benefit. No single parameter or device fully captures fluid status. An integrated, serial, multi-parametric assessment supported by appropriate monitoring platforms is therefore preferable to reliance on any single measure. At present, these tools support fluid stewardship rather than representing interventions with independently proven survival benefit. Future priorities include defining organ-specific congestion thresholds and testing whether monitoring-informed protocols improve outcomes.