Rationale behind the use of venous volume measurement (dynamic filling index, DFI) in testing cardiac load-responsiveness. With acute reduction of pump flow (e.g. from 4 to 2 l/min), the patient remains hemodynamically stable if cardiac load-responsiveness is sufficient (upper left). If cardiac load-responsiveness is blunted, the heart is not able to take over output, and venous volume will increase (upper right). Venous volume modulates the relation between pump speed (s) and flow (f) in the assisted circulation, which can be quantified by the DFI, i.e. the slope of the curve Δf/Δs. The DFI will not increase with diminished ELS flow when cardiac load-responsiveness is good (lower left), but will increase if cardiac load-responsiveness is decreased (lower right). CVP, central venous pressure; ABP, arterial blood pressure; RV, right ventricle; LV, left ventricle; SVR, systemic vascular resistance.