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Figure 4 Relationship between beat-to-beat single-beat M_w (SBM_w) (top panel) maximum power divided by the square of the end-diastolic volume (PWR_max/EDV²) (middle panel) or single-beat elastance (SBE_es) (bottom panel) and end-diastolic volume during the same vena caval occlusion runs performed before (control) and during aortic constriction. End-diastolic volumes are normalized to the value of the first steady-state beat recorded. Values of the three single-beat indexes are normalized to the control steady-state value. Single-beat M_w estimates are insensitive to both preload and afterload. PWR_max/EDV² is significantly preload sensitive (p < 0.05) but is significantly afterload sensitive only for EDVs less than 65% of baseline (*p < 0.05). Single-beat E_es is highly variable from beat to beat.