B2M and hydrops fetalis: To better clarify the link between the selected features and HF severity, we evaluated the linear correlation of the 5 selected features with the levels of VO2/Kg, and demonstrated a significant inverse correlation for A1AC (R-value = −0.586, p = 0.004), B2M (R-value= −0.604, p = 0.003), C9 (R-value = −0.498, p = 0.018) and NRP2 (R-value = −0.469, p = 0.028) (Figure 3), while gelsolin did not result significantly correlated with the levels of VO2/Kg.