Bidimensional VMR representation derived by NIRS signals. Bidimensional VMR patterns as assessed by NIRS signals for the sample population. Each red circle represents the instantaneous concentration of CO2Hb (horizontal axis) and O2Hb (vertical axis). The concentration values are normalized with respect to their maximum value during the BH phase. The dotted lines depict the first and third quadrants bisectors. The reactivity pattern is always comprised into the region delimited by the two bisectors, evidencing a greater increase in the O2Hb level with respect to the CO2Hb concentration level.