Fig. 5. The relations between Q (i.e. , the alfentanil–sevoflurane concentration ratio) and I(Q) (i.e. , the interaction) for ventilation (top ) and heart rate (bottom ). For both effects, synergistic interactions were observed over the range of concentration ratios from 0 to 1, with maximum synergistic interactions (Imaxin table 2) occurring at Q values of approximately 0.7 (Qmaxin table 2).