Fig. 2.

The Greco model (A), the Machado model (B), the Plummer model (C), and the Carter model (D) show isoboles of the response surface. The abscissa is a concentration of vecuronium (µM) and the ordinate, that of rocuronium (µM). The numbers in the plots are the fractional increase of tetanic fade. The thick, solid line in panel A is shown as an example of the Loewe additivity line at a 0.9 effect. If all the Loewe additivity lines would be drawn, these would reveal that the isoboles are upward concave. Therefore, it is concluded that interactions between vecuronium and rocuronium are consistently synergistic.