Fig. 5. Isoflurane concentrations (mean ± SD) required to produce immobility increase with increasing stimulation voltage. However, for a given voltage greater than 1 V, the concentration differences between 0.1- and 10-s interstimulus interval (ISI ) pulses remained unchanged. The finding of an unchanging concentration difference across an isoflurane range from 0.0 to nearly 1.0 MAC (the minimum alveolar concentration producing immobility in 50% of subjects receiving noxious stimulation) suggests that isoflurane may not be altering the process of temporal summation itself. The 50-V ISI pulses provided supramaximally intense stimulation.