Fig. 3.
A train-of-four (TOF) consists of four equal stimuli delivered at 0.5-s intervals. The TOF ratio is calculated by comparing the magnitude of the fourth evoked response or twitch (T4) to that of the first response (T1). During partial nondepolarizing block, T4 decreases preferentially, such that there is fade (i.e., TOF ratio less than 1.0). In the illustration above, the TOF ratio = 0.40 (40%).