The effect of a tetanic stimulus on the response to subsequent tetanic stimulation.

We evaluated the effect of tetanic stimulation (5 s, 50 Hz, 60 mA) on the response to tetanic stimulation delivered 2 min (TET@2) and 5 min (TET@5) later in 22 anesthetized patients receiving a vecuronium infusion. Once a consistent mechanomyographic train-of-four ratio was obtained at the adductor pollicis muscle, the first (baseline) tetanic stimulus was delivered. Tetanic sequences were repeated randomly after 2- and 5-min intervals. Tetanic fade was consistent among the study periods: the tetanic fade ratio for the first second of tetanus was 0.35 during baseline tetanic stimulation, 0.32 during TET@2 (P = NS by paired t-test), and 0.33 during TET@5 (P = NS). The respective fade ratios for the entire 5-s period were 0.15, 0.15 (P = NS), and 0.14 (P = NS). In contrast, the peak tetanic height increased from 10.8 mm at baseline to 11.3 mm at TET@2 (P < 0.05), and 11.3 mm at TET@5 (P < 0.05). We conclude that, in light of the consistency exhibited by tetanic fade, the small change in twitch height is clinically insignificant.