The effect of single twitch and train-of-four stimulation on twitch forces during stable neuromuscular block

Objective.We investigated whether the response to a single twitch (ST) stimulus or the first response (T1) toa train-of-four (TOF; 4 stimuli at 2 Hz) stimulus followinga stimulus interval of 10 s (i.e., the time between two consecutive ST or TOF stimuli) is influenced by the preceding stimulus in the presence of a stable 50% neuromuscular block. In addition, we determined whether ST and TOF stimulation yield different results under these circumstances. Methods.Twitch forces were measured in both tibialis anterior muscles of six cats. In the presence of a stable 50% neuromuscular block the stimulation pattern (ST or TOF) or stimulus interval (3.3, 10 or 30 s) was varied every 30min. A linear mixed model was used for statistical analysis. Results.ST forces with a stimulus interval of 3.3 s were 10.3%(95% CI: 7.3–13.3%) smaller than those with a stimulus interval of 10 s. For T1 forces this effect was 15.2% (95% CI: 12–18.4%). There was no significant difference between twitch forces with stimulus intervals of 30 and 10 s. For a stimulus interval of 3.3 s the ST forces exceeded the T1 forces by 7.6% (95% CI: 4.4–10.8%); no significant differences were found between the ST and T1 forces for stimulus intervals of 10 and 30 s. Conclusions.The ST or T1force during stimulation with a stimulus interval of 10 s or more during a stable 50% neuromuscular block in the tibialis anterior muscle of the cat is not affected by the preceding stimulus. In addition, ST and T1 forces do not differ when employing a stimulus interval of 10 s or more under these circumstances. Our results thus indicate that the known differences between ST and T1 forces after a bolus injection of a muscle relaxant can not be explained by differences in acetylcholine release when the stimulus interval exceeds10 s. This revised version was published online in July 2006 with corrections to the Cover Date.