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

Objective. We investigated whether the response to a single twitch (ST) sti mulus or the first response (T1) to a train-of-four (TOF; 4 stimuli at 2 Hz ) stimulus following a stimulus interval of 10 s (i.e., the time between tw o consecutive ST or TOF stimuli) is influenced by the preceding stimulus in the presence of a stable 50% neuromuscular block. In addition, we determin ed whether ST and TOF stimulation yield different results under these circu mstances. Methods. Twitch forces were measured in both tibialis anterior mu scles 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 30 min. 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 forc es this effect was 15.2% (95% CI: 12-18.4%). There was no significant diffe rence between twitch forces with stimulus intervals of 30 and 10 s. For a s timulus 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 T 1 forces for stimulus intervals of 10 and 30 s. Conclusions. The ST or T1 f orce during stimulation with a stimulus interval of 10 s or more during a s table 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 n ot differ when employing a stimulus interval of 10 s or more under these ci rcumstances. Our results thus indicate that the known differences between S T and T1 forces after a bolus injection of a muscle relaxant can not be exp lained by differences in acetylcholine release when the stimulus interval e xceeds 10 s.