Background: Shortening the onset time of neuromuscular blocking drugs is important in Rapid sequence induction(RSI).
Many anaesthetists use rocuronium  in place of succinylcholine for RSI.
High dose rocuronium is ideal for RSI, but studies on rocuronium onset have revealed individual differences such as large standard deviation(SD) or variations in the distribution. Therefore, it is difficult to achieve rapid onset in all cases.
We hypothesized that rapid injection of rocuronium diluted with saline would shorten the onset time and
reduce the individual differences. Further, we investigated the factors on the onset time and which of degrees. 

Metods: Eighty-two patients randomly allocated to the control group or the other three groups, according to the dose of rocuronium and the presence of saline dilution: rocuronium 0.9mg/kg(Group A=Control, n=19), 0.9mg/kg in 25ml saline(Group B, n=21), 1.2mg/kg (Group C, n=20), 1.2mg/kg in 25ml saline(Group D, n=20). Anaesthesia was induced with narcotic analgesic and propofol, and rocuronium was administrated as follows whthin 5 seconds. Neuromuscular blockade was assessed using acceleromyography(AMG) at the adductor pollicis muscle with train-of-four(TOF) stimulation. Onset time was measured as the time from the start of rocuronium injection until TOF count 0. Multiple regression analysis was used to investigated to identify clinical characteristics associated with onset time. Significance was designed at p<0.05.

Resulets: The measured onset time was significantly shorter in the Group D than the others(Median90s(86-109), p<0.05), and Individual differences were reduced in the Group D(IQR=22.5, SD=21). Rapid injection with 25ml saline shorten the onset time by 20 seconds, conversely the older the age get, the slower the time of onset(multiple regression analysis,respectively p=0.01, p=0.03). (advanced age slow the onset time)
Scatter plot of 80 cases formed three clusters at regular(about 60 seconds) intervals, onset is in proportion to the number of times of circulation not to the time. In other words, Onset is tend to express? In stages.

Conclusion: Rapid injection of rocuronium In 25ml saline provided fastar onset of neuromuscular blockade and reduction of individual differences. Age and rapid injection with saline were independent factors of rocuronium onset time.
It is not clear that what is proper dose of Roc.

Rapid injection of rocuronium bromide with saline dilution provides fastar onset of neuromuscular blockade and reduction of individual differences. Randamized controlled trial of factors on the onset time of non-depolarizing muscle relaxants