We found that elderly patients had a significantly longer onset time (mean 297 vs. 199 seconds) and duration of action (mean 89 vs. 77 minutes) of cisatracurium 0.15 mg/kg compared with younger adults. However, no difference was found in intubating conditions using the IDS score or the Fuchs-Buder scale when the TOF count was 0.

Excellent intubating conditions were reported in 66% of the elderly patients after administration of cisatracurium 0.15 mg/kg, and no difference was seen between young and elderly. However, a videolaryngoscope was used in 45% of the elderly compared with 16% of the younger patients (P = 0.02) which may have influenced the assessment of tracheal intubating conditions, and this is a limitation in our study. Thus, videolaryngoscopy was used as first choice more often in the elderly but no difference was found in IDS scores including the number of attempts. Other limitations included the assessment of tracheal intubating conditions by multiple clinicians, possibly introducing interrater variability, as well as assessment of intubating conditions using the IDS score which is based on the employment of direct laryngoscopy. However, previous studies of intubating conditions have successfully used the IDS comparing direct laryngoscopy with the use of a videolaryngoscope [18]. The strength of our study lies in the neuromuscular monitoring, carried out according to research guidelines and managed by few, specifically trained, investigators providing exact data on onset time and duration of action [15].

Other studies have compared onset time and duration of action of cisatracurium in younger and elderly patients. Cisatracurium 0.15 mg/kg administered during total intravenous anesthesia resulted in a similar onset time; 250 seconds and 260 seconds in young and elderly patients, respectively [5]. Administration of cisatracurium 0.1 mg/kg during inhalational anesthesia resulted in a prolonged onset time in the elderly patients (240 vs 180 seconds (P < 0.01)) in one study [19] whereas a similar study reported no difference in onset time approximately 310 seconds vs 250 seconds in elderly and younger patients, respectively [3]. None of the three studies reported differences in duration of action comparing the young with the elderly patients [3, 5, 19]. When interpreting these previous data in relation to our results, it is important to underline that the two studies administered smaller doses of cisatracurium (0.1 mg/kg) [3, 19] and their designs compared age groups with a wider range i.e. the elderly patients had a mean age of approximately 70 years [3, 5, 19].

The observed difference in onset time may be related to an increased circulation time and reduction in cardiac output as suggested in previous studies on elderly patients [3, 20]. The pharmacokinetic profile of cisatracurium however, has been reported to be only minimally affected in elderly patients [3] and some studies report wide standard deviations of the duration of action of cisatracurium [3, 19]. It is possible that we were able to detect a difference in duration of action because of our study design, involving two age groups with a relatively large difference. The mean difference in duration of action of 12 minutes however, had a 95% confidence interval ranging from 2.5 to 20.5 minutes, meaning that the difference may be only 2.5 minutes, which is of minor clinical relevance.

The occurrence of excellent intubating conditions in two thirds of the elderly in our study is in contrast to a recent study on elderly patients administered rocuronium 0.6 mg/kg where excellent intubating conditions occurred in less than 30% [21]. In both studies intubating conditions were assessed upon a TOF count of 0 monitored at the ulnar nerve. Also, a recent study reports that increasing the dose of rocuronium from 0.6 to 1.0 mg/kg was associated with a larger proportion of excellent tracheal intubating conditions when TOF count was 0 [22]. The difference in intubating conditions may reflect the different muscle sensitivity towards NMBAs [23] and the difference in potency between cisatracurium and rocuronium. It is possible that a TOF count of 0 detected at the ulnar nerve after cisatracurium 0.15 mg/kg provides better intubating conditions than rocuronium 0.6 mg/kg.

Cisatracurium is a potent non-depolarizing NMBA with a longer onset time than less potent NMBAs such as rocuronium, which in the elderly has an onset time of 135 seconds [8]. In elderly patients who have an increased risk of prolonged duration of action of NMBAs [1] however, cisatracurium could be a rational choice due to its almost organ independent elimination [3]. In relation to this, one important finding from this study was that both onset time and duration of action were prolonged in the elderly and offered no advantage compared to rocuronium 0.6 mg/kg where duration of action is around 80 minutes [8]. In addition, it is possible to reverse a rocuronium-induced NMB with sugammadex within few minutes, which is not possible for cisatracurium [24]. Finally, the dose of cisatracurium was based on ideal body weight and it is important to emphasize that cisatracurium 0.15 mg/kg based on actual body weight would cause an even longer duration of action.

Regardless of the type of NMBA administered, this study illustrates the importance of objectively monitoring the neuromuscular blockade especially in the elderly, both for guidance of optimal timing for intubation, but also to reduce the risk of postoperative residual blockade [1, 25].

As an alternative to cisatracurium, a bolus of remifentanil can be administered for facilitating tracheal intubation. However, in a study on elderly patients excellent intubating conditions were found in less than 40% of the patients after administration of remifentanil 2 μg/kg despite the use of a videolaryngoscope in 50% of cases [21]. Also, it is relevant to emphasize that tracheal intubation was commenced already two minutes after administration of the study drug.

In conclusion, this study found that cisatracurium 0.15 mg/kg had a significantly longer onset time and duration of action in elderly compared with younger patients. No difference was found in intubating conditions at a TOF count of 0.