In this study, we assessed the effect of the two factors, the use of nitrous oxide and the patient health status, on the end-tidal sevoflurane concentrations at the completion of the intravenous cannulation and on the time required for intravenous cannulation. We found that the use of nitrous oxide had a significant effect on the end-tidal sevoflurane concentration but no effect on the time required for cannulation. It was also revealed that the patient health status had no significant effect on either of the two variables.

The optimal timing for intravenous cannulation after the loss of eyelash reflex is affected by various factors, such as the volume of the breathing system fresh gas flow rate, sevoflurane concentration in alveoli and ventilation / perfusion mismatch [1].

It has been reported that epileptiform activity during general anesthesia in children with developmental disabilities affect BIS values [10,11,12]. The ictal and interictal EEG activity such as δ waves might be different from those of healthy patients [9, 12]. BIS values were used only for the depth of anesthesia during surgery, not for induction of anesthesia. In addition, inhalational anesthetics might influence GABA receptors in the CNS. It has been shown that the MAC of inhalational anesthetics can be up to 20% lower in cerebral palsy patients [11]. However, there were no physiological signs or monitoring methods to determine the optimal timing of intravenous cannulation after loss of the eyelash reflex.

Anesthesiologists are usually anxious to obtain an intravenous route as soon as possible at induction of anesthesia in order to administer agents for faster intubation. Attempt of intravenous cannulation just after the loss of the eyelash reflex may lead to premature attempt for the patients, which may cause body movement, laryngospasm, attempt failure, and coughing [2].

Recently, ketamine was categorized into narcotic drugs legally, and was becoming not popular for pediatric patients in Japan. Ketamine and local anesthetic creams were not used in this study. On the other hand, sevoflurane is widely used inhalational anesthetic in pediatric anesthesia for mask induction. And nitrous oxide, used in this study, is a well-established acting analgesic and sedative agent [1, 2, 13]. We expected that the combination of sevoflurane and nitrous oxide was beneficial to make the optimum timing for intravenous cannulation shorter without causing complications [13, 14]. However, when stopping the admin of nitrou oxide, nitrous oxide enters the alveoli rapidly from the blood. The dilution of the alveolar air-gas mixture leads to a decrease in the concentration of alveolar sevoflurane. Subsequently the end-tidal sevoflurane concentration might be decreased. This phenomenon might counteract the benefit of nitrous oxide.

Compared to patients in previous studies, none of the patients in our study had hemodynamic and/or respiratory complications such as laryngospasm and desaturation. This could be explained by higher concentration of sevoflurane (5–8%) for anesthetic induction, which could ensure safe depth [1,2,3,4]. However, our retrospective results showed that the time for intravenous cannulation did not shorten in spite of nitrous oxide inhalation. We should not stop nitrous oxide until the venous route was obtained.

Our study has some limitations. Attempts of intravenous cannulation depend each anesthesiologist and their experiences. Due to the condition of blood vessels, cannulation could be difficult and it might take longer time. Also, the number of cannulation attempts could have been difficult to record. As a next step, further prospective studies are required with larger sample sizes, limited number of anesthesiologists and continuing nitrous oxide inhalation until securing venous cannulation.

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