Effects of remifentanil on sevoflurane requirement for intubation in children

I read the study published by L. He et al. (1) with great interest. On reading the article, I have a few questions and comments.

The authors managed to pre-oxygenate all 130 children between 3 to 8 years with 100% oxygen, using a tight fitting mask, but without using any sedative premedication. In my practice, this is almost unachievable. I have always been encouraged to minimise emotional upset to a child during induction of anaesthesia. This leads me to the question as to why a sedative premedication was not used.  

If children were emotionally upset by this method of pre-oxygenation, they would most probably cry. This may affect the end-tidal gas readings, sevoflurane uptake, time for induction and ultimately, the total amount of remifentanil received by the patient at the end-point of inhalational induction. Also, the end-point of inhalational induction was measured by loss of eyelash reflex, which is not a sensitive test, in my opinion. The time required for induction and the total amount of remifentanil infused may have also been affected by the assisted ventilation, which was needed to maintain the end-tidal CO₂ between 4–4.7 kPA. Thus, sevoflurane uptake may have been augmented.

I do not agree with the authors' explanation that the nasal catheter is more accurate than the one at the elbow of the breathing circuit. All end-tidal measurements. without the definitive sealed airway, are subject to error. In my opinion,  nasal catheters are far less accurate, due to the inspired and expired gases mixing freely in the nostril, especially in a child who is crying.

Intravenous access was established in all children and intravenous atropine administered, before inhalational induction. If intravenous access was possible without causing emotional upset to a child, then I think that most anaesthetists would probably prefer to preform an intravenous induction.

The composition of the fresh gas flow used for induction is not mentioned in the study.

The study shows that in remi_0.3 group, as little as 20% sevoflurane is enough for good intubation conditions, as compared to the ones without any remifentanil. This low end-tidal concentration of sevoflurane may have been associated with awareness. Remifentanil is not a hypnotising agent, and hence monitoring the depth of anaesthesia to judge the ceiling effect of remifentanil would have been more helpful. As stated in the discussion of this study, it may be possible to intubate children with higher doses of remifentanil only, but whether or not this would be good clinical practice is not clear, as there is no reference quoted for this statement.

In remi_0.2 and remi_0.3 groups, there was a statistically significant drop in heart rate and the mean arterial pressure. In remi_0.3 group, the heart rate and the mean arterial pressure remained low even after the intubation, as compared to the baseline. The authors claim these changes were clinically insignificant but it may have been useful to explain how they defined cardiovascular compromise.

It would be interesting to know the valid non-cardiac indications of such a complicated, potentially dangerous and probably expensive method of inhalational induction. A study published by Min et al. (2) describes the use remifentanil bolus for co-induction with 5% inhaled sevoflurane. The ED₅₀ and ED₉₅ estimated, using this method, is likely to be more accurate, as the concentration of one of the two induction agents i.e. sevoflurane (usually the main agent responsible for errors due to its less modifiable uptake in the lungs) remained constant, thus reducing the error. In the current study, both the induction agents were likely to have been administered in a variable amount. and therefore subject to more error. Furthermore, administering the remifentanil in a slow bolus form (2), rather than as an infusion, may be a more practical method of administration in clinical practice.

There are a number of different inhalational induction methods in clinical practice, where muscle relaxants and remifentanil are not used in this age group. Therefore, I agree with the authors that this method would need to be tested, using valid and consistent methods of assessment. However, I think that the method described, would need to show significant advantage over other methods in clinical practice. 

Priti Gandre

ST4

Barts and The London School of Anaesthesia

References:

1.       L. He, X. Wang, X-F. Zhang, S-R. Tang. Effect of different doses of remifentanil on the end-tidal concentration of sevoflurane required for tracheal intubation in children. Anaesthesia 2009; 64: 850-5.

2.       Min SK, Kwak YL, Park SY et al. The optimal dose of remifentanil for intubation during sevoflurane induction without neuromuscular blockade in children. Anaesthesia 2007; 62: 446-50.