Send letter to journal:
Re: Nitrous oxide can be made more effective and predictable using a closed breathing circuit

Dear Sir,

We read with interest the study by Babl et al “limited analgesia efficacy of nitrous oxide for painful procedures in children” published in the EMJ November 2008 Vol 25 No 11.

Central to the protocol and management is the use of clinical assessment to determine adequacy and depth of analgesia/ sedation. We agree with this wholeheartedly and support this method as it emphasises the importance of conscious sedation as an adjunct to therapy, rather than unconscious sedation to compensate for inadequate analgesia. However it is worth recognising that the inspired concentration (in your study stated as 50-70% N20) does not relate to effector site concentration (in this case the central nervous system) unless a steady state has been achieved, the period of equilibration being approximately ten minutes. Without a closed breathing circuit being used there will inevitably be a significant entrainment of air resulting in a further reduction in real versus expected nitrous oxide concentration. This means that the nitrous oxide concentration achieved in the children in your study will be significantly lower than the settings on the machines would suggest and be unpredictably variable.

In our experience using a closed mask system (such as a T – piece or close fitting demand mask) and end tidal gas monitoring provides a breath by breath analysis of end tidal nitrous oxide and oxygen concentration. These are analogous to arterial blood levels and thus reflect much more closely the effector site concentration.

As a second point we note that all 8 patients who received codeine (which has a sedative effect) within two hours of sedation were not excluded from the study, although it was stated in the protocol that those given sedatives would be excluded.

We agree with your findings that nitrous oxide is a useful adjunct in conscious sedation in painful procedures in children. We suggest this could be made more effective and predictable delivering nitrous oxide within an anaesthetic closed circuit in the Emergency department using end tidal gas monitoring.

Send letter to journal:
Re: nitrous oxide can be made more effective and predictable using a closed breathing circuit

Dear Sir,

We read with interest the study by Babl et al “limited analgesia efficacy of nitrous oxide for painful procedures in children” published in the EMJ November 2008 Vol 25 No 11.

Central to the protocol and management is the use of clinical assessment to determine adequacy and depth of analgesia/ sedation. We agree with this wholeheartedly and support this method as it emphasises the importance of conscious sedation as an adjunct to therapy, rather than unconscious sedation to compensate for inadequate analgesia. However it is worth recognising that the inspired concentration (in your study stated as 50-70% N20) does not relate to effector site concentration (in this case the central nervous system) unless a steady state has been achieved, the period of equilibration being approximately ten minutes. Without a closed breathing circuit being used there will inevitably be a significant entrainment of air resulting in a further reduction in real versus expected nitrous oxide concentration. This means that the nitrous oxide concentration achieved in the children in your study will be significantly lower than the settings on the machines would suggest and be unpredictably variable.

In our experience using a closed mask system (such as a T – piece or close fitting demand mask) and end tidal gas monitoring provides a breath by breath analysis of end tidal nitrous oxide and oxygen concentration. These are analogous to arterial blood levels and thus reflect much more closely the effector site concentration.

As a second point we note that all 8 patients who received codeine (which has a sedative effect) within two hours of sedation were not excluded from the study, although it was stated in the protocol that those given sedatives would be excluded.

Send letter to journal:
Re: Nitrous is not enough

Dear Sir

RE: The Efficacy of N2O

We read with interest the recent article by Babl et al but feel the paper, its provocative title and accompanying editorial comment could be misleading and potentially damaging to the use of this useful agent.

In our experience, ‘intra procedural’ pain scoring by the patient during adequately administered 70% nitrous would in most cases be impossible. Indeed the study design suggests that these pain scores were in fact taken ‘immediately after’ completion of the procedure. As the analgesic effects of nitrous oxide rapidly reverse at the end of administration, there seems a risk that the authors were measuring the after effects of a painful procedure and the ongoing pain of the condition, without effective analgesia.

Given that a quarter of these procedures were for fractures severe enough to require manipulation, that nearly 90% of patients had no other adjuvant analgesia and those who had received parenteral opiates were excluded (although this was not defined as a priori), it was not surprising pain scores remained high. As the authors point out, we know that ‘N2O is highly affective when accompanied by additional analgesic interventions for fracture reduction’ and so we would also question the ethics of not providing this standard of care.