Nitrous oxide can be made more effective and predictable using a closed breathing circuit.

We read with interest the study by Babl et al published in the EMJ in November 2008. 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% nitrous oxide) 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 10 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 will 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 eight patients who received codeine (which has a sedative effect) within 2 h 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.