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What is “relative analgesia?”
  1. M C Howes1
  1. 1Accident and Emergency Department, Royal Preston Hospital, Preston PR2 9HT, UK;
  1. Correspondence to:
 Dr G J Browne
  1. G J Browne2,
  2. A Frampton2,
  3. L T Lam2,
  4. M G Cooper2,
  5. L G Lane2
  1. 2Department of Emergency Medicine, The Children’s Hospital at Westmead, Sydney, Australia

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    I welcome the paper by Frampton et al1 describing their experiences of nurse administered nitrous oxide, which adds further evidence to the literature2 supporting this technique as a useful and safe method of easing the suffering of children during their attendance at an emergency department.

    I feel that the use of the term “relative analgesia” is somewhat confusing; this is not a term previously encountered in the literature describing sedative/analgesic techniques. The United States guidance (their reference 2) does not use this term when defining sedation levels nor do the current UK3,4 and Australasian5 guidance and definitions. To introduce a new term may prevent accurate comparisons of techniques in the literature.

    I would also welcome description of two other outcomes measures that readers would find important when considering a sedative/analgesic technique: adequacy of sedation and parent/operator satisfaction. The authors do describe 10 cases (4.4%) requiring additional sedation but not whether the remaining children were adequately sedated, or inadequately sedated but the procedure was completed anyway. Nitrous oxide has two useful properties: analgesia and sedation. In the context of this study “relative analgesia” could mean “inadequate analgesia” or “sedation (without analgesia)”.


    Authors’ reply

    Howes’ concern about the term “relative analgesia” pertaining to our recent description of nitrous oxide analgesia in children is noted. This is actually a term that has been used to describe nitrous oxide analgesia for many years. It first appeared in the dental literature and was used originally to describe situations where continuous flow/variable concentration nitrous oxide was administered, often via a nasal mask.1–5 Other authors looking at the risk of aspiration using nitrous oxide analgesia used the term “relative analgesia” when studying 50% nitrous oxide/oxygen (Entonox).6,7 The term does not appear to have been used in any of the emergency medicine literature pertaining to nitrous oxide that we have seen.

    The term continues to be used in contemporary literature 8,9 and in 2001 Lahoud et al10 described relative analgesia as having three elements: patient remaining conscious deliver 100% O2 if needed. Certainly we found in our study that distraction techniques are easily done in conjunction with this method of analgesia and form an important part of it. We have used the term “relative analgesia” in our institution for many years, which is why it was included in our study. The term has also persisted in the name of the equipment used to administer continuous flow/variable concentration with the Quantiflex RA machine originally manufactured by Cyprane, Keighley, England and now by Matrix Medical, New York.

    We agree with Howes that there is enough confusion in the semantics of the literature on sedation/analgesic techniques without rejuvenating old terminology. However, perhaps the term “relative analgesia” may be useful in describing analgesia by inhalational techniques alone, which are becoming more common using agents such as nitrous oxide, methoxyflurane, and nitrous oxide/sevoflurane mixtures.11 Nitrous oxide provides analgesia, anxiolysis, and mild amnesia obtained with maintenance of verbal contact and predominantly intact laryngeal reflexes. No other single agent does this.


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