Relief of Post-operative Pain: Comparison of a 25% Nitrous-oxide and Oxygen Mixture with Morphine

Abstract

Nitrous oxide has had a wide use as an analgesic in midwifery and in dental surgery, but its use does not appear to have been investigated for the relief of post-operative wound pain.We describe here a study designed to compare the relative potency of nitrous oxide and morphine for relief of pain in patients during the first 48 hours after upper abdominal operations. Post-operative pain in such patients restricts breathing to a marked degree, and the relief by analgesics of this restriction can be assessed by simple tests of vital capacity in the manner described by Overholt (1930), Bromage (1955), and Masson (1962. Previous work on volunteers (Chapman et al., 1943 ;Sonnenschein et al., 1948 ;Delisle Burns et al., 1960;Dundee and Moore, 1960;Parkhouse et al., 1960;Dundee et al., 1962) suggested that 25% nitrous oxide in oxygen was the most suitable concentration for this study. Material and MethodsThe investigation was limited to patients under the age of 60 who had operation wounds in or extending to the upper abdomen. Prior to operation the patients were seen, the nature of the trial was explained, and their co-operation obtained; vital capacity and peak expiratory flow readings were then measured and the nitrous oxide and oxygen breathing apparatus was demonstrated.If vital capacity is measured before operation and again after operation, when the patient is fully recovered from the anaesthetic, its diminution gives some measure of the impairment of respiratory movement. This can be taken as some measure of the degree of pain. If this pain is then relieved by an analgesic the vital capacity increases in proportion to the efficacy of the drug. A perfect analgesic will restore the vital capacity to virtually the pre-operative value, whereas a drug having no effect will make no difference to the vital capacity. We therefore used changes in vital capacity, and, to a less extent, those in peak expiratory flow rate, as a way of assessing the analgesic effects of nitrous oxide and morphine.Vital Capacity.-This was measured by the Wright respirometer used with a mouth-piece and Ruben valve, and with a nose-clip applied. After practice readings the mean of three readings was noted. Although the Wright respirometer is less accurate than bulkier apparatus for vital capacity measurement, it was thought to give satisfactory comparative readings since the same respirometer was used in every case.Peak Expiratory Flow Rate.-The Wright peak flow meter was used and the mean of three readings taken after practice " puffs." 25% Nitrous Oxide in Oxygen Mixture.-The mixture was delivered to the patient from a specially calibrated Walton 5 machine using a reservoir bag and a non-rebreathing circuit with a Ruben valve. The " pressure " control was adjusted so that the reservoir bag did not collapse during inspiration while

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Nitrous oxide in emergency medicine

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