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In our opinion, the article “Occupational exposure of midwives to nitrous oxide on delivery suites”1 is in need of some remarks.
In the paper a serious problem seems to be the presence of nitrous oxide in samples collected at the beginning of the shift.
Many years ago, when N2O in urine was first evaluated, we frequently observed “uncommon” concentration of N2O in urine of exposed and unexposed subjects. The phenomenon was kept under control and disappeared when urine samples were treated with a small quantity of H2SO4 (0.2 ml). For this reason, we suggested the following:2 “... Approximately 10 ml of urine were collected from all the subjects at the end of the exposure period in 120 ml gastight glass vials with airtight plugs. Caps were rapidly replaced in the vials to prevent any significant loss of N2O. The vials contained 0.2 ml sulfuric acid in order to avoid the in vitro production of N2O (probably due to microflora activity).3 ...”.
Another point we consider very important is that the subjects must void the bladder rapidly in areas known to be free of nitrous oxide, otherwise a significant contamination of samples can occur.
In conclusion, we think that among the simple precautions that should be taken to avoid significant errors (avoiding collection of urine samples in places contaminated with N2O, carrying out collection rapidly, and using airtight collection vials in order to avoid any major loss of dissolved anaesthetic), one point should be emphasised in view of its importance: storage of urine before analysis can produce an endogenous formation of N2O originating from the oxidation processes of the nitrogen compounds present in biological liquids. Experiments performed to study this phenomenon have shown that the process is inhibited if the urine is kept acid. If, as a precaution, a few drops of strong acid are added to each collection vial before urine samples are collected, neoformation of nitrous oxide will be avoided and the urine samples may then be stored as long as required prior to the analysis.
Professor Imbriani and colleagues report experiments which showed that endogenous formation of N2O was inhibited if urine is kept acid. The convenience of adding 0.2 ml of sulphuric acid to vials recommends its routine use in practice and we do not disagree with this recommendation.
The likelihood that the pre-shift urine measurements which we reported arise from this phenomenon rather than other factors should be judged in the light of the following considerations:
All pre-shift urine samples were collected in areas free of nitrous oxide.
The period between sample collection and deposit in a freezer was approximately the same for each sample. Despite this 24 midwives had zero N2O in their pre-shift samples and 22 had non-zero values, of whom 12 had very high values.
The period between deposit in a freezer and analysis varied between samples but biological activity should not occur in the freezer.
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