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Second gas effect of N₂O

Chicago, Illinois

To the Editor:

Nishikawa et al.¹ recently addressed the issue of the second gas effect of N₂O on oxygen uptake. The authors reported an increase in FETO₂ and arterial oxygen tension (PaO₂) five minutes after N₂O inhalation, which they interpreted as such a second gas effect.

There are, however, some serious methodological issues in their experiments that not only cloud the authors' interpretation of their findings, but also provide no definitive evidence of a second gas effect of N₂O on oxygen uptake.

Instead of using a straight combination of high-flow, non-rebreathing 67% N₂O and 33% oxygen to ventilate the patient during the experiment, the authors have chosen to ventilate the lungs with a Servo 900C ventilator equipped with a gas monitor for O₂, N₂, and N₂O. The recording of the respiratory gas fractional concentration depicted in Figure 1 demonstrates exactly the problem of gas mixing and maintaining 33% of the inspired FIO₂ throughout the course of the experiment. During four minutes of gas mixing after the start of N₂O inhalation (between 15 and 19 min), the inspired-expired oxygen concentration differences almost disappeared, and a much higher oxygen concentration was demonstrated at the end of mixing (at 19 min) which indicated that the patient received an oxygen concentration higher than 33%. At the end of gas mixing (at 19 min), inspired and expired oxygen concentration differences started again, but with smaller differences in the beginning, which indicate that the functional residual capacity of the lung has been filled with a higher O₂ concentration similar to the inspired concentration during the mixing. This coincides very well with their statement at the end of their discussion, in which they admitted that there was a small change in FIO₂ immediately after N₂O administration due to a delay in adjusting the oxygen fraction to 0.33 in several patients. Therefore, the FETO₂ and PaO₂ increase after N₂O administration was definitely not related to the N₂O administration, but was related to the higher oxygen supply during the initial mixing period.

Reference

1 Nishikawa K, Kunimoto F, Isa Y, et al. Second gas effect of N₂O on oxygen uptake. Can J Anesth 2000; 47: 506–10.[Abstract/Free Full Text]

New, York, USA

The answer to the question raised by Dr. Lin's letter is definitely "no". We actually measured FiO₂ and FETCO₂ concentration every minute after N₂–N₂0 exchange. As shown in Figure 2 (data at one and five minutes), we did not observe any statistically significant change in

FiO₂ compared to the control value (FiO₂, 0.33). As we mentioned in the discussion, ETCO₂ data are a bit variable in each case, however, we never saw such high O₂ concentrations as speculated by Dr. Lin. Needless to say, had we found such high O₂ concentrations, those patients would have been excluded from the analysis.

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