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LETTERS TO THE EDITOR

The Second Gas Effect Is a Valid Concept

Department of Anaesthetics and Intensive Care Medicine University of Wales College of Medicine Cardiff, United Kingdon

B. Korman, BSC, MBBS, MD, FANZCA

Department of Anaesthesia Royal Perth Hospital Perth, Western Australia

We were interested to read the article by Sun et al. (1), who claim that the second gas effect is not a valid concept.

The authors showed no statistical difference for F_A/F_I of enflurane when administered with and without 80% nitrous oxide. However, we believe they have fallen into the trap of equating "no statistical difference" to "there is no difference," a commonly-made Type II error (2).

Using their 5 min data, we calculated the 95% confidence limits of the mean difference as -0.06 to 0.10. These comfortably include the difference of 0.075, which we computed for F_A/F_I of isoflurane with and without nitrous oxide and used in our recent explanation of the second gas effect in the water analog (3). Those computations were for the "constant-inflow" case (4), i.e., fixed inspired ventilation, also used by Sun et al. (1). The second gas effect is considerably more marked (5) in the "constant-outflow" case (4), used in the original description (6), where it is the expired ventilation which is kept constant. Therefore, the authors studied the case in which they were least likely to demonstrate the second gas effect.

Although we support the contention that the second gas effect is clinically unimportant, it continues to attract interest and therefore requires a clear explanation (3,4).

References

1. Sun X, Su F, Shi Y, et al. The "second gas effect" is not a valid concept. Anesth Analg 1999;88:188–92.[Abstract/Free Full Text]
2. Zar JH. Biostatistical analysis. 2nd ed. Englewood Cliffs, NJ:Prentice Hall, 1984:43–5.
3. Mapleson WW, Korman B. Concentration and second gas-effects in the water analogue. Br J Anaesth 1998;81:837–43.[Abstract/Free Full Text]
4. Korman B, Mapleson WW. Concentration and second gas effects: can the accepted explanation be improved? Br J Anaesth 1997;78:618–25 (erratum 1997;79:268).
5. Mapleson WW. Inert gas-exchange theory using an electric analogue. J Appl Physiol 1964;19:1193–9.[Abstract/Free Full Text]
6. Epstein RM, Rackow H, Salanitre E, et al. Influence of the concentration effect on the uptake of anesthetic mixtures: the second gas effect. Anesthesiology 1964;25:364–71.[Web of Science][Medline]

Response

Department of Anesthesiology Harbor-University of California Los Angeles Medical Center Torrance, CA 90509-2910

Theoretically, disproportionate removal of any gas from any space will alter the composition of the remaining gases, whether the movement is from the tidal volume to the functional residual capacity or from the alveoli to blood. On physical and mathematical models, therefore, "concentration and second gas effects" must be real (1,2). However, such effects are difficult to verify in vivo (3), and any verifiable effects are clinically "unimportant" to those giving and receiving anesthesia. In practice, therefore, the second gas effect is not a valid concept to exploit in clinical anesthesia.

Any drug given in large doses will likely hasten, and any anesthetic beneficially added to another will likely augment, the effects. Generations of physician and nonphysician anesthetists have mistaken this nonspecific "large dose effect" for a specific "concentration effect" and a nonspecific additive or synergistic effect for a specific "second gas effect." In view of this, we prefer a conservative stance when dealing with statistical risks in clinical studies. We appreciate the high risk of missing marginal differences; however, we are not convinced that we have actually fallen to the statistical trap of nullifying a tree in a forest. Nevertheless, we enjoy their expert scholarly dissertations.

References

1. Mapleson WW, Korman B. Concentration and second-gas effects in a water analogue. Anaesth 1998;81:837–43.
2. Korman B, Mapleson WW. Concentration and second-gas effects: can the accepted explanation be improved? Br J Anaesth 1997;78:618–25 (erratum 1997;79:268).
3. Sun X, Su F, Shi Y, et al. The "second gas effect" is not a valid concept. Anesth Analg 1999;88:188–92.

This Article Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a colleague Similar articles in this journal Similar articles in Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Permissions Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Mapleson, W. W. Articles by Sun, X.-G. Search for Related Content PubMed PubMed Citation Articles by Mapleson, W. W. Articles by Sun, X.-G.