Concentration and second gas effects: can the accepted explanation be improved?

Abstract

During induction with high inspired concentrations of nitrous oxide, net uptake of gas produces a contraction in volume and a concentrating effect. In turn, this results in concentration and second gas effects. Most explanations of these effects are based on the common "rectangle" diagram devised by Stoelting and Eger and contain several inconsistencies which are explored here in order to produce a more accurate description. It is shown that in the standard diagram gas uptake is incomplete, there is ambiguity over functional residual capacity (FRC), equilibration with blood is inadequately represented and there is no representation of recirculation of anaesthetic. Compensation for loss of volume may be by means of an increased inspired ventilation, decreased expired ventilation or reduction in lung volume. Numerous accounts in the literature (including those based on the standard diagram) focus on the former mechanism at constant FRC. This has produced an unbalanced picture in which it is often implied that extra gas is routinely drawn into the lungs to replace that taken up. Significant compensation by this means cannot occur, for example when a constant volume ventilator is used. In discussing concentration and second gas effects, it is necessary to give a balanced view of the alternative mechanisms of compensation or to revert, as above, to a simple statement of the principle of conservation of volume.

Clinical evaluation of the Mapleson theoretical ideal fresh gas flow sequence at the start of low‐flow anaesthesia with isoflurane, sevoflurane and desflurane*

“…The suggested scheme is also the most reliable in achieving mean end-expired concentration equal to 1 MAC (Table 2) after the first 2 min of isoflurane. It would be interesting to evaluate the effect of nitrous oxide, as its hypnotic properties would be especially useful in this context, although computer simulation would be complicated by the second gas effect [9,12].…”

Can Mathematical Modeling Explain the Measured Magnitude of the Second Gas Effect?

“…Years later, Korman and Mapleson showed that the effect is primarily one of volume “shrinkage” and produced modifications of the Stoelting-Eger diagram that explained the phenomenon more accurately, including equilibration of the alveolar compartment with blood. 3 …”

Low flow, minimal flow and closed circuit system inhalational anesthesia in modern clinical practice

“…Finally, due to the pharmacological characteristics of the last inhaling anesthetics, together with the advantages offered by low flow or metabolic flow circuits, volatile anesthesia seems to be more advantageous than intravenous anesthesia, especially in regard to waking time and awareness prevention. (10,11) Technology …”