British Journal of Anaesthesia, Vol 78, Issue 5 618-625, Copyright © 1997 by The Board of Management and Trustees of the British Journal of Anaesthesia
B. Korman and W. W. Mapleson
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.
COMMENTARIES
Concentration and second gas effects: can the accepted explanation be improved?
Department of Anaesthesia, Royal Perth Hospital, Perth, Western Australia; Department of Anaesthetics and Intensive Care Medicine, University of Wales College of Medicine, Cardiff
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