Second Gas Effect

The factors that govern the concentration effect also influence the concentration of any gas given concomitantly with nitrous oxide (i.e., second gas effect).^{[15] [18]} The loss of volume associated with the uptake of nitrous oxide concentrates the second gas (see Fig. 5-3A ). Replacement of the gas taken up by an increase in inspired ventilation augments the amount of second gas in the lung (see Fig. 5-3B ).

Both the concentration effect and the second gas effect have been demonstrated in dogs^[18] and humans.^[19] Humans received 4% desflurane in 5% nitrous oxide or 65% nitrous oxide. The FA/FI for nitrous oxide rose more rapidly when 65% nitrous oxide was inspired than when 5% was inspired (i.e., concentration effect) ( Fig. 5-4 ). Similarly, the FA/FI ratio for desflurane rose more rapidly when 65% nitrous oxide was inspired than when 5% was inspired (i.e., second gas effect).