Effect of the mode of administration of inhaled anaesthetics on the interpretation of the F_A/F_I curve - A GasMan® simulation

The effects of blood solubility, cardiac output and ventilation on the rise of the alveolar towards the inspired concentration, the F_A/F _I curve, of an inhaled anaesthetic are often thought to reflect how these factors affect wash-in of the central nervous system compartment and, therefore, speed of induction because F_A is the partial pressure ultimately attained in the central nervous system (F_VRG). These classical F_A/F_I curves assumed a constant F_I . We used GasMan® to examine whether changes in solubility, cardiac output and ventilation affect the relationship between the F_A/FI curve and FVRG differently while either FI or FA are kept constant. Using GasMan® , we studied the effects of solubility (desflurane vs isoflurane), cardiac output (5 vs 10 l.min^-1) and minute ventilation (4 vs 8 l.min^-1) on F_A/ F_I , F_A/F_Iand F_VRG with either F_I kept constant or F_A kept constant (at 1 minimum alveolar concentration). High fresh gas flows were used to avoid rebreathing, so that the delivered concentration matched F_I . Despite similar effects on the F_A/F_I curve, the effects on F _VRG differed. With constant F_I, lower solubility or higher ventilation results in a higher F_VRG and a higher cardiac output results in a lower F_VRG . With constant F_A , solubility has only a minimal effect on F_VRG ; an increase in cardiac output hastens the rise of F_VRG to the same plateau value; and a change in ventilation has minimal effect on F_VRG . Despite similar effects on the F_A/F_I curve, the effects of solubility, cardiac output and ventilation on the F_VRG are different when either F_I or F_A are kept constant. With the F_I kept constant, induction of anaesthesia is slower with a higher cardiac output, but with F _A kept constant, induction of anaesthesia is faster with a higher cardiac output. The introduction of an end-expired closed-loop feedback administration of inhaled anaesthetics makes this distinction clinically relevant.