Effect of the mode of administration of inhaled anaesthetics on the interpretation of the [F.sub.A]/[F.sub.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.sub.A]/[F.sub.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.sub.A] is the partial pressure ultimately attained in the central nervous system ([F.sub.VRG]). These classical [F.sub.A]/[F.sub.I] curves assumed a constant [F.sub.I]. We used GasMan® to examine whether changes in solubility, cardiac output and ventilation affect the relationship between the [F.sub.A]/[F.sub.I] curve and [F.sub.VRG] differently while either [F.sub.I] or [F.sub.A] are kept constant. Using GasMan®, we studied the effects of solubility (desflurane vs isoflurane), cardiac output (5 vs 10 l.[min.sup.-1]) and minute ventilation (4 vs 8 l.[min.sup.-1]) on [F.sub.A], [F.sub.I], [F.sub.A]/[F.sub.I] and [F.sub.VRG] with either [F.sub.I] kept constant or [F.sub.A] kept constant (at 1 minimum alveolar concentration). High fresh gas flows were used to avoid rebreathing, so that the delivered concentration matched [F.sub.I]. Despite similar effects on the [F.sub.A]/[F.sub.I] curve, the effects on [F.sub.VRG] differed. With constant [F.sub.I], lower solubility or higher ventilation results in a higher [F.sub.VRG] and a higher cardiac output results in a lower [F.sub.VRG]. With constant [F.sub.A], solubility has only a minimal effect on [F.sub.VRG]; an increase in cardiac output hastens the rise of [F.sub.VRG] to the same plateau value; and a change in ventilation has minimal effect on [F.sub.VRG]. Despite similar effects on the [F.sub.A]/[F.sub.I] curve, the effects of solubility, cardiac output and ventilation on the [F.sub.VRG] are different when either [F.sub.I] or [F.sub.A] are kept constant. With the [F.sub.I] kept constant, induction of anaesthesia is slower with a higher cardiac output, but with [F.sub.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. Key Words: anaesthetics, volatile, isoflurane, desflurane, wash-in curves, simulation, anaesthesia, GasMan®