Effect of the Mode of Administration of Inhaled Anaesthetics on the Interpretation of the FA/FI Curve – a GasMan® Simulation

Abstract

The effects of blood solubility, cardiac output and ventilation on the rise of the alveolar towards the inspired concentration, the FA/FI 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 If is the partial pressure ultimately attained in the central nervous system (FVRG). These classical FA/FI curves assumed a constant FI. We used GasMan® to examine whether changes in solubility, cardiac output and ventilation affect the relationship between the FA/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 FA, FI FA/FI. and FVRG with either FI kept constant or FA kept constant (at 1 minimum alveolar concentration). High fresh gas flows were used to avoid rebreathing, so that the delivered concentration matched FI. Despite similar effects on the FA/FI curve, the effects on FVRG differed. With constant FI, lower solubility or higher ventilation results in a higher FVRG and a higher cardiac output results in a lower FVRG With constant FA, solubility has only a minimal effect on FVRG; an increase in cardiac output hastens the rise of FVRG to the same plateau value; and a change in ventilation has minimal effect on FVRG. Despite similar effects on the FA/FI curve, the effects of solubility, cardiac output and ventilation on the FVRG are different when either FI or FA are kept constant. With the FI kept constant, induction of anaesthesia is slower with a higher cardiac output, but with FA 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.