CIRCUIT ABSORPTION OF HALOTHANE, ISOFLURANE, AND SEVOFLURANE

Uptake of inhaled anesthetics may be measured as the amount of anesthe tic infused to maintain a constant alveolar concentration of anestheti c. This method assumes that the patient absorbs all of the infused ane sthetic, and that none is lost to circuit components. Using a standard anesthetic circuit with a 3-L rebreathing bag simulating the lungs, a nd simulating metabolism by input of carbon dioxide, we tested this as sumption for halothane, isoflurane, and sevoflurane. Our results sugge st that after washin of anesthetic sufficient to eliminate a material difference between inspired and end-tidal anesthetic, washin to other parts of the circuit (probably the ventilator) and absorbent (soda lim e) continued to remove anesthetic for up to 15 min. From 30 min to 180 min of anesthetic administration, circuit components absorbed trivial amounts of isoflurane (12 +/- 13 ml vapor at 1.5 minimum alveolar ane sthetic concentration, slightly more sevoflurane (39 +/- 15 ml), and s till more halothane (64 +/- 9 ml). During this time, absorbent degrade d sevoflurane (321 +/- 31 mL absorbed by circuit components and degrad ed by soda lime). The amount degraded increased with increasing input of carbon dioxide (e.g., the 321 +/- 31 mL increased to 508 +/- 48 mt when carbon dioxide input increased from 250 mL/min to 500 mL/min). Me asurement of anesthetic uptake as a function of the amount of anesthet ic infused must account for these findings. Implications: Systems that deliver inhaled anesthetics may also remove the anesthetic. Initially , anesthetics may diffuse into delivery components and the interstices of material used to absorb carbon dioxide. Later, absorbents may degr ade some anesthetics (e.g., sevoflurane). Such losses may compromise m easurements of anesthetic uptake.